Synchronous bilateral breast cancer irradiation: clinical and dosimetrical issues using volumetric modulated arc therapy and simultaneous integrated boost

Objectives

The aim of the present retrospective analysis was to evaluate dosimetric parameters, feasibility and outcome for Synchronous Bilateral Breast Cancer (SBBC) patients treated with adjuvant radiotherapy (RT) by Volumetric Modulated Arc Therapy (VMAT).

Methods

From September 2011 to April 2016, 1100 Breast Cancer (BC) patients were referred to our institution to receive adjuvant breast RT, and those with SBBC were selected for the present analysis. A total of 16 patients were identified. A total dose of 50 Gy in 25 fractions was prescribed to the Planning Target Volume of the whole bilateral breast (PTV_BN) with or without the supraclavicular and infraclavicular nodes, while a total dose of 60 Gy in 25 fractions was prescribed to the surgical bed (PTV_boost). Several V _xGy and Dx% parameters were analyzed for the PTVs, together with Conformity and Homogeneity indexes (CI, HI), and for the critical Organs at risk (OARs), lungs and heart first.

Results

With a median follow-up of 24 months, no acute or late side effects more than grade 2 were observed. All patients are alive without any sign of disease. For target dose coverage, our observed inter-patients averages (±1 sd) were V _{95% Dp} = 96.7 ± 1.6% (96.3 ± 1.8%) to the left (right) PTV_BN, V _{95% Dp} = 98.6 ± 2.7% (99.4 ± 0.9%) to the left (right) PTV_boost, and D _2% = 64.4 ± 1.8 Gy (65.0 ± 2.0 Gy) to the left (right) PTV_boost, respectively. With regard to the heart, the inter-patient average of D _mean was 8.3 ± 3.3 Gy. For the lungs, the inter-patient average of D _mean, V _5 Gy and V _20 Gy were 11.8 ± 2.3 Gy, 78.9 ± 15.3% and 15.7 ± 5%, respectively.

Conclusions

The present retrospective analysis showed the feasibility, tolerability and safety of VMAT in the treatment of SBBC patients. Further studies are necessary to confirm these preliminary data.

     

A dosimetric comparison of RapidArc and IMRT with hypofractionated simultaneous integrated boost to the prostate for treatment of prostate cancer

Objective:

To compare the dosimetric results and treatment delivery efficiency among RapidArc® (Varian Medical Systems, Palo Alto, CA), 7-field intensity-modulated radiotherapy (7-f IMRT) and 9-field IMRT (9-f IMRT) with hypofractionated simultaneous integrated boost to the prostate.

Methods:

RapidArc, 7-f IMRT and 9-f IMRT plans were created for 21 consecutive patients treated for high-risk prostate cancer using the Eclipse™ treatment planning system (Varian Medical Systems). All plans were designed to deliver 70.0 Gy in 28 fractions to the prostate planning target volume (PTV) while simultaneously delivering 50.4 Gy in 28 fractions to the pelvic nodal PTV. Target coverage and sparing of organs at risk (OARs) were compared across techniques. The total number of monitor units (MUs) and the treatment time were used to assess treatment delivery efficiency.

Results:

RapidArc resulted in slightly superior conformity and homogeneity of prostate PTV, whereas all plans were comparable with respect to dose to the nodal PTV. Although OARs sparing for RapidArc and 7-f IMRT plans were almost equivalent, 9-f IMRT achieved better sparing of the rectum and bladder than RapidArc and 7-f IMRT. RapidArc provided the highest treatment delivery efficiency with the lowest MUs and shortest treatment time.

Conclusion:

RapidArc resulted in similar OAR sparing to 7-f IMRT, whereas 9-f IMRT provided the best OAR sparing. Treatment delivery efficiency is significantly higher for RapidArc.

Advances in knowledge:

This study validated the feasibility and limitations of RapidArc in the treatment of high-risk prostate cancer with complex pelvic target volumes.Radiotherapy has played an important role in the treatment of locally advanced prostate cancer. Several randomised controlled trials have demonstrated that high-dose radiotherapy improves prostate-specific antigen control, and a recently published meta-analysis [1] showed that high-dose radiotherapy is superior to conventional-dose radiotherapy in preventing biochemical or clinical failure and prostate cancer-specific death. However, dose escalation has been limited by toxicity in conventional techniques. Therefore, prostate cancer is one of the most common tumour sites treated with intensity-modulated radiation therapy (IMRT), which enables the delivery of highly conformal dose distribution to the target while reducing the dose to critical organs. IMRT also has the ability to produce inhomogeneous dose distribution, which allows for simultaneous differential dose delivery to multiple tumour targets (simultaneous integrated boost). Despite the obvious benefits of IMRT, there are some disadvantages. The potential downsides of IMRT include the increased time required for radiotherapy delivery and increased monitor units (MUs) needed compared with conventional three-dimensional conformal radiation therapy.Volumetric-modulated arc therapy (VMAT) is a relatively new rotational radiation therapy technique based on the idea of delivering IMRT with continuous dynamic modulation of the dose rate, field aperture and gantry speed. Compared with IMRT, the potential benefit of VMAT is the increase in delivery efficiency, including a shorter treatment time and a lower number of MUs.Several recent studies have compared VMAT with IMRT for prostate radiotherapy [2–13]. Although shortened treatment time is a common finding, there are inconsistencies in the dosimetric outcome. Many studies considering relatively simple target volumes that included prostate only or prostate with seminal vesicles found that VMAT achieved equal or better normal tissue sparing over IMRT [2,3,5,6,8–10,12]. However, very few studies have focused on more complex pelvic target volumes, including the prostate, seminal vesicles and pelvic lymph nodes [4,7,11,13]. Some of these studies found largely equivalent sparing of organs at risk (OARs) between VMAT and IMRT [7,13]. However, other planning studies have reported contradictory results. Yoo et al [4] noted superior OARs sparing with IMRT to VMAT. Myrehaug et al [11] found VMAT to have no consistent dosimetric advantage over IMRT. Thus, those studies have yielded mixed results. Our study aims to expand such studies to quantitatively evaluate VMAT for prostate cancer cases with complex pelvic target volumes and simultaneous integrated boost techniques.RapidArc® is one of the VMAT techniques implementing the progressive resolution optimisation algorithm in the Eclipse™ planning system by Varian Medical Systems (Palo Alto, CA). In the present study, we compare the performance of RapidArc, 7-field IMRT (7-f IMRT) and 9-field IMRT (9-f IMRT) with hypofractionated simultaneous integrated boost to the prostate for patients with high-risk prostate cancer. This study focused on the evaluation of the dosimetric results and treatment delivery efficiency.     

Multiplanar arc boost radiation therapy for prostate cancer

Localized prostatic carcinoma may be treated with either radical surgery or radiation therapy. Radiation therapy techniques for localized prostatic carcinoma include mega-voltage external irradiation or interstitial implantation, usually with iodine-125 seeds. Two external-beam techniques, multiplanar arc and biplanar arc, are additional options for the treatment of localized prostatic carcinoma. Film dosimetry measurements were made in pelvic phantoms to compare the isodose distributions of various external-beam radiation therapy techniques for boost treatment of prostate target volumes. Idealized calculations were performed to determine the isodose distribution of an I-125 implant. A comparison of these techniques shows that the multiplanar and biplanar arc techniques produce isodose distributions that may be useful in the treatment of prostate carcinoma.     

Automated volumetric modulated arc therapy planning for whole pelvic prostate radiotherapy

Background

For several tumor entities, automated treatment planning has improved plan quality and planning efficiency, and may enable adaptive treatment approaches. Whole-pelvic prostate radiotherapy (WPRT) involves large concave target volumes, which present a challenge for volumetric arc therapy (VMAT) optimization. This study evaluates automated VMAT planning for WPRT-VMAT and compares the results with manual expert planning.

Methods

A system for fully automated multi-criterial plan generation was configured for each step of sequential-boost WPRT-VMAT, with final “autoVMAT” plans being automatically calculated by the Monaco treatment planning system (TPS; Elekta AB, Stockholm, Sweden). Configuration was based on manually generated VMAT plans (manualVMAT) of 5 test patients, the planning protocol, and discussions with the treating physician on wishes for plan improvements. AutoVMAT plans were then generated for another 30 evaluation patients and compared to manualVMAT plans. For all 35 patients, manualVMAT plans were optimized by expert planners using the Monaco TPS.

Results

AutoVMAT plans exhibited strongly improved organ sparing and higher conformity compared to manualVMAT. On average, mean doses (D_mean) of bladder and rectum were reduced by 10.7 and 4.5?Gy, respectively, by autoVMAT. Prostate target coverage (V_95%) was slightly higher (+0.6%) with manualVMAT. In a blinded scoring session, the radiation oncologist preferred autoVMAT plans to manualVMAT plans for 27/30 patients. All treatment plans were considered clinically acceptable. The workload per patient was reduced by > 70 min.

Conclusion

Automated VMAT planning for complex WPRT dose distributions is feasible and creates treatment plans that are generally dosimetrically superior to manually optimized plans.

     

旋转调强与固定野逆向调强放疗在颅脑多发转移瘤中的剂量学比较研究

目的 比较旋转调强(RapidArc)与固定野调强(IMRT)放疗在颅脑多发转移瘤中的剂量学差异。方法 针对10例多发脑转移瘤患者分别设计3种放疗计划:固定野逆向调强(IMRT),RapidArc单弧旋转调强(RA1),双弧旋转调强(RA2)。在保证计划均满足临床要求前提下,分别比较3种计划的靶区剂量分布、危及器官及靶区外正常组织的受照剂量、机器跳数以及治疗时间,探讨其剂量学差异。结果 3种计划均满足临床要求,在靶区适形度和均匀性方面,RA2计划优于IMRT(Z＝-2.803、-2.094,P<0.05)和RA1(Z＝-2.448、-2.191,P＜0.05),RA1计划与IMRT计划差别不大。RA1、RA2计划中的双侧晶体、双侧眼球、脑干的最大剂量均显著低于IMRT(Z＝-2.803～-2.191,P <0.05)。RA2计划评估的双侧视神经最大剂量均显著低于IMRT(Z＝-2.293、-2.701,P<0.05)。RA1、RA2计划中的机器跳数相对于IMRT平均分别减少了43%和24%,缩短了治疗时间。结论 单弧和双弧旋转调强计划均可达到或优于IMRT计划的靶区剂量分布,能更好地降低部分危及器官的受照剂量,同时可以显著降低机器跳数和治疗实施时间。     

旋转调强与固定野逆向调强放疗在颅脑多发转移瘤中的剂量学比较研究

Objective To evaluate the performace of fixed field Intensity modulated radiation therapy (IMRT) and RapidArc in the radiotherapy for multiple intracranial metastases.Methods The clinical data of 10 patients with multiple intracranial metastases,8 male and 2 female,aged 65-73,were used to design 3 plans:fixed field IMRT,RapidArc with single Arc (RA1),and RapidArc with double Arc (Arc 2).Dose-volume-histogram analysis was used to compare dose results,monitor unit,and delivery time.Results All 3 plans met the clinical requirements.The best target conformity and homogeneity were observed in the RA2 plan (Z = -2.803,- 2.904,P ＜ 0.05) and there were no statistical differences between the IMRT plan and RA1 plan.The maximum doses to the lens,eyes,and brainstem of the two RapidArc plans were all significantly lower than those of the IMRT plan(Z = -2.803--2.191 ,P ＜0.05),and the maximum dose to the optic nerves of the RA2 plan was significantly lower than that of the IMRT plan (Z = -2.293,-2.701 ,P ＜0.05).Compared with the IMRT plan,the average monitor units of the RA1 and RA2 plans were reduced by 29% and 24%,respectively,and the delivery time of these plans were significantly shorter by 84% and 69%,respectively.Conclusions Compared to the IMRT plan,RapidArc plans with single or double Arcs show similar or better effects in the target dose distribution,reduction of irradiation doses on organs at risk and,moreover,significant decrease of the monitor units and delivery time.     

Ultra hypofractionated extended nodal irradiation using volumetric modulated arc therapy for oligorecurrent pelvic nodal prostate cancer

Prostate cancer (PCa) may recur after primary treatment but no standard of care exists for patients with pelvic nodal relapse. Based on obervational data, Extended Nodal Irradiation (ENI) might be associated with fewer treatment failures than Stereotactic Ablative Radiotherapy (SABR) to the involved node(s) alone. Ultra hypofractionated ENI is yet to be evaluated in this setting, but it could provide a therapeutic advantage if PCa has a low α/β ratio in addition to patient convenience/resource benefits. This volumetric modulated arc therapy (VMAT) planning study developed a class solution for 5-fraction Extended Nodal Irradiation (ENI) plus a simultaneous integrated boost (SIB) to involved node(s). Ten patients with oligorecurrent nodal disease after radical prostatectomy/post-operative prostate bed radiotherapy were selected. Three plans were produced for each dataset to deliver 25 Gy in 5 fractions ENI plus SIBs of 40, 35 and 30 Gy. The biologically effective dose (BED) formula was used to determine the remaining dose in 5 fractions that could be delivered to re-irradiated segments of organs at risk (OARs). Tumour control probability (TCP) and normal tissue complication probability (NTCP) were calculated using the LQ-Poisson Marsden and Lyman-Kutcher-Burman models respectively. Six patients had an OAR positioned within planning target volume node (PTVn), which resulted in reduced target coverage to PTV node in six, five and four instances for 40, 35 and 30 Gy SIB plans respectively. In these instances, only 30 Gy SIB plans had a median PTV coverage >90% (inter-quartile range 90-95). No OAR constraint was exceeded for 30 Gy SIB plans, including where segments of OARs were re-irradiated. Gross tumour volume node (GTVn) median TCP was 95.7% (94.4-96), 90.7% (87.1-91.2) and 78.6% (75.8-81.1) for 40, 35 and 30 Gy SIB plans respectively, where an α/β ratio of 1.5 was assumed. SacralPlex median NTCP was 43.2% (0.7-61.2), 12.1% (0.6-29.7) and 2.5% (0.5-5.1) for 40, 35 and 30 Gy SIB plans respectively. NTCP for Bowel_Small was <0.3% and zero for other OARs for all three plan types. Ultra hypofractionated ENI planning for pelvic nodal relapsed PCa appears feasible with encouraging estimates of nodal TCP and low estimates of NTCP, especially where a low α/β ratio is assumed and a 30 Gy SIB is delivered. This solution should be further evaluated within a clinical trial and compared against SABR to involved node(s) alone.     

Dosimetric effects of weight loss or gain during volumetric modulated arc therapy and intensity-modulated radiation therapy for prostate cancer

Weight loss or gain during the course of radiation therapy for prostate cancer can alter the planned dose to the target volumes and critical organs. Typically, source-to-surface distance (SSD) measurements are documented by therapists on a weekly basis to ensure that patients' exterior surface and isocenter-to-skin surface distances remain stable. The radiation oncology team then determines whether the patient has undergone a physical change sufficient to require a new treatment plan. The effect of weight change (SSD increase or decrease) on intensity-modulated radiation therapy (IMRT) or volumetric modulated arc therapy (VMAT) dosimetry is not well known, and it is unclear when rescanning or replanning is needed. The purpose of this study was to determine the effects of weight change (SSD increase or decrease) on IMRT or VMAT dose delivery in patients with prostate cancer and to determine the SSD change threshold for replanning. Whether IMRT or VMAT provides better dose stability under weight change conditions was also determined. We generated clinical IMRT and VMAT prostate and seminal vesicle treatment plans for varying SSDs for 10 randomly selected patients with prostate cancer. The differences due to SSD change were quantified by a specific dose change for a specified volume of interest. The target mean dose, decreased or increased by 2.9% per 1-cm SSD decrease or increase in IMRT and by 3.6% in VMAT. If the SSD deviation is more than 1 cm, the radiation oncology team should determine whether to continue treatment without modifications, to adjust monitor units, or to resimulate and replan.     

Assessing the feasibility of volumetric-modulated arc therapy using simultaneous integrated boost (SIB-VMAT): An analysis for complex head-neck,high-risk prostate and rectal cancer cases

Intensity-modulated radiotherapy (IMRT) allowed the simultaneous delivery of different doses to different target volumes within a single fraction, an approach called simultaneous integrated boost (SIB). As consequence, the fraction dose to the boost volume can be increased while keeping low doses to the elective volumes, and the number of fractions and overall treatment time will be reduced, translating into better radiobiological effectiveness. In recent years, volumetric-modulated arc therapy (VMAT) has been shown to provide similar plan quality with respect to fixed-field IMRT but with large reduction in treatment time and monitor units (MUs) number. However, the feasibility of VMAT when used with SIB strategy has few investigations to date. We explored the potential of VMAT in a SIB strategy for complex cancer sites. A total of 15 patients were selected, including 5 head-and-neck, 5 high-risk prostate, and 5 rectal cancer cases. Both a double-arc VMAT and a 7-field IMRT plan were generated for each case using Oncentra MasterPlan treatment planning system for an Elekta Precise linac. Dosimetric indexes for targets and organs at risk (OARs) were compared based on dose-volume histograms. Conformity index, homogeneity index, and dose-contrast index were used for target analyses. The equivalent uniform doses and the normal tissue complication probabilities were calculated for main OARs. MUs number and treatment time were analyzed to score treatment efficiency. Pretreatment dosimetry was performed using 2-dimensional (2D)-array dosimeter. SIB-VMAT plans showed a high level of fluence modulation needed for SIB treatments, high conformal dose distribution, similar target coverage, and a tendency to improve OARs sparing compared with the benchmark SIB-IMRT plans. The median treatment times reduced from 13 to 20 minutes to approximately 5 minutes for all cases with SIB-VMAT, with a MUs reduction up to 22.5%. The 2D-array ion-chambers' measurements reported an agreement of more than 95% for a criterion of 3% to 3 mm. SIB-VMAT was able to combine the advantages of conventional SIB-IMRT with its highly conformal dose distribution and OARs sparing and the advantages of 3D-conformal radiotherapy with its fast delivery.     

Feasibility study of volumetric modulated arc therapy with constant dose rate for endometrial cancer

To investigate the feasibility, efficiency, and delivery accuracy of volumetric modulated arc therapy with constant dose rate (VMAT-CDR) for whole-pelvic radiotherapy (WPRT) of endometrial cancer. The nine-field intensity-modulated radiotherapy (IMRT), VMAT with variable dose-rate (VMAT-VDR), and VMAT-CDR plans were created for 9 patients with endometrial cancer undergoing WPRT. The dose distribution of planning target volume (PTV), organs at risk (OARs), and normal tissue (NT) were compared. The monitor units (MUs) and treatment delivery time were also evaluated. For each VMAT-CDR plan, a dry run was performed to assess the dosimetric accuracy with MatriXX from IBA. Compared with IMRT, the VMAT-CDR plans delivered a slightly greater V₂₀ of the bowel, bladder, pelvis bone, and NT, but significantly decreased the dose to the high-dose region of the rectum and pelvis bone. The MUs decreased from 1105 with IMRT to 628 with VMAT-CDR. The delivery time also decreased from 9.5 to 3.2 minutes. The average gamma pass rate was 95.6% at the 3%/3 mm criteria with MatriXX pretreatment verification for 9 patients. VMAT-CDR can achieve comparable plan quality with significant shorter delivery time and smaller number of MUs compared with IMRT for patients with endometrial cancer undergoing WPRT. It can be accurately delivered and be an alternative to IMRT on the linear accelerator without VDR capability.     

Hypofractionation with simultaneous integrated boost for early breast cancer

Purpose

To evaluate the feasibility of hypofractionation with SIB in all settings in Germany to prepare a multicenter treatment comparison.

Methods

Eligible patients had histopathologically confirmed breast cancer operated by BCS. Patients received WBI 40.0 Gy in 16 fractions of 2.5 Gy. A SIB with 0.5 Gy per fraction was administered to the tumor bed, thereby giving 48.0 Gy in 16 fractions to the boost-PTV sparing heart, LAD, lung, contralateral breast. The primary study objective was feasibility, administration of specified dose in 16 fractions within 22–29 days with adherence to certain dose constraints (heart; LAD; contralateral breast); secondary endpoints were toxicity, QoL.

Results

151 patients were recruited from 7 institutions between 07/11-10/12. 10 patients met exclusion criteria prior to irradiation. All but two patients (99 %) received the prescribed dose in the PTVs. Adherence to dose constraints and time limits was achieved in 89 % (95 % CI 82 % to 93 %). 11 AE were reported in 10 patients; five related to concurrent endocrine therapy. Two of the AEs were related to radiotherapy: grade 3 hot flushes in two cases. QoL remained unchanged.

Conclusion

Hypofractionation with a SIB is feasible and was well tolerated in this study.

     

A predictive model for determining rectum and bladder dose constraints in prostate volumetric modulated arc therapy

Generic dose-volume constraints of the rectum/bladder (R/B) are used in inverse planning to reduce doses to these organs for patients undergoing prostate radiotherapy. A retrospective study was undertaken to assess correlations between the overlap of the R/B with the planning target volume (PTV) and the dose received during planning to organs at risk (OARs). Data for 105 prostate cancer patients who had volumetric modulated arc therapy (VMAT) to the intact prostate and proximal seminal vesicles at Nepean Cancer Care Centre from 2011 to 2015 were analyzed. R/B volume, R/B-PTV overlap volume, and R/B-PTV overlap percent metrics were collected with VMAT planning objectives. Characteristics were evaluated for correlation with different planning outcomes. The percentage overlap between the R/B and PTV were highly correlated to the doses to the relevant OAR, with a coefficient of determination (R²) of 0.63 for the rectum volume percentage receiving more than 75 Gy (RV_75Gy) and R² of 0.91 for the bladder volume percentage receiving more than 70 Gy (BV_70Gy). We identified a cut-off value of 10.14% (sensitivity 84.62%, specificity 80.43%) as predictive of RV_75Gy < 10% and a cut-off of 7.95% (sensitivity 97.62%, specificity 92.06%) as predictive of BV_70Gy < 15%. A 95% prediction interval assisted in identifying individualized R/B planning goals. The R/B-PTV percentage overlap has a high reliability in estimating sparing of the R/B. This prediction model can be used to improve planning efficiency and create customised automated OAR planning goals in prostate VMAT plans. By doing this, the radiation doses received by these OARs can be minimized.     

宫颈癌术后放疗中自动计划与人工计划的剂量学比较

目的 比较容积弧形调强（VMAT）人工计划与自动计划的剂量学参数,评估自动计划在临床宫颈癌术后放疗应用的可行性以及剂量学特点。方法 选取23例宫颈癌术后病例,其中Ⅱ_A期8例,Ⅱ_B期15例,使用Pinnacle³放射治疗计划系统分别设计人工VMAT和自动VMAT计划。比较两种计划靶区的D_mean、D₉₅、适形指数（CI）、均匀性指数（HI）、危及器官（OAR）的剂量体积直方图（DVH）、平均优化时间、调试次数以及机器跳数（MU）。结果 自动计划得到的靶区D_mean、CI以及HI均优于人工计划（t=4.65~14.92,P<0.05）,而D₉₅无明显差异（P>0.05）;自动计划得到的OAR各参数均优于人工计划（t=3.30~14.42,P<0.05）;自动计划明显减少了计划的平均优化时间（72 min,t=3.85,P<0.05）和调试次数（2次,t=5.41,P<0.05）;自动计划和人工计划的平均机器跳数分别为（819±53）和（638±41） MUs,自动计划跳数增加了181 MUs。结论 使用Pinnacle³自动设计的宫颈癌术后VMAT计划具有临床可行性,并且自动VMAT计划与人工VMAT计划相比提高了计划质量,减少了计划设计时间、人为因素对于计划质量的影响。     

Comparative dosimetry of volumetric modulated arc therapy and limited-angle static intensity-modulated radiation therapy for early-stage larynx cancer

To compare relative carotid and normal tissue sparing using volumetric-modulated arc therapy (VMAT) or intensity-modulated radiation therapy (IMRT) for early-stage larynx cancer. Seven treatment plans were retrospectively created on 2 commercial treatment planning systems for 11 consecutive patients with T1-2N0 larynx cancer. Conventional plans consisted of opposed-wedged fields. IMRT planning used an anterior 3-field beam arrangement. Two VMAT plans were created, a full 360° arc and an anterior 180° arc. Given planning target volume (PTV) coverage of 95% total volume at 95% of 6300 cGy and maximum spinal cord dose below 2500 cGy, mean carotid artery dose was pushed as low as possible for each plan. Deliverability was assessed by comparing measured and planned planar dose with the gamma (γ) index. Full-arc planning provided the most effective carotid sparing but yielded the highest mean normal tissue dose (where normal tissue was defined as all soft tissue minus PTV). Static IMRT produced next-best carotid sparing with lower normal tissue dose. The anterior half-arc produced the highest carotid artery dose, in some cases comparable with conventional opposed fields. On the whole, carotid sparing was inversely related to normal tissue dose sparing. Mean γ indexes were much less than 1, consistent with accurate delivery of planned treatment. Full-arc VMAT yields greater carotid sparing than half-arc VMAT. Limited-angle IMRT remains a reasonable alternative to full-arc VMAT, given its ability to mediate the competing demands of carotid and normal tissue dose constraints. The respective clinical significance of carotid and normal tissue sparing will require prospective evaluation.     

Impact of pitch angle setup error and setup error correction on dose distribution in volumetric modulated arc therapy for prostate cancer

In volumetric modulated arc therapy (VMAT) for prostate cancer, a positional and rotational error correction is performed according to the position and angle of the prostate. The correction often involves body leaning, and there is concern regarding variation in the dose distribution. Our purpose in this study was to evaluate the impact of body pitch rotation on the dose distribution regarding VMAT. Treatment plans were obtained retrospectively from eight patients with prostate cancer. The body in the computed tomography images for the original VMAT plan was shifted to create VMAT plans with virtual pitch angle errors of ±1.5° and ±3°. Dose distributions for the tilted plans were recalculated with use of the same beam arrangement as that used for the original VMAT plan. The mean value of the maximum dose differences in the dose distributions between the original VMAT plan and the tilted plans was 2.98 ± 0.96 %. The value of the homogeneity index for the planning target volume (PTV) had an increasing trend according to the pitch angle error, and the values of the D ₉₅ for the PTV and D _2ml, V ₅₀, V ₆₀, and V ₇₀ for the rectum had decreasing trends (p < 0.05). However, there was no correlation between differences in these indexes and the maximum dose difference. The pitch angle error caused by body leaning had little effect on the dose distribution; in contrast, the pitch angle correction reduced the effects of organ displacement and improved these indexes. Thus, the pitch angle setup error in VMAT for prostate cancer should be corrected.     

Establishing an optimized patient-specific verification program for volumetric modulated arc therapy

Quality assurance (QA) of volumetric modulated arc therapy (VMAT) increases the workload significantly. We compared the results from 4 verification methods to establish an efficient VMAT QA. Planning for VMAT treatments was carried out for 40 consecutive patients. Pretreatment verifications were carried out with ion chamber array Physikalish-Technische Werkstätten (PTW729), electronic portal dosimetry (EPID), ion chamber measurements, and independent dose calculation with Diamond program. 2D analyses were made using the gamma analysis (3 mm distance to agreement and 3% dose difference relative to maximum, 10% dose threshold). Average point dose difference calculated by Eclipse relative to ion chamber measurements and Diamond were 0.1%±0.9% and 0.6%±2.2%, respectively. Average pass rate for PTW729 was 99.2%±1.9% and 98.3%±1.3% for EPID. The total required time (linac occupancy time given in parentheses) for each QA method was: PTW729 43.5 minutes (26.5 minutes), EPID 14.5 minutes (2.5 minutes), ion chamber 34.5 minutes (26.5 minutes), and Diamond 12.0 minutes (0 minute). The results were consistent and allowed us to establish an optimized protocol, considering safety and accuracy as well as workload, consisting of 2 verification methods: EPID 2D analysis and independent dose calculation.     

The effect of bladder contrast on dose calculation in volumetric modulated arc therapy planning in patients treated for postoperative prostate cancer

Purpose

The aim of this study was to evaluate the effect of contrast agent on dose calculation in volumetric modulated arc therapy (VMAT) in the post-prostatectomy setting.

Methods and material

Ten patients were studied. Each patient received planning computed tomography (CT) images with contrast agent. All of the plans were done on virtually simulated contrast-free CT scans. The plan approved by the radiation oncologist was replicated to the contrast CT series. In both of the plans the same monitor unit was used. The doses calculated from the two plans were compared in regard to target volumes and organs at risk. A paired sample t-test was used to evaluate the differences in cumulative dose volume histogram between the two plans.

Results

We showed that the use of contrast agent may cause significant differences in dose distribution. There was a significant decrease in doses received by planning target volume (PTV70), rectum V65 Gy, rectum V40 Gy, bladder V65 Gy, penile bulb V40 Gy in plans with contrast-enhanced CT sets. The decrease in mean, maximum and minimum doses received by PTV70 also contributed to the significant decrease in conformity index.

Conclusions

Using a contrast agent at the time of CT simulation may cause significant differences in dose distribution. For this reason, the plan should always be carried out on non-contrast CT data sets to avoid additional errors in the treatment planning process.