Investigation of Simple IMRT Delivery Techniques for Non-Small Cell Lung Cancer Patients with Respiratory Motion Using 4DCT

Techniques for generating simplified IMRT treatment plans for treating non-small cell lung cancer (NSCLC) patients with respiratory motion were investigated. To estimate and account for respiratory motion, 4-dimensional computed tomography (4DCT) datasets from 5 patients were used to design 5-field 6-MV ungated step-and-shoot intensity modulated radiotherapy (IMRT) plans delivering a dose of 66 Gy to the planning target volume (PTV). For each patient, 2 plans were generated using the mean intensity and the maximum intensity of 10 CT datasets from different breathing phases. The plans also utilized different margins around the clinical target volume/internal target volume (CTV/ITV) to account for tumor motion. To reduce the treatment time and ensure accurate dose delivery to moving targets, the number of intensity levels was minimized while maintaining dose coverage to PTV and minimizing dose to organs at risk (OARs). Dose-volume histograms (DVHs), dosimetric metrics, and outcome probabilities were evaluated for all plans. Plans using the averaged CT image dataset were inferior, requiring larger margins around the PTV, with a maximum of 1.5 cm, to ensure coverage of the tumor, and therefore increased the dose to OARs located in proximity of the tumor. The plans based on superimposed CT image datasets achieved full coverage of the tumor, while allowing tight margins around the PTV and minimizing the dose to OARs. A small number of intensity-levels (3 to 5), resulting in IMRT plans with a total of 13 to 30 segments, were sufficient for homogeneous PTV coverage, without affecting the sparing of OARs. In conclusion, a technique involving treatment planning with the superimposed CT scans of all respiratory phases, and the application of IMRT with only a small number of segments was feasible despite significant tumor motion; however, greater patient numbers are needed to support the statistical significance of the results presented in this work.     

Postoperative Radiotherapy for Prostate Cancer

PURPOSE: To determine the extent of target motion in postprostatectomy radiotherapy (RT) and the value of intensity-modulated radiotherapy (IMRT) compared to three-dimensional conformal radiotherapy (3D-CRT). PATIENTS AND METHODS: 20 patients underwent CT scans in supine position with both a full bladder (FB) and an empty bladder (EB) before RT and at three dates during the RT series. Displacements of the CTV (clinical target volume) center of mass and the posterior border were determined. 3D-CRT and IMRT treatment plans were compared regarding homogeneity, conformity, and dose to organs at risk. RESULTS: In the superior-inferior direction, larger displacements were found for EB compared to FB scans; anterior-posterior and right-left displacements were similar. With an initial rectum volume of < 115 cm(3), 90% of displacements at the posterior border were within a margin of 6 mm. The non-target volume irradiated in the high-dose area doubled in 3D-CRT versus IMRT plans (80 cm(3) vs. 38 cm(3) encompassed by the 95% isodose). Bladder dose was significantly lower with IMRT, but no advantage was found for the integral rectal dose. An adequate bladder filling was paramount to reduce the dose to the bladder. CONCLUSION: Postprostatectomy RT can be recommended with FB due to an improved CTV position consistency and a lower dose to the bladder. With improved non-target tissue and bladder volume sparing, IMRT is an option for dose escalation. However, this analysis did not find an advantage concerning the integral rectal dose with IMRT versus 3D-CRT.     

Comparative dosimetric study of two strategies of intensity-modulated radiotherapy in nasopharyngeal cancer

This study compared the target volume coverage and normal tissues sparing of simultaneous integrated boost (SIB-IMRT, 1-phase) and sequential-IMRT (2-phase) for nasopharyngeal carcinoma (NPC). Fourteen consecutive patients with newly diagnosed primary NPC were enrolled in this study. The CT images were transferred to a commercial planning system for structural delineation. The gross tumor volume (GTV) included gross nasopharyngeal tumor and involved lymph nodes of more than 1-cm diameter. The clinical target volume (CTV) modeled two regions considered to represent different risks. CTV1 encompassed the GTV with 5–10-mm margin of adjacent tissues. CTV2 encompassed ipsilateral or contralateral elective nodal regions at risk of harboring microscopic tumor. A commercial IMRT treatment planning system (Eclipse Version 7.1) was used to provide treatment planning. Seven fixed-gantry (0°, 50°, 100°, 150°, 210°, 260°, 310°) angles were designated. The 14 patients were treated with sequential-IMRT, and treatment was then replanned with an SIB strategy to compare the dosimetric difference. For the sequential strategy, the dose delivered to CTV1/CTV2 in the first course was 54 Gy (1.8 Gy × 30 Fr); while CTV1 was boosted by an additional 16.2 Gy (1.8 Gy × 9 Fr) in the second course. For SIB-IMRT, the dose prescribed to CTV1 was 69.7 Gy (2.05 Gy × 34 Fr); 56.1 Gy was given to CTV2 (1.65 Gy × 34 Fr). A statistical analysis of the dose-volume-histogram of target volumes and critical organs was performed. Paired Student’s t-test was used to compare the dosimetric differences between the two techniques. The mean dose to CTV1 was 101.7 ± 2.4% and 102.3 ± 3.1% of the prescribed dose for SIB-IMRT and sequential-IMRT, respectively. The mean CTV2 dose was 109.8 ± 4.7% of the prescribed dose for SIB-IMRT and 112.6 ± 6.0% of the prescribed dose for sequential-IMRT. The maximal dose to the spinal cord was 4489 ± 495 cGy and 3547 ± 767 cGy for SIB and sequential-IMRT (p = 0.0001), respectively. The maximal dose to brain stem was significantly higher using SIB technique (5284 ± 551 cGy) than sequential-IMRT (4834 ± 388 cGy) (p = 0.0001). The mean dose to the parotid gland and ear apparatus was significantly lower using SIB-IMRT. The mean dose to the right/left parotids was 2865 ± 320 cGy/2903 ± 429 cGy and 3567 ± 534 cGy/3476 ± 489 cGy for SIB and sequential-IMRT, respectively (p = 0.0001). Target coverage was the same for both techniques; the dose distribution in the elective nodal area with SIB was superior to that with sequential-IMRT. SIB-IMRT provides better sparing of parotid gland and inner ear structures. Extra caution should be taken when applying SIB-IMRT since critical organs close to the boost volume may receive higher doses.     

Physical and clinical implications of radiotherapy treatment of prostate cancer using a full bladder protocol

Purpose

To assess the dosimetric and clinical implication when applying the full bladder protocol for the treatment of the localized prostate cancer (PCA).

Patients and Methods

A total of 26 consecutive patients were selected for the present study. Patients underwent two series of CT scans: the day of the simulation and after 40 Gy. Each series consisted of two consecutive scans: (1) full bladder (FB) and (2) empty bladder (EB). The contouring of clinical target volumes (CTVs) and organs at risk (OAR) were compared to evaluate organ motion. Treatment plans were compared by dose distribution and dose?Cvolume histograms (DVH).

Results

CTV shifts were negligible in the laterolateral and superior?Cinferior directions (the maximum shift was 1.85 mm). Larger shifts were recorded in the anterior?Cposterior direction (95% CI, 0.83?C4.41 mm). From the dosimetric point of view, shifts are negligible: the minimum dose to the CTV was 98.5% (median; 95%CI, 95?C99%). The potential advantage for GU toxicity in applying the FB treatment protocol was measured: the ratio between full and empty bladder dose?Cvolume points (selected from our protocol) is below 0.61, excluding the higher dose region where DVHs converge.

Conclusion

Having a FB during radiotherapy does not affect treatment effectiveness, on the contrary it helps achieve a more favorable DVH and lower GU toxicities.     

The Use of 4DCT to Reduce Lung Dose: A Dosimetric Analysis

Dosimetric studies on respiratory movement suggest several advantages toward the use of 4-dimensional computed tomography (4DCT) in radiation treatment planning. 4DCT is a method to obtain a series of CT scans each representing a different respiratory phase. The use of 4DCT has provided substantial information on tumor movement in the lung, allowing for the creation of custom planning margins explicitly including respiratory motion. These custom motion margins may result in an increase in the amount of normal lung in the field; however, it is believed less normal lung is irradiated than if generic motion margins were used. Clinical data regarding dose to normal lung by using 4DCT remain rather limited. Thus, a study presenting figures on the change in normal lung dose between planned free breathing CT and 4DCT cases would be useful to the dosimetry community. We have generated plans comparing fast spiral CT and 4DCT in regard to tumor coverage and the resulting dose to normal lung for the clinical target volume (CTV) and planning target volume (PTV) expansions used at our institution. These data were analyzed for free breathing and 4D plans of 6 lung cancer patients using intensity modulated radiation therapy (IMRT). We compared doses to normal lung tissue between free breathing and 4DCT plans.     

The Use of 4DCT to Reduce Lung Dose: A Dosimetric Analysis

Dosimetric studies on respiratory movement suggest several advantages toward the use of 4-dimensional computed tomography (4DCT) in radiation treatment planning. 4DCT is a method to obtain a series of CT scans each representing a different respiratory phase. The use of 4DCT has provided substantial information on tumor movement in the lung, allowing for the creation of custom planning margins explicitly including respiratory motion. These custom motion margins may result in an increase in the amount of normal lung in the field; however, it is believed less normal lung is irradiated than if generic motion margins were used. Clinical data regarding dose to normal lung by using 4DCT remain rather limited. Thus, a study presenting figures on the change in normal lung dose between planned free breathing CT and 4DCT cases would be useful to the dosimetry community. We have generated plans comparing fast spiral CT and 4DCT in regard to tumor coverage and the resulting dose to normal lung for the clinical target volume (CTV) and planning target volume (PTV) expansions used at our institution. These data were analyzed for free breathing and 4D plans of 6 lung cancer patients using intensity modulated radiation therapy (IMRT). We compared doses to normal lung tissue between free breathing and 4DCT plans.     

A robust volumetric arc therapy planning approach for breast cancer involving the axillary nodes

We quantify the robustness of a proposed volumetric-modulated arc therapy (VMAT) planning and treatment technique for radiotherapy of breast cancer involving the axillary nodes. The proposed VMAT technique is expected to be more robust to breast shape changes and setup errors, yet maintain the improved conformity of VMAT compared to our current standard technique that uses tangential intensity-modulated radiation therapy (IMRT) fields. Treatment plans were created for 10 patients. To account for anatomical variation, planning was carried out on a computed tomography (CT) with an expanded breast, followed by segment weight optimization (SWO) on the original planning CT (VMAT + SWO). For comparison purposes, tangential field IMRT plans and conventional VMAT (cVMAT) plans were also created. Anatomical changes (expansion and contraction of the breast) and setup errors were simulated to quantify changes in target coverage, target maximum, and organ-at-risk (OAR) doses. Finally, robustness was assessed by calculating the actual delivered dose for each fraction using cone-beam CT images acquired during treatment. Target coverage of VMAT + SWO was shown to be significantly more robust compared to cVMAT technique, against anatomical variations and setup errors. Sensitivity of the clinical target volume (CTV) V95% is ?5%/cm of expansion for the proposed technique, which is identical to the IMRT technique and much lower than the ?22%/cm for cVMAT. Results are similar for setup errors. OAR doses are mostly insensitive to anatomical variations and the OAR sensitivity to setup variations does not depend on the planning technique. The results are confirmed by dose distributions recalculated on cone-beam CT, showing that for VMAT + SWO the CTV V95% remains within 2.5% of the planned value, whereas it deviates by up to 7% for cVMAT. A practical VMAT planning technique is developed, which is robust to daily anatomical variations and setup errors.     

Comparative dosimetric study of two strategies of intensity-modulated radiotherapy in nasopharyngeal cancer

This study compared the target volume coverage and normal tissues sparing of simultaneous integrated boost (SIB-IMRT, 1-phase) and sequential-IMRT (2-phase) for nasopharyngeal carcinoma (NPC). Fourteen consecutive patients with newly diagnosed primary NPC were enrolled in this study. The CT images were transferred to a commercial planning system for structural delineation. The gross tumor volume (GTV) included gross nasopharyngeal tumor and involved lymph nodes of more than 1-cm diameter. The clinical target volume (CTV) modeled two regions considered to represent different risks. CTV1 encompassed the GTV with 5–10-mm margin of adjacent tissues. CTV2 encompassed ipsilateral or contralateral elective nodal regions at risk of harboring microscopic tumor. A commercial IMRT treatment planning system (Eclipse Version 7.1) was used to provide treatment planning. Seven fixed-gantry (0°, 50°, 100°, 150°, 210°, 260°, 310°) angles were designated. The 14 patients were treated with sequential-IMRT, and treatment was then replanned with an SIB strategy to compare the dosimetric difference. For the sequential strategy, the dose delivered to CTV1/CTV2 in the first course was 54 Gy (1.8 Gy × 30 Fr); while CTV1 was boosted by an additional 16.2 Gy (1.8 Gy × 9 Fr) in the second course. For SIB-IMRT, the dose prescribed to CTV1 was 69.7 Gy (2.05 Gy × 34 Fr); 56.1 Gy was given to CTV2 (1.65 Gy × 34 Fr). A statistical analysis of the dose-volume-histogram of target volumes and critical organs was performed. Paired Student’s t-test was used to compare the dosimetric differences between the two techniques. The mean dose to CTV1 was 101.7 ± 2.4% and 102.3 ± 3.1% of the prescribed dose for SIB-IMRT and sequential-IMRT, respectively. The mean CTV2 dose was 109.8 ± 4.7% of the prescribed dose for SIB-IMRT and 112.6 ± 6.0% of the prescribed dose for sequential-IMRT. The maximal dose to the spinal cord was 4489 ± 495 cGy and 3547 ± 767 cGy for SIB and sequential-IMRT (p = 0.0001), respectively. The maximal dose to brain stem was significantly higher using SIB technique (5284 ± 551 cGy) than sequential-IMRT (4834 ± 388 cGy) (p = 0.0001). The mean dose to the parotid gland and ear apparatus was significantly lower using SIB-IMRT. The mean dose to the right/left parotids was 2865 ± 320 cGy/2903 ± 429 cGy and 3567 ± 534 cGy/3476 ± 489 cGy for SIB and sequential-IMRT, respectively (p = 0.0001). Target coverage was the same for both techniques; the dose distribution in the elective nodal area with SIB was superior to that with sequential-IMRT. SIB-IMRT provides better sparing of parotid gland and inner ear structures. Extra caution should be taken when applying SIB-IMRT since critical organs close to the boost volume may receive higher doses.     

CT图像重建视野大小对放射治疗计划剂量计算及体积评估的影响

目的 探讨CT图像重建视野(FOV)大小对放射治疗计划剂量计算及体积评估可能存在的影响。方法 对16例鼻咽癌患者的CT原始扫描数据分别行45 cm常规FOV和65 cm扩展视野(EFOV)重建并传输至放射治疗计划系统,所有病例均在常规FOV重建的CT图像上勾画肿瘤体积(GTV)、临床靶区(CTV)及脑干、晶体、腮腺、脊髓等危及器官,并制定7野等角动态调强放射治疗计划(GTV处方剂量70 Gy)。两种重建方法图像按照医学数字影像通信3.0标准(DICOM 3.0)坐标方式融合后,拷贝常规FOV图像上的靶区及危及器官至EFOV图像,并将治疗计划移植至EFOV图像,治疗计划中心为两种重建方法图像的同一DICOM坐标,利用剂量体积直方图(DVH)工具计算两种重建方法图像上GTV、CTV和脑干、晶体、腮腺、脊髓的体积、最大剂量(D_max)、平均剂量(D_mean)及最小剂量(D_min)。将入组病例的每个治疗计划7野分别导入常规45 cm FOV和65 cm EFOV重建的二维通量图验证设备Mapchek 1175的模体,距离通过协议(DTA)分析5 cm深度平面绝对剂量的计算和实测结果通过率。结果 两种重建方法图像上的靶区和危及器官的体积差异具有统计学意义,所有入组病例靶区和危及器官在常规FOV图像上的体积均大于EFOV图像上的体积。较小体积的晶体最大剂量D_max常规FOV与EFOV图像之间差异有统计学意义(t =-3.14, P<0.007),其余靶区及危及器官的最大剂量D_max差异无统计学意义。CTV和GTV平均剂量D_mean在EFOV图像上大于FOV图像,差异有统计学意义(t=-6.45、-5.65, P< 0.001),危及器官的平均剂量D_mean和靶区及危及器官最小剂量D_min差异均无统计学意义。两种重建方法图像上治疗计划的7野通过率之间差异无统计学意义。结论 在放射治疗CT模拟定位过程中图像重建FOV的大小对于靶区及部分危及器官的体积及剂量计算结果和治疗计划的评价存在影响;观察和验证二维通量图通过率,两者之间的差异并不显著。     

CT图像重建视野大小对放射治疗计划剂量计算及体积评估的影响

目的 探讨CT图像重建视野(FOV)大小对放射治疗计划剂量计算及体积评估可能存在的影响.方法 对16例鼻咽癌患者的CT原始扫描数据分别行45 cm常规FOV和65 cm扩展视野(EFOV)重建并传输至放射治疗计划系统,所有病例均在常规FOV重建的CT图像上勾画肿瘤体积(GTV)、临床靶区(CTV)及脑干、晶体、腮腺、脊髓等危及器官,并制定7野等角动态调强放射治疗计划(GTV处方剂量70 Gy).两种重建方法图像按照医学数字影像通信3.0标准(DICOM 3.0)坐标方式融合后,拷贝常规FOV图像上的靶区及危及器官至EFOV图像,并将治疗计划移植至EFOV图像,治疗计划中心为两种重建方法图像的同一DICOM坐标,利用剂量体积直方图(DVH)工具计算两种重建方法图像上GTV、CTV和脑干、晶体、腮腺、脊髓的体积、最大剂量(Dmax)、平均剂量(Dmean)及最小剂量(Dmin).将入组病例的每个治疗计划7野分别导入常规45 cm FOV和65 cmEFOV重建的二维通量图验证设备Mapchek 1175的模体,距离通过协议(DTA)分析5 cm深度平面绝对剂量的计算和实测结果通过率.结果 两种重建方法图像上的靶区和危及器官的体积差异具有统计学意义,所有入组病例靶区和危及器官在常规FOV图像上的体积均大于EFOV图像上的体积.较小体积的晶体最大剂量Dmax常规FOV与EFOV图像之间差异有统计学意义(t=-3.14,P<0.007),其余靶区及危及器官的最大剂量Dmax差异无统计学意义.CTV和GTV平均剂量Dmean在EFOV图像上大于FOV图像,差异有统计学意义(t=-6.45、-5.65,P<0.001),危及器官的平均剂量Dmean和靶区及危及器官最小剂量Dmin差异均无统计学意义.两种重建方法图像上治疗计划的7野通过率之间差异无统计学意义.结论 在放射治疗CT模拟定位过程中图像重建FOV的大小对于靶区及部分危及器官的体积及剂量计算结果和治疗计划的评价存在影响;观察和验证二维通 量图通过率,两者之间的差异并不显著.     

CT图像重建视野大小对放射治疗计划剂量计算及体积评估的影响

目的 探讨CT图像重建视野(FOV)大小对放射治疗计划剂量计算及体积评估可能存在的影响.方法 对16例鼻咽癌患者的CT原始扫描数据分别行45 cm常规FOV和65 cm扩展视野(EFOV)重建并传输至放射治疗计划系统,所有病例均在常规FOV重建的CT图像上勾画肿瘤体积(GTV)、临床靶区(CTV)及脑干、晶体、腮腺、脊髓等危及器官,并制定7野等角动态调强放射治疗计划(GTV处方剂量70 Gy).两种重建方法图像按照医学数字影像通信3.0标准(DICOM 3.0)坐标方式融合后,拷贝常规FOV图像上的靶区及危及器官至EFOV图像,并将治疗计划移植至EFOV图像,治疗计划中心为两种重建方法图像的同一DICOM坐标,利用剂量体积直方图(DVH)工具计算两种重建方法图像上GTV、CTV和脑干、晶体、腮腺、脊髓的体积、最大剂量(Dmax)、平均剂量(Dmean)及最小剂量(Dmin).将入组病例的每个治疗计划7野分别导入常规45 cm FOV和65 cmEFOV重建的二维通量图验证设备Mapchek 1175的模体,距离通过协议(DTA)分析5 cm深度平面绝对剂量的计算和实测结果通过率.结果 两种重建方法图像上的靶区和危及器官的体积差异具有统计学意义,所有入组病例靶区和危及器官在常规FOV图像上的体积均大于EFOV图像上的体积.较小体积的晶体最大剂量Dmax常规FOV与EFOV图像之间差异有统计学意义(t=-3.14,P<0.007),其余靶区及危及器官的最大剂量Dmax差异无统计学意义.CTV和GTV平均剂量Dmean在EFOV图像上大于FOV图像,差异有统计学意义(t=-6.45、-5.65,P<0.001),危及器官的平均剂量Dmean和靶区及危及器官最小剂量Dmin差异均无统计学意义.两种重建方法图像上治疗计划的7野通过率之间差异无统计学意义.结论 在放射治疗CT模拟定位过程中图像重建FOV的大小对于靶区及部分危及器官的体积及剂量计算结果和治疗计划的评价存在影响;观察和验证二维通 量图通过率,两者之间的差异并不显著.     

Dosimetric comparison of intensity modulated radiotherapy and intensity modulated proton therapy in the treatment of recurrent nasopharyngeal carcinoma

Background and purposeTo compare the dosimetric performance of Intensity Modulated Proton Therapy (IMPT) and Intensity Modulated Radiotherapy (IMRT) in terms of target volume coverage and sparing of neurological organs-at-risk (OARs) in salvaging recurrent nasopharyngeal carcinoma (rNPC). The maximum dose to the internal carotid artery (ICA) and nasopharyngeal (NP) mucosa, which are associated with potential carotid blowout and massive epistaxis, were also evaluated.Materials and methodsIMRT and IMPT treatment plans were created for twenty patients with locally advanced rNPC. Planning Target Volume (PTV) was used to account for the setup and spatial error/uncertainty in the IMRT planning. Robust optimization on Clinical Target Volume (CTV) coverage with consideration of range and setup uncertainty was employed to produce two IMPT plans with 3-field and 4-field arrangements. The planning objective was to deliver 60 Gy to the PTV (IMRT) and CTV (IMPT) without exceeding the maximum lifetime cumulative Biologically Effective Dose (BED) of the neurological OARs (applied to the Planning organs-at-risk volume). The target dose coverage as well as the maximum dose to the neurological OARs, ICA, and NP mucosa were compared.ResultsCompared with IMRT, 3-field IMPT achieved better coverage to GTV V100% (83.3% vs. 73.2%, P <0.01) and CTV V100% (80.5% vs. 72.4%, P <0.01), and lower maximum dose to the critical OARs including the spinal cord (19.2 Gy vs. 22.3 Gy, P <0.01), brainstem (30.0 Gy vs. 32.3 Gy, P <0.01) and optic chiasm (6.6 Gy vs. 9.8 Gy, P <0.01). The additional beam with the 4-fields IMPT plans further improved the target coverage from the 3-field IMPT (CTV V98%: 85.3% vs. 82.4%, P <0.01) with similar OAR sparing. However, the target dose was highly non-uniform with both IMPT plans, leading to a significantly higher maximum dose to the ICA (～68 Gy vs. 62.6 Gy, P <0.01) and NP mucosa (～72 Gy vs. 62.8 Gy, P <0.01) than IMRT.ConclusionIMPT demonstrated some dosimetric advantage over IMRT in treating rNPC. However, IMPT could also result in very high dose hot spots in the target volume. Careful consideration of the ICA and NP mucosal complications is recommended when applying IMPT on rNPC patients.     

Intensity modulation with respiratory gating for radiotherapy of the pleural space.

We wanted to describe a technique for the implementation of intensity-modulated radiotherapy (IMRT) with a real-time position monitor (RPM) respiratory gating system for the treatment of pleural space with intact lung. The technique is illustrated by a case of pediatric osteosarcoma, metastatic to the pleura of the right lung. The patient was simulated in the supine position where a breathing tracer and computed tomography (CT) scans synchronized at end expiration were acquired using the RPM system. The gated CT images were used to define target volumes and critical structures. Right pleural gated IMRT delivered at end expiration was prescribed to a dose of 44 Gy, with 55 Gy delivered to areas of higher risk via simultaneous integrated boost (SIB) technique. IMRT was necessary to avoid exceeding the tolerance of intact lung. Although very good coverage of the target volume was achieved with a shell-shaped dose distribution, dose over the targets was relatively inhomogeneous. Portions of target volumes necessarily intruded into the right lung, the liver, and right kidney, limiting the degree of normal tissue sparing that could be achieved. The radiation doses to critical structures were acceptable and well tolerated. With intact lung, delivering a relatively high dose to the pleura with acceptable doses to surrounding normal tissues using respiratory gated pleural IMRT is feasible. Treatment delivery during a limited part of the respiratory cycle allows for reduced CT target volume motion errors, with reduction in the portion of the planning margin that accounts for respiratory motion, and subsequent increase in the therapeutic ratio.     

Dosimetric difference amongst 3 techniques: TomoTherapy,sliding-window intensity-modulated radiotherapy (IMRT), and RapidArc radiotherapy in the treatment of late-stage nasopharyngeal carcinoma (NPC)

To investigate the dosimetric difference amongst TomoTherapy, sliding-window intensity-modulated radiotherapy (IMRT), and RapidArc radiotherapy in the treatment of late-stage nasopharyngeal carcinoma (NPC). Ten patients with late-stage (Stage III or IV) NPC treated with TomoTherapy or IMRT were selected for the study. Treatment plans with these 3 techniques were devised according to departmental protocol. Dosimetric parameters for organ at risk and treatment targets were compared between TomoTherapy and IMRT, TomoTherapy and RapidArc, and IMRT and RapidArc. Comparison amongst the techniques was done by statistical tests on the dosimetric parameters, total monitor unit (MU), and expected delivery time. All 3 techniques achieved similar target dose coverage. TomoTherapy achieved significantly lower doses in lens and mandible amongst the techniques. It also achieved significantly better dose conformity to the treatment targets. RapidArc achieved significantly lower dose to the eye and normal tissue, lower total MU, and less delivery time. The dosimetric advantages of the 3 techniques were identified in the treatment of late-stage NPC. This may serve as a guideline for selection of the proper technique for different clinical cases.     

An evaluation of adaptive planning by assessing the dosimetric impact of weight loss throughout the course of radiotherapy in bilateral treatment of head and neck cancer patients

The purpose of this study was to investigate the dosimetric impact of weight loss in head and neck (H&N) patients and examine the effectiveness of adaptive planning. Data was collected from 22 H&N cancer patients who experienced weight loss during their course of radiotherapy. The robustness of Intensity Modulated Radiation Therapy (IMRT) and Volumetric Modulated Arc Therapy (VMAT) treatment plans were compared including the potential need for replanning. The dosimetric impact of weight loss was evaluated by calculating a verification plan for each patient on an assessment CT scan taken during the course of treatment. Using a regression analysis, significance was tested for the dosimetric change in target volumes and 10 specific organs at risk (OAR) using an anatomical separation difference in the H&N at corresponding levels. For both the IMRT and VMAT plans, a significant correlation was found for the dose to 5% of the high risk Planning Target Volume (PTV) (D₅), dose to 95% of the intermediate risk PTV and Clinical Target Volume (CTV) (D₉₅), and the percentage of the pharynx receiving 65 Gy. An independent t-test was also performed for each metric in the VMAT and IMRT plans showing the dose to 95% of the intermediate risk PTV as significant. No quantitative method for finding the threshold of anatomical separation difference requiring a replan was established. Based on the increase in dose to organs at risk and increased target coverage due to separation loss, it was concluded that adaptive radiotherapy may not always be necessary when alignment of bony anatomy and remaining soft tissue is within tolerance. Physician judgment and preference is needed in such situations.     

Surface and superficial dose dosimetric verification for postmastectomy radiotherapy

In patients given postmastectomy radiotherapy (PMRT), the chest wall is a very thin layer of soft tissue with a low-density lung tissue behind. Chest wall treated in this situation with a high-energy photon beam presents a high dosimetric uncertainty region for both calculation and measurement. The purpose of this study was to measure and to evaluate the surface and superficial doses for patients requiring PMRT with different treatment techniques. An elliptic cylinder cork and superflab boluses were used to simulate the lung and the chest wall, respectively. Sets of computed tomography (CT) images with different chest wall thicknesses were acquired for the study phantom. Hypothetical clinical target volumes (CTVs) were outlined and modified to fit a margin of 1–3 mm, depending on the chest wall thickness, away from the surface for the sets of CT images. The planning target volume (PTV) was initially created by expanding an isotropic 3-mm margin from the CTV, and then a margin of 3 mm was shrunk from the phantom surface to avoid artifact-driven results in the beam-let intensity. Treatment techniques using a pair of tangential wedged fields (TWFs) and 4-field intensity-modulated radiation therapy (IMRT) were designed with a prescribed fraction dose (D_p) of 180 cGy. Superficial dose profiles around the phantom circumference at depths of 0, 1, 2, 3, and 5 mm were obtained for each treatment technique using radiochromic external beam therapy (EBT) films. EBT film exhibits good characteristics for dose measurements in the buildup region. Underdoses at the median and lateral regions of the TWF plans were shown. The dose profiles at shallow depths for the TWF plans show a dose buildup about 3 mm at the median and lateral tangential incident regions with a surface dose of about 52% of D_p. The dose was gradually increased toward the most obliquely tangential angle with a maximum dose of about 118% of D_p. Dose profiles were more uniform in the PTV region for the 4-F IMRT plans. Most of the PTV region had doses >94% of D_p at depths >1 mm. The mean surface dose was about 65% of D_p for the 4-F IMRT plans. The maximum dose for the 4-F IMRT plans was <118.4% of D_p. The application of added bolus has to consider the treatment technique, tumor coverage, and possible skin reactions. For PMRT, if the chest surface and wall are treated adequately, at least 3 mm bolus should be added to the chest wall when tangential beams and 6-MV photon energy are arranged. However, when the surface and superficial regions are not high-risk areas, an IMRT plan with tangential beams and 6-MV photon energy can provide uniform dose distributions within the PTV, spare the skin reaction, and deliver sufficient doses to the chest wall at depths >1 mm.     

The clinical implementation of respiratory-gated intensity-modulated radiotherapy.

The clinical use of respiratory-gated radiotherapy and the application of intensity-modulated radiotherapy (IMRT) are 2 relatively new innovations to the treatment of lung cancer. Respiratory gating can reduce the deleterious effects of intrafraction motion, and IMRT can concurrently increase tumor dose homogeneity and reduce dose to critical structures including the lungs, spinal cord, esophagus, and heart. The aim of this work is to describe the clinical implementation of respiratory-gated IMRT for the treatment of non-small cell lung cancer. Documented clinical procedures were developed to include a tumor motion study, gated CT imaging, IMRT treatment planning, and gated IMRT delivery. Treatment planning procedures for respiratory-gated IMRT including beam arrangements and dose-volume constraints were developed. Quality assurance procedures were designed to quantify both the dosimetric and positional accuracy of respiratory-gated IMRT, including film dosimetry dose measurements and Monte Carlo dose calculations for verification and validation of individual patient treatments. Respiratory-gated IMRT is accepted by both treatment staff and patients. The dosimetric and positional quality assurance test results indicate that respiratory-gated IMRT can be delivered accurately. If carefully implemented, respiratory-gated IMRT is a practical alternative to conventional thoracic radiotherapy. For mobile tumors, respiratory-gated radiotherapy is used as the standard of care at our institution. Due to the increased workload, the choice of IMRT is taken on a case-by-case basis, with approximately half of the non-small cell lung cancer patients receiving respiratory-gated IMRT. We are currently evaluating whether superior tumor coverage and limited normal tissue dosing will lead to improvements in local control and survival in non-small cell lung cancer.     

Dosimetric comparison of 3D conformal,IMRT, and V-MAT techniques for accelerated partial-breast irradiation (APBI)

The purpose is to dosimetrically compare the following 3 delivery techniques: 3-dimensional conformal radiation therapy (3D-CRT), intensity-modulated arc therapy (IMRT), and volumetric-modulated arc therapy (V-MAT) in the treatment of accelerated partial-breast irradiation (APBI). Overall, 16 patients with T1/2N0 breast cancer were treated with 3D-CRT (multiple, noncoplanar photon fields) on the RTOG 0413 partial-breast trial. These cases were subsequently replanned using static gantry IMRT and V-MAT technology to understand dosimetric differences among these 3 techniques. Several dosimetric parameters were used in plan quality evaluation, including dose conformity index (CI) and dose-volume histogram analysis of normal tissue coverage. Quality assurance studies including gamma analysis were performed to compare the measured and calculated dose distributions. The IMRT and V-MAT plans gave more conformal target dose distributions than the 3D-CRT plans (p < 0.05 in CI). The volume of ipsilateral breast receiving 5 and 10 Gy was significantly less using the V-MAT technique than with either 3D-CRT or IMRT (p < 0.05). The maximum lung dose and the ipsilateral lung volume receiving 10 (V₁₀) or 20 Gy (V₂₀) were significantly less with both V-MAT and IMRT (p < 0.05). The IMRT technique was superior to 3D-CRT and V-MAT of low dose distributions in ipsilateral lung (p < 0.05 in V₅ and D₅). The total mean monitor units (MUs) for V-MAT (621.0 ± 111.9) were 12.2% less than those for 3D-CRT (707.3 ± 130.9) and 46.5% less than those for IMRT (1161.4 ± 315.6) (p < 0.05). The average machine delivery time was 1.5 ± 0.2 minutes for the V-MAT plans, 7.0 ± 1.6 minutes for the 3D-CRT plans, and 11.5 ± 1.9 minutes for the IMRT plans, demonstrating much less delivery time for V-MAT. Based on this preliminary study, V-MAT and IMRT techniques offer improved dose conformity as compared with 3D-CRT techniques without increasing dose to the ipsilateral lung. In terms of MU and delivery time, V-MAT is significantly more efficient for APBI than for conventional 3D-CRT and static-beam IMRT.     

Comparison of a simple dose-guided intervention technique for prostate radiotherapy with existing anatomical image guidance methods

Objectives

A simple dose-guided intervention technique for prostate radiotherapy using an isodose overlay method combined with soft-tissue-based corrective couch shifts has been proposed previously. This planning study assesses the potential clinical impact of such a correction strategy.

Methods

10 patients, each with 8–11 on-treatment CT studies (n=97), were assessed using this technique and compared with no intervention, bony anatomy intervention and soft-tissue intervention methods. Each assessment technique used a 4-mm action level for intervention. Outcomes were evaluated using measures of sensitivity, specificity and dosimetric effect, and compared across intervention techniques. Dosimetric effect was defined as the change in dosimetric coverage by the 95% isodose from the no intervention case of an evaluation construct called the verification target volume.

Results

Bony anatomy, soft tissue and dosimetric overlay-based interventions demonstrated sensitivity of 0.56, 0.73 and 1.00 and specificity of 0.64, 0.20 and 0.66, respectively. A detrimental dosimetric effect was shown in 7% of interventions for each technique, with benefit in 30%, 35% and 55% for bony anatomy, soft tissue and dosimetric overlay techniques, respectively.

Conclusion

Used in conjunction with soft-tissue-based corrective couch shifts, the dosimetric overlay technique allows effective filtering out of dosimetrically unnecessary interventions, making it more likely that any intervention made will result in improved target volume coverage.Image-guided radiotherapy (IGRT) aims to improve treatment delivery accuracy by visualising the patient''s anatomy immediately prior to treatment, comparing this with the localisation data, usually a CT scan, and identifying and compensating for inaccuracies in the set up or target position that would compromise treatment efficacy [1]. In cancer of the prostate, potential inaccuracies include misalignments of the patient, e.g. caused by pelvic rotation or skin drag against the treatment couch, or changes in internal anatomy as a result of motion caused by bladder or rectal filling.Planar megavoltage (MV) imaging using electronic portal imaging devices has long been used to verify bony anatomy position [2] and, in recent years, the increased availability of kilovoltage and three-dimensional MV in-room imaging systems has enabled soft-tissue visualisation [3-6]. Image-based correction using translational couch shifts is now routine practice in modern radiotherapy centres, with bony anatomy, fiducial marker and soft-tissue-based assessment protocols being well documented [7-12].In prostate radiotherapy, moving from bony anatomy to soft-tissue-based assessment and intervention changes the approach from a surrogate for target position to tracking the target itself. Logically this should improve treatment accuracy, since the effect of internal motion on prostate position should be directly taken into account. However, clinical intervention strategies assume that any breach of a defined action level always requires a corrective shift and takes no account of the expected dose distribution in the patient.Systematic and random error components of the margin between the clinical target volume (CTV) and the planning target volume (PTV) mean that dosimetric coverage of the CTV will not be compromised if, despite changes in position, it remains within the International Commission on Radiation Units and Measurements (ICRU) 50/62-compliant 95% dose “cloud” [13,14]. In such a case, clinical intervention would not be necessary. Using dose-guided radiotherapy, the coverage of the daily verification CTV (vCTV) could be assessed against the expected daily dose distribution. An informed decision on the need to intervene could then be made based on probable dosimetric coverage, taking account of remaining uncertainties.Such an online dose-guided technique could be performed using a full dose recalculation based on the daily on-treatment anatomy immediately prior to treatment delivery. However, the implementation of any online dose-guided intervention poses a number of logistical problems: it would be time consuming, require prompt access to treatment planning stations, be prone to error because of the short decision time available and is a significant role extension for treatment staff more used to anatomical matching techniques. An alternative technique would be to use a sufficiently accurate surrogate for a full dose calculation, allowing dose-based judgements without the need for a potentially time-consuming calculation while a patient is in the treatment position.A previous paper proposed a dosimetric overlay method for dose-based assessment in image-guided radiotherapy of the prostate [15]. The technique involved the use of an overlay of the treatment plan 95% isodose over an on-treatment verification CT scan, achieved by a quick CT reference point registration between the verification CT scan and the localisation planning scan. The isodose could then be used in lieu of a full recalculation of the dose distribution on the pre-treatment scan and used to assess the adequacy of CTV coverage on that day. The paper showed that the technique was a feasible and acceptable means of assessment for prostate radiotherapy and that uncertainties between a full recalculation and this overlay isodose for a given patient anatomy were quantifiable and reasonable.This paper describes a planning study performed to determine the efficacy of the dosimetric evaluation technique described in Smyth et al [15] compared with both existing bony anatomy and soft tissue-matching and intervention protocols. Issues around future clinical implementation of the dosimetric overlay technique will also be discussed.