Feasibility of using MRI alone for 3D radiation treatment planning in brain tumors

OBJECTIVE: The aim of this study was to establish whether radiation treatment planning using MRI alone could replace CT-based planning for brain tumors while retaining the dosimetric accuracy. This would help to provide a single imaging modality for both target delineation as well as treatment planning, thus saving time and resources. METHODS: Twenty-five patients with brain tumors were scanned on a spiral CT scanner and 1.5 T MRI scanner. Three treatment plans were generated for all patients. The first plan was generated using the CT scan images with inhomogeneity correction (CT + IC); the second plan used the CT scan without inhomogeneity correction (CT-IC) and the third plan was generated using the MRI scan (MRI alone). RESULTS: The maximum distortion in the MRI phantom study was less than 1 mm. There were no statistically significant differences in any of the target coverage parameters analysed in this study. Similarly, the maximum antero-posterior and lateral dimensions for the CT-based and MRI-based planning did not show any statistical difference. CONCLUSION: MRI-based treatment planning for brain lesions is feasible and gives equivalent dosimetric results compared to CT-based treatment planning.     

CT及3.0T MRI在食管癌IMRT中的价值

目的 探讨CT及3.0T MRI在食管癌IMRT计划中的价值。方法 选取2013-2015年本院放疗科首次确诊并行根治性IMRT的食管癌患者35例,分别在CT及MRI T2WI与DWI融合图像上勾画靶区,以相同处方剂量和OAR限制剂量分别制定计划,比较2种计划的靶区体积、处方剂量及OAR受量差异。配对t检验差异。结果 2种计划的剂量分布及计划参数均达到了临床处方剂量要求。基于3.0T MRI计划的病灶长度、VGTV和VPTV均小于CT计划(P=0.00、0.03、0.03)。2种计划的PGTV、PTV-PGTV的D2%、D98%、D50%、HI、CI差异均无统计学意义(P均>0.05)。基于3.0T MRI计划的双肺平均剂量明显低于CT计划(P=0.00),双肺实际受量亦与CT计划相近(P均>0.05)。2种计划的脊髓最大耐受量及心脏耐受量相近(P均>0.05)。结论 基于CT及3.0T MRI计划所勾画的靶区及参数均能满足临床需求,但基于3.0T MRI计划的靶区体积更小,可能会使部分OAR潜在获益。     

MRI-based treatment planning with electron density information mapped from CT images: a preliminary study

Nowadays magnetic resonance imaging (MRI) has been profoundly used in radiotherapy (RT) planning to aid the contouring of targets and critical organs in brain and intracranial cases, which is attributable to its excellent soft tissue contrast and multi-planar imaging capability. However, the lack of electron density information in MRI, together with the image distortion issues, precludes its use as the sole image set for RT planning and dose calculation. The purpose of this preliminary study is to probe the feasibility and evaluate an MRI-based radiation dose calculation process by providing MR images the necessary electron density (ED) information from a patient's readily available diagnostic/staging computed tomography (CT) images using an image registration model. To evaluate the dosimetric accuracy of the proposed approach, three brain and three intracranial cases were selected retrospectively for this study. For each patient, the MR images were registered to the CT images, and the ED information was then mapped onto the MR images by in-house developed software generating a modified set of MR images. Another set of MR images with voxel values assigned with the density of water was also generated. The original intensity modulated radiation treatment (IMRT) plan was then applied to the two sets of MR images and the doses were calculated. The dose distributions from the MRI-based calculations were compared to that of the original CT-based calculation. In all cases, the MRI-based calculations with mapped ED yielded dose values very close (within 2%) to that of the CT-based calculations. The MRI-based calculations with voxel values assigned with water density indicated a dosimetric error of 3-5%, depending on the treatment site. The present approach offers a means of utilizing MR images for accurate dose calculation and affords a potential to eliminate the redundant simulation CT by planning a patient's treatment with only simulation MRI and any available diagnostic/staging CT data.     

Comparison of MRI-based and CT/MRI fusion-based postimplant dosimetric analysis of prostate brachytherapy

PURPOSE: The aim of this study was to compare the outcomes between magnetic resonance imaging (MRI)-based and computed tomography (CT)/MRI fusion-based postimplant dosimetry methods in permanent prostate brachytherapy. METHODS AND MATERIALS: Between October 2004 and March 2006, a total of 52 consecutive patients with prostate cancer were treated by brachytherapy, and postimplant dosimetry was performed using CT/MRI fusion. The accuracy and reproducibility were prospectively compared between MRI-based dosimetry and CT/MRI fusion-based dosimetry based on the dose-volume histogram (DVH) related parameters as recommended by the American Brachytherapy Society. RESULTS: The prostate volume was 15.97+/-6.17 cc (mean+/-SD) in MRI-based dosimetry, and 15.97+/-6.02 cc in CT/MRI fusion-based dosimetry without statistical difference. The prostate V100 was 94.5% and 93.0% in MRI-based and CT/MRI fusion-based dosimetry, respectively, and the difference was statistically significant (p=0.002). The prostate D90 was 119.4% and 114.4% in MRI-based and CT/MRI fusion-based dosimetry, respectively, and the difference was statistically significant (p=0.004). CONCLUSION: Our current results suggested that, as with fusion images, MR images allowed accurate contouring of the organs, but they tended to overestimate the analysis of postimplant dosimetry in comparison to CT/MRI fusion images. Although this MRI-based dosimetric discrepancy was negligible, MRI-based dosimetry was acceptable and reproducible in comparison to CT-based dosimetry, because the difference between MRI-based and CT/MRI fusion-based results was smaller than that between CT-based and CT/MRI fusion-based results as previously reported.     

Magnetic resonance-based treatment planning for prostate intensity-modulated radiotherapy: creation of digitally reconstructed radiographs

PURPOSE: To develop a technique to create magnetic resonance (MR)-based digitally reconstructed radiographs (DRR) for initial patient setup for routine clinical applications of MR-based treatment planning for prostate intensity-modulated radiotherapy. METHODS AND MATERIALS: Twenty prostate cancer patients' computed tomography (CT) and MR images were used for the study. Computed tomography and MR images were fused. The pelvic bony structures, including femoral heads, pubic rami, ischium, and ischial tuberosity, that are relevant for routine clinical patient setup were manually contoured on axial MR images. The contoured bony structures were then assigned a bulk density of 2.0 g/cm(3). The MR-based DRRs were generated. The accuracy of the MR-based DDRs was quantitatively evaluated by comparing MR-based DRRs with CT-based DRRs for these patients. For each patient, eight measuring points on both coronal and sagittal DRRs were used for quantitative evaluation. RESULTS: The maximum difference in the mean values of these measurement points was 1.3 +/- 1.6 mm, and the maximum difference in absolute positions was within 3 mm for the 20 patients investigated. CONCLUSIONS: Magnetic resonance-based DRRs are comparable to CT-based DRRs for prostate intensity-modulated radiotherapy and can be used for patient treatment setup when MR-based treatment planning is applied clinically.     

Dosimetric feasibility of cone-beam CT-based treatment planning compared to CT-based treatment planning

PURPOSE: Cone-beam computed tomography (CBCT) images are currently used for positioning verification. However, it is yet unknown whether CBCT could be used in dose calculation for replanning in adaptive radiation therapy. This study investigates the dosimetric feasibility of CBCT-based treatment planning. METHODS AND MATERIALS: Hounsfield unit (HU) values and profiles of Catphan, homogeneous/inhomogeneous phantoms, and various tissue regions of patients in CBCT images were compared to those in CT. The dosimetric consequence of the HU variation was investigated by comparing CBCT-based treatment plans to conventional CT-based plans for both phantoms and patients. RESULTS: The maximum HU difference between CBCT and CT of Catphan was 34 HU in the Teflon. The differences in other materials were less than 10 HU. The profiles for the homogeneous phantoms in CBCT displayed reduced HU values up to 150 HU in the peripheral regions compared to those in CT. The scatter and artifacts in CBCT became severe surrounding inhomogeneous tissues with reduced HU values up to 200 HU. The MU/cGy differences were less than 1% for most phantom cases. The isodose distributions between CBCT-based and CT-based plans agreed very well. However, the discrepancy was larger when CBCT was scanned without a bowtie filter than with bowtie filter. Also, up to 3% dosimetric error was observed in the plans for the inhomogeneous phantom. In the patient studies, the discrepancies of isodose lines between CT-based and CBCT-based plans, both 3D and IMRT, were less than 2 mm. Again, larger discrepancy occurred for the lung cancer patients. CONCLUSION: This study demonstrated the feasibility of CBCT-based treatment planning. CBCT-based treatment plans were dosimetrically comparable to CT-based treatment plans. Dosimetric data in the inhomogeneous tissue regions should be carefully validated.     

A study on adaptive IMRT treatment planning using kV cone-beam CT.

BACKGROUND AND PURPOSE: Changes in tumor size during the course of radiotherapy warrant performing adaptive radiotherapy (ART). This work investigates the feasibility and usefulness of acquiring on-board cone-beam CT (CBCT) for ART for patients with bulky head and neck tumors treated with IMRT and for prostate patients with potentially significant target position variations during the treatment course. MATERIALS AND METHODS: A phantom designed for CT quality assurance was used to compare the dosimetric and geometric accuracy between conventional CT and CBCT from a linear accelerator's on-board imager. Patient planning CT and CBCT images were acquired before treatment and at mid-course. The IMRT plans made on the CT were applied to the CBCT and dose-volume histograms were calculated. RESULTS: In both phantom and patient studies, the dose-volume histograms (DVHs) based on CBCT images were in excellent agreement with DVHs based on planning CT images. Minimum, maximum and mean doses agreed very well. In a patient study, doses for targets and normal tissues from the same IMRT plans calculated on CBCT images agreed within 1-3% with those calculated on planning CT images. CONCLUSIONS: CBCT images can be used to accurately predict dosimetric results. It is feasible to use CBCT to determine dosimetric consequences resulting from tumor shrinkage and patient geometry changes. An additional planning CT may be necessary to perform IMRT re-planning at present in order to accurately delineate tumor and organs. The CBCT has potential to become a very useful tool for on-line ART.     

Importance of the CT/MRI fusion method as a learning tool for CT-based postimplant dosimetry in prostate brachytherapy.

BACKGROUND AND PURPOSE: To compare the CT-based and CT/MRI fusion-based postimplant dosimetry after permanent prostate brachytherapy and to evaluate the improvement in CT-based dosimetry by physicians with or without experience in using the CT/MRI fusion method. PATIENTS AND METHODS: Thirty-eight consecutive patients agreed to participate in a prospective study. The prostate contours from CT/MRI fusion are the gold standard for determining the prostate volume and dose volume histogram (DVH). CT-based postimplant dosimetries were performed by two physicians. Observer 1 was a radiologist who had never used CT/MRI fusion method for postimplant dosimetric analysis. Observer 2 was a radiation oncologist experienced in postimplant analysis using the CT/MRI fusion method. The prostate dosimetry was evaluated by prostate D90 and V100. RESULTS: No significant difference was observed in the mean prostate volumes between the two observers and the CT/MRI fusion data. However, the correlation coefficient value for observer 2 (R(2)=0.932) was greater than that for observer 1 (R(2)=0.793). The D90 and V100 values as evaluated by the two observers were significantly underestimated in comparison to those evaluated using the CT/MRI fusion methods. The DVH related parameters were underestimated more frequently by observer 1 than by observer 2: (prostate D90: 99.56% for observer 1, 102.97% for observer 2, 109.37% for CT/MRI fusion. Prostate V100: 88.12% for observer 1, 90.14% for observer 2, 91.91% for CT/MRI fusion). CONCLUSIONS: The difference in the mean value in D90 and V100 by observer 1 was significantly greater than that for observer 2. These findings suggest that the CT/MRI fusion method provides accurate feedback which thereby improves CT-based postimplant dosimetry for prostate brachytherapy.     

千伏级锥形束CT图像对食管癌放疗剂量计算可行性研究

目的 探讨千伏级锥形束CT图像(KVCBCT)对食管癌放疗剂量计算的可行性。方法 比较Trilogy加速器采集的Catphan600模体KVCBCT图像CT值稳定性及离轴曲线重合度并建立CT值-密度转换曲线用于剂量计算。在模体和 10例食管癌患者常规CT上设计逆向调强计划,并将计划移植到相对应的KVCBCT图像上。基于KVCBCT的转换曲线计算靶区及周围组织受量,并与常规CT计算结果行配对t检验。结果 KVCBCT的CT值变化范围均在1.6%内,且未发现有随时间变化趋势。常规CT和KVCBCT的CT值离轴曲线重合度较高,大部分成像区域差别在1%内。模体调强计划常规CT与KVCBCT所得剂量统计参数最大差别为1.33%,对食管癌患者调强计划两者剂量差别在3.65%内。等中心层面剂量分布有较好一致性。结论 使用食管癌KVCBCT图像进行剂量计算是准确可行的,在临床上具有实际应用意义。     

Accuracy of seed reconstruction in prostate postplanning studied with a CT- and MRI-compatible phantom.

BACKGROUND AND PURPOSE: Postimplant dosimetry of prostate seed implants is usually performed by seed localisation on transversal CT or MR images. In order to obtain reliable dosimetric evaluation data, it is important that seeds are reconstructed accurately. Currently, there is no comparative data available on seed localisation accuracy of CT-and MRI-based reconstructions, mainly due to the lack of a suitable QA tool. In this study, we developed a CT-and MRI compatible prostate phantom to investigate the intrinsic accuracy of seed detection for both imaging modalities. PATIENTS AND METHODS: A 60 seed geometry was created according to a clinically meaningful plan, including rotated and shifted seeds. After implantation of the seeds in the phantom, CT and MRI scans with 3, 4 and 5mm slice thickness were performed. The seed locations were reconstructed in the treatment planning system and compared with the known reference positions. RESULTS: Due to the comparable density and relaxation times of the phantom material to prostate tissue, the seeds are visualised similarly as on real patient images. The observed mean reconstruction uncertainties were in general smaller for CT (0.9+/-0.6, 0.9+/-0.6, 2.1+/-0.8 mm on 3, 4 and 5mm scans, respectively), than for MRI (Philips 1.5 T: 2.1+/-1.4, 1.6+/-1.2, 1.9+/-0.9 mm on 3, 4 and 5 mm scans, respectively, and Siemens 1.5 T: 2.3+/-0.8, 2.0+/-1.6, 1.6+/-0.8 mm on 3, 4 and 5mm scans, respectively). CONCLUSIONS: For our clinical sequences of both CT and MRI, the mean deviation of the reconstructed seed positions were all within acceptable limits for clinical use (<2.3 mm). The phantom was found to be a suitable quality assurance tool to assess the reliability and accuracy of the seed reconstruction procedure. Moreover, as the phantom material has the same imaging characteristics as real prostate tissue, it is a useful device to define proper MRI sequences.     

MRI-guided prostate radiation therapy planning: Investigation of dosimetric accuracy of MRI-based dose planning

Background and purpose

Dose planning requires a CT scan which provides the electron density distribution for dose calculation. MR provides superior soft tissue contrast compared to CT and the use of MR-alone for prostate planning would provide further benefits such as lower cost to the patient. This study compares the accuracy of MR-alone based dose calculations with bulk electron density assignment to CT-based dose calculations for prostate radiotherapy.

Materials and methods

CT and whole pelvis MR images were contoured for 39 prostate patients. Plans with uniform density and plans with bulk density values assigned to bone and tissue were compared to the patient’s gold standard full density CT plan. The optimal bulk density for bone was calculated using effective depth measurements. The plans were evaluated using ICRU point doses, dose volume histograms, and Chi comparisons. Differences in spatial uniformity were investigated for the CT and MR scans.

Results

The calculated dose for CT bulk bone and tissue density plans was 0.1 ± 0.6% (mean ± 1 SD) higher than the corresponding full density CT plan. MR bulk bone and tissue density plans were 1.3 ± 0.8% lower than the full density CT plan. CT uniform density plans and MR uniform density plans were 1.4 ± 0.9% and 2.6 ± 0.9% lower, respectively. Paired t-tests performed on specific points on the DVH graphs showed that points on DVHs for all bulk electron density plans were equivalent with two exceptions. There was no significant difference between doses calculated on Pinnacle and Eclipse. The dose distributions of six patients produced Chi values outside the acceptable range of values when MR-based plans were compared to the full density plan.

Conclusions

MR-alone bulk density planning is feasible provided bone is assigned a density, however, manual segmentation of bone on MR images will have to be replaced with automatic methods. The major dose differences for MR bulk density plans are due to differences in patient external contours introduced by the MR couch-top and pelvic coil.     

Towards individualised radiotherapy for Stage I seminoma.

BACKGROUND AND PURPOSE: Adjuvant radiotherapy is currently standard treatment of Stage I seminoma (SOS). The use of computerised tomogram (CT) planning is compared with traditional planning for greater treatment individualisation. MATERIAL AND METHODS: Two plans were generated for each of 10 patients: one using traditional rectangular para-aortic fields, and one using conformal fields. The primary target volume compared was the dosimetric coverage of the inferior vena cava and aorta. RESULTS: The dosimetric analysis of traditional plans showed that they provided reasonable dosimetric coverage of the CTV. However, if 1cm is used for uncertainty based on nodal coverage then the periphery of the PTV could be significantly under-dosed. The CT based plan delivered improved dosimetry to the vessel PTV compared with the traditional field (CT D 95=24.7 Gy, traditional D 95=23.6 Gy, P=0.002). CT-based plans were significantly wider than traditional plans (CT=11.8 cm, traditional=9 cm, P=0.002). The CT plan tended to irradiate relatively small volumes of the kidneys to higher doses. CONCLUSIONS: Traditional para-aortic fields may deliver suboptimal dosimetry to an anatomically defined PTV. Our CT-based fields tend to be wider than traditional fields, and provide improved dosimetry to vessels based target volumes. Given that traditional fields are often delivering significantly less than the prescribed dose to the target volume, and that marginal relapses cause a high proportion of treatment failure, there is a suggestion that CT-based plans may avoid under-dosage and geographical miss sometimes seen with traditional plans.     

Dosimetric comparison of four target alignment methods for prostate cancer radiotherapy

PURPOSE: The aim of this study was to compare the dosimetric consequences of 4 treatment delivery techniques for prostate cancer patients treated with intensity-modulated radiotherapy (IMRT). METHODS AND MATERIALS: During an 8-week course of radiotherapy, 10 patients underwent computed tomography (CT) scans 3 times per week (243 total) before daily treatment with a CT-linear accelerator. Treatment delivery was simulated by realigning a fixed-margin treatment plan on each CT scan and calculating doses. The alignment methods were those based on the following: skin marks, bony registration, ultrasonography (US), and in-room CT. For the last two methods, prostate was the alignment target. The dosimetric effects of these alignment methods on the prostate, seminal vesicles, rectum, and bladder were compared. The average daily minimum dose to 0.1 cm3 was used as the metric for target coverage. RESULTS: Skin and bone alignments provided acceptable prostate coverage for only 70% of patients, US alignment for 90%, and CT alignment for 100%. CT-based alignment of the prostate provided seminal vesicle (SV) coverage of > or = 69 Gy for all patients; US and bone alignments provided SV coverage of > or = 60 Gy. This SV coverage may be acceptable for early-stage cancer (equivalent SV dose = 55.8 Gy at 1.8 Gy per fraction), but unacceptable for late-stage cancer (SV dose = 75.6 Gy). At 75.6 Gy, the acceptable rate for SV coverage was 40% for skin and bone alignments, 70% for US, and 80% for CT. CONCLUSIONS: Direct target alignment methods (US and CT) provided better target coverage. CT-guided alignment provided the best and most consistent dosimetric coverage. A larger planning target volume margin is needed for SV coverage when the alignment target is the prostate.     

利用0.13cc电离室对头颈部肿瘤调强放疗计划的剂量学验证

目的:利用0.13cc电离室对头颈部肿瘤调强适形放射治疗(IMRT)计划进行剂量学验证.方法:将20例头颈部肿瘤患者的IMRT计划分别移植到经过CT扫描的调强体模,生成验证计划,将0.13cc电离室放置到调强体模中在加速器下执行验证计划,在治疗计划系统中算出电离室所在区域的吸收剂量为计划剂量,按验证计划照射测量到的电离室吸收剂量为实测剂量,将二者进行比较得出误差.相对误差=(计划剂量-实测剂量)/实测剂量.百分误差超过±5％,说明计划在执行中剂量误差过大,计划需要修正.结果:20例患者中有17例患者验证的误差在±5％以内,表明计划通过;有3例患者误差超过±5％以内,计划需重新修改,计划通过率为85％.结论:剂量学验证确定IMRT治疗剂量的置信度,保证治疗计划的准确实施,提供了临床评价治疗计划的依据.     

Rectal dose reduction with IMRT for prostate radiotherapy

Dose escalation in radiation therapy has led to increased control rates with some clinical trial evidence that rectal toxicity may be reduced when using intensity-modulated radiotherapy (IMRT) over 3D conformal radiotherapy (3DCRT) for dose-escalated prostate radiotherapy. However, IMRT for prostate patients is not yet standard in many Australian radiation oncology centres. This study investigates dosimetric changes that can be observed between IMRT and 3DCRT in prostate radiotherapy. Fifteen patients were selected for analysis. Two target definitions were investigated – prostate-only and prostate plus seminal vesicles (p + SVs). A five-field 3DCRT and seven-field IMRT plan were created for each patient and target definition. The planning target volume coverage was matched for both plans. Doses to the rectum, bladder and femoral heads were compared using dose volume histograms. The rectal normal tissue complication probabilities (NTCPs) were calculated and compared for the 3DCRT and IMRT plans. The delivery efficiency was investigated. The IMRT plans resulted in reductions in the V25, V50, V60, V70 and V75 Gy values for both the prostate-only and p + SVs targets. Rectal NTCP was reduced with IMRT for three different sets of model parameters. The reductions in rectal dose and NTCP were much larger for the p + SVs target. Delivery of IMRT plans was less efficient than for 3DCRT plans. IMRT resulted in superior plans based on dosimetric and biological endpoints. The dosimetric gains with IMRT were greater for the more complex p + SVs target. The gains made came at the cost of decreased delivery efficiency.     

Treatment planning issues related to prostate movement in response to differential filling of the rectum and bladder

Conventional stimulation for patients with localized prostatic carcinoma often includes opacification of the dose limiting adjacent normal tissues. However, CT-based treatment planning is performed with the bladder and the rectum naturally filled or emptied. These latter conditions more closely approximate those in place at treatment Comparison of these CT-based treatment plans to simulator films taken with the rectum and bladder opacified yielded indirect evidence of movement of the prostate gland by 0.5 cm or more in 31 of 50 consecutive patients. The range of motion was 0 to 2 cm with an average of 0.5 cm (1.0 cm in the 31 patients). Six additional patients (five with local recurrence following I-125 seed implantation) were analyzed separately using CT scans. Registered CT images (3 mm slices) taken with the rectum and bladder full and/or empty provided direct evidence of prostate movement in 3 of the 6 patients. The dosimetric consequences of this movement are demonstrated using 3-dimensional dose distributions.     

国产加速器IMRT和VMAT的剂量学测试

目的 应用TG119报告中的测试例对联影公司URT-Linac 506C加速器和URT计划系统FF模型进行全面的评估。方法 主要研究调强放疗(IMRT)和容积调强弧形治疗(VMAT)计划剂量目标、点剂量和剂量分布计算准确性。测试模体采用AAPM TG119号报告测试例模体,主要包括模拟多靶区、模拟前列腺肿瘤、模拟头颈肿瘤和模拟C形靶区(简单)等。使用指形电离室和胶片,在URT计划系统(临床实验版本)和URT-Linac 506C加速器上分别优化计算和测量剂量,并分析计划剂量和测量值之间的偏差。结果 URT计划系统中的4个测试案例的计划剂量目标均满足TG119标准要求。对于不同案例点剂量验证,IMRT计划最大误差为2.62%,VMAT计划最大误差为 3.90%,均满足TG119 报告中点剂量误差小于4.5%的要求。复合剂量分布胶片测量结果的γ通过率IMRT计划均>97.50%,VMAT计划均>93.27%。结论 URT-Linac 506C加速器和URTT放疗计划系统FF计算模型性能试验结果均达到TG119报告验证标准。     

Radiotherapy treatment planning of prostate cancer using magnetic resonance imaging alone.

PURPOSE: Accurate anatomical delineation of the gross tumour volume (GTV) is crucial for effective radiotherapy (RT) treatment of prostate cancers. Although reference to pelvic magnetic resonance (MR) for improved delineation of the prostate is a regular practice in some clinics, MR has not replaced CT due to its geometrical distortions and lack of electron-density information. The possibility and practicality of using MR only for RT treatment planning were studied. MATERIALS AND METHODS: The addition of electron-density information to MR images for conformal radiotherapy (CRT) planning of the prostate was quantified by comparing dose distributions created on the homogeneous density- and bulk-density assigned images to original CT for four patients. To quantify the MR geometrical distortions measurements of a phantom imaged in CT (Siemens Somatom Plus 4) and FLASH 3D T1-weighted MR (1.5 T whole body Siemens Magnetom Vision) were compared. Dose statistics from CRT treatment plans made on CT and MR for five patient data were compared to determine if MR-only treatment plans can be made. RESULTS: The differences between dose-plans on bulk-density assigned images when compared to CT were less than 2% when water and bone values were assigned. Dose differences greater than 2% were observed when images of homogeneous-density assignment were compared to the CT. Phantom measurements showed that the distortions in the FLASH 3D T1-weighted MR averaged 2 mm in the volume of interest for prostate RT planning. For the CT and MR prostate planning study, doses delivered to the planning target volume (PTV) in CT and MR were always inside a 93-107% dose range normalised to the isocentre. Also, the doses to the organs-at-risk in the MR images were similar to the doses delivered to the volumes in the registered CT image when the organ volumes between the two images were similar. CONCLUSIONS: Negligible differences were observed in dose distribution between CRT plans using bone+water CT number bulk-assigned image and original CT. Also, the MR distortions were reduced to negligible amounts using large bandwidth MR sequence for prostate CRT planning. MR treatment planning was demonstrated using a large bandwidth sequence and bulk-assigned images. The development of higher quality, low distortion MR sequence will allow regular practice of this technique.     

Effects of geometric distortion in 0.2T MRI on radiotherapy treatment planning of prostate cancer.

BACKGROUND AND PURPOSE: To evaluate the impact of two different methods of geometric distortion correction of MR images from a Siemens Magnetom Open Viva 0.2T resistive MR unit on the process of external beam radiotherapy treatment planning for prostate cancer. PATIENTS AND METHODS: A method for correction of system related and object induced distortions and one for correction of purely system related distortions have been evaluated. The latter used information extracted from MR images of a 3D phantom specifically designed for geometric distortion evaluation. An active shim procedure was performed prior to all phantom and patient scans. For each of five patients five standard treatment plans were compared using uncorrected and corrected MR images alone (density=water) and CT images alone. Finally internal anatomical landmarks were used for image registration between MR images (corrected and uncorrected) and CT images to evaluate the impact of distortion correction on the image registration process. RESULTS: Maximum distortions of 28 mm (mean 2.2 mm) were found within the FOV in frequency encode direction. Maximum distortions could be reduced by a factor of two (mean factor four) by our phantom measurement based technique. Distortion patterns were found to be stable and reproducible over several weeks with this MR unit. For 4/5 patients, relative doses at the normalization point as calculated on the distortion corrected MR images only (all tissues taken water equivalent) were all within 1% of the corresponding value from the standard CT-based plan (actual Hounsfield units). The largest differences in isocentric dose found in one case were 3.1% MR uncorrected vs. CT and 2.6% MR corrected vs. CT. Typical sites of internal anatomical landmarks chosen for image registration show distortions up to 3 mm. CONCLUSIONS: Object induced distortions are negligible at such low field strengths compared to system related distortions. Treatment plans for prostate cancer do not seem to differ significantly from "standard" plans calculated on CT images when calculated on distortion corrected MR images, even if all tissues are assigned the electron density of water. Distortion correction of MR images can theoretically improve the starting point for image registration of MR and CT images.