Impact of radioimmunoscintigraphy on definition of clinical target volume for radiotherapy after prostatectomy.

The goal of this study was to evaluate the role of radioimmunoscintigraphy (RIS) directed against prostate-specific membrane antigen in modifying postprostatectomy prostate fossa clinical target volume (CTV) definition. METHODS: The records of 25 postprostatectomy patients who received external-beam radiotherapy after prostatectomy and who underwent vessel-based RIS/planning CT registration were reviewed. For each patient, the CTV that would have been treated (CTV(pre)) before this registration was compared with that defined after the registration (CTV(post)). In addition, using a standard dose of 66 Gy in 2-Gy fractions, the corresponding bladder and rectum dose volume histograms were compared using 2 endpoints: volume receiving > or =60 Gy (V60) and area under the curve (AUC). RESULTS: The mean CTV(pre) vs. CTV(post) volumes were 24.4 vs. 35.0 cm(3), respectively (P = 0.032). The V60 results for CTV(pre) and CTV(post) were 32.7 vs. 41.0 cm(3), respectively, for the rectum (P = 0.168) and 33.9 vs. 46.6 cm(3), respectively, for the bladder (P = 0.015). The AUC results for CTV(pre) and CTV(post) were 4,027 vs. 4,516 Gy x cm(3), respectively, for the rectum (P = 0.396) and 4,782 vs. 5,561, respectively, for the bladder (P = 0.119). No Radiation Therapy Oncology Group grade 3, 4, or 5 (acute or late, gastrointestinal, or genitourinary) toxicity was observed. Two-year biochemical failure-free survival (with failure defined as 2 consecutive prostate-specific antigen rises above 0.2ng/mL) was 87% for the cohort. CONCLUSION: Incorporating RIS uptake resulted in significant modifications in CTV definition. The consequences of these modifications on the rectum V60 or AUC or on the bladder AUC were not significant, although the bladder V60 did increase. However, observed toxicity was low, with acceptable short-term biochemical control, suggesting that treatment to the modified CTV was tolerable.     

Postprostatectomy target-normal structure overlap volume differences using computed tomography and radioimmunoscintigraphy images for radiotherapy treatment planning

PURPOSE: The purpose of this study was to analyze regions of uptake in normal structures on postprostatectomy radioimmunoscintigraphy (RIS) images by evaluating differences in the overlap volumes of prostate fossa clinical target volume (CTV) and planning target volume (PTV) using correlative computed tomography (CT) images. MATERIALS AND METHODS: The electronic records of 13 patients who received external beam radiotherapy postprostatectomy and who underwent a vessel-based RIS/CT registration were reviewed. For each patient, the RIS-defined CTV (CTV(RIS)) was compared (in terms of the overlap volume with the surrounding bladder, rectum, pubic symphysis, and penile bulb) with the CT-defined CTV(pre) before this registration and also with CTV(post) (the final target volume used for treatment). Similar analyses were done for PTV(RIS), PTV(pre), and PTV(post) defined in each case to be the corresponding CTV + 1-cm margin. RESULTS: CTV(RIS) overlapped significantly more with the bladder, rectum, and symphysis, but not with the penile bulb, than did either the CTV(pre) or CTV(post). However, the corresponding PTV analyses revealed no significant differences between any of the overlap volumes of any of the PTVs with the bladder, rectum, and penile bulb, but did reveal a significant difference between the PTV(RIS) and PTV(post) overlap volumes with the symphysis compared with PTV(pre) overlap volumes with the symphysis. CONCLUSIONS: On RIS images, there appear to be areas of uptake in the bladder, rectum, and pubic symphysis but not the penile bulb; however, the dosimetric consequences of this uptake for radiation treatment planning are minimal on the bladder, rectum, and penile bulb, but require segmentation for dose reduction to the pubic symphysis.     

The Incidence of Inclusion of the Sigmoid Colon and Small Bowel in the Planning Target Volume in Radiotherapy for Prostate Cancer

BACKGROUND AND PURPOSE: In radiotherapy for prostate cancer, the rectum is considered the dose-limiting organ. The incidence of overlap between the sigmoid colon and/or small bowel and the planning target volume (PTV) as well as the dose to sigmoid colon and small bowel were investigated. PATIENTS AND METHODS: The CT data of 75 prostate cancer patients were analyzed. The clinical target volume (CTV) consisted of prostate and seminal vesicles. The PTV was defined as a three-dimensional expansion of the CTV with a 10-mm margin in craniocaudal and a 7-mm margin in the other directions. All patients were planned to a mean CTV dose of at least 76 Gy. Minimum CTV dose was set at 70 Gy. Dose inhomogeneity within the CTV was kept between 12% and 17%. Sigmoid colon was defined upward from the level where the rectum turned in a transverse plane. Contrast-filled small bowel was contoured on all slices where it was visible. The presence of sigmoid colon and/or small bowel in close vicinity to or overlapping with the PTV was recorded. For each case, the dose to the sigmoid colon and small bowel was calculated. RESULTS: The PTV was found to overlap with the sigmoid colon in 60% and with the small bowel in 19% of the cases. In these patients, mean maximum dose to the sigmoid colon was 76.2 Gy (5th-95th percentile: 70.0-80.7 Gy). Mean maximum dose to the small bowel was 74.9 Gy (5th-95th percentile: 68.0-80.0 Gy). CONCLUSION: When systematically investigating the anatomic position of sigmoid colon and small bowel in patients accepted for prostate irradiation, parts of both organs were often observed in close vicinity to the PTV. Apart from the rectum, these organs may be dose-limiting in prostate radiotherapy.     

Dose-volume and radiobiological dependence on the calculation grid size in prostate VMAT planning

This study evaluated the effects of dose-volume and radiobiological dependence on the calculation grid size in prostate volumetric-modulated arc therapy (VMAT) planning. Ten patients with prostate cancer were selected for this retrospective treatment planning study. Prostate VMAT plans were created for the patients using the 6 MV photon beam produced by a Varian TrueBEAM linac with the calculation grid size equal to 1, 2, 2.5, 3, 4, and 5?mm. Dose-volume histograms (DVHs) of targets and organs at risk were generated for different grid sizes. We calculated the radiobiological parameters of the tumor control probability (TCP) of clinical target volume (CTV) and planning target volume (PTV), and the normal tissue complication probability (NTCP) of organs at risk (rectal wall, rectum, bladder wall, bladder, left femur, and right femur). The homogeneity, conformity, and gradient indexes of CTV and PTV were calculated for different grid sizes. The TCP of PTV was found decreasing with a rate of 0.06%/mm when the calculation grid size increased from 1 to 5?mm. On the other hand, both NTCPs of rectal wall and rectum were found decreasing with rates of 0.03%/mm and 0.05%/mm, respectively, with an increase of grid size. The homogeneity index of PTV increased with a rate of 0.57/mm of the calculation grid size, whereas the conformity index of PTV decreased with a rate of 0.0075/mm. The gradient index of PTV was found increasing with a rate equal to 0.05/mm. In prostate VMAT planning, variations of dose-volume and radiobiological parameters with calculation grid size on PTV, rectal wall, and rectum were more significant than those of CTV and other organs at risk such as bladder wall, bladder, and femurs. Results in this study are important in the treatment planning quality assurance when the calculation grid size is varied to compromise a shorter dose computing time.     

Intensity-modulated radiotherapy for a prostate patient with a metal prosthesis

When treating prostate patients having a metallic prosthesis with radiation, a 3D conformal radiotherapy (3DCRT) treatment plan is commonly created using only those fields that avoid the prosthesis in the beam’s-eye view (BEV). With a limited number of portals, the resulting plan may compromise the dose sparing of the rectum and bladder. In this work, we investigate the feasibility of using intensity-modulated radiotherapy (IMRT) to treat prostate patients having a metallic prosthesis. Three patients, each with a single metallic prosthesis, who were previously treated at the University of Chicago Medical Center for prostate cancer, were selected for this study. Clinical target volumes (CTV = prostate + seminal vesicles), bladder, and rectum volumes were identified on CT slices. Planning target volumes (PTV) were generated in 3D by a 1-cm expansion of the CTVs. For these comparative studies, treatment plans were generated from CT data using 3DCRT and IMRT treatment planning systems. The IMRT plans used 9 equally-spaced 6-MV coplanar fields, with each field avoiding the prosthesis. The 3DCRT plans used 5 coplanar 18-MV fields, with each field avoiding the prosthesis. A 1-cm margin around the PTV was used for the blocks. Each of the 9-field IMRT plans spared the bladder and rectum better than the corresponding 3DCRT plan. In the IMRT, plans, a bladder volume receiving 80% or greater dose decreased by 20–77 cc, and a volume rectal volume receiving 80% or greater dose decreased by 24–40 cc. One negative feature of the IMRT plans was the homogeneity across the target, which ranged from 95% to 115%.     

Computed Tomogram prior to Prostatectomy Advantage in Defining Planning Target Volumes for Postoperative Adjuvant Radiotherapy in Patients with Stage C Prostate Cancer?

PURPOSE: To evaluate the influence of a preoperative computed tomogram (CT) on delineation of the planning target volume (PTV) for adjuvant radiation therapy of pT3 pN0 prostate cancer. PATIENTS AND METHODS: PTVs of ten patients who had an additional preoperative CT examination were contoured by three independent radiation oncologists. PTV included the former prostatic bed and seminal vesicles with a safety margin. First PTVs were drawn without knowledge of the preoperative CTs and in a second attempt, this procedure was repeated with these CTs available for visual comparison. Changes in PTV dimensions for every patient were analyzed. RESULTS: In 93% of all PTVs there was a decision to increase the PTV after viewing the preoperative CT images. Mean PTV length increased from 7.3 to 8.4 cm and PTV volumes expanded 26% from 244 to 308 cm3. These differences were statistically significant for all three participating radiation oncologists. CONCLUSION: Planning target volume definition probably is a critical factor in adjuvant radiation therapy after radical prostatectomy. As there is a considerable incertainty in PTV definition a preoperative CT is helpful and therefore may have beneficial influence on results.     

Einfluss von bildgestützter translatorischer Isozentrumskorrektur auf die Dosisverteilung bei 3-D-Konformationsbestrahlung der Prostata

BACKGROUND AND PURPOSE: Interfractional prostate motion during radiotherapy due to variation in rectal distension can have negative consequences. The authors investigated the dosimetric consequences of a linear translational position correction based on image guidance when a three-dimensional conformal treatment technique was used. MATERIAL AND METHODS: Planning CTs of seven patients with empty and distended rectum were analyzed. A reference plan for the planning target volume (PTV) and the boost were calculated on the CT dataset with the empty rectum with a standard four-field technique. The treatment plan was transferred to the CT with the distended rectum for an uncorrected setup (referenced to bony anatomy) and a corrected setup after position correction of the isocenter. The dosimetric consequences were analyzed. RESULTS: Organ motion decreased the coverage of the prostate by the 95% isodose during simulated single treatment fractions by up to -21.0 percentage points (%-p; boost plan) and by up to -14.9%-p for the seminal vesicles (PTV plan). The mean rectum dose increased by up to 18.3%-p (PTV plan). Linear translational correction (mean 6.4 +/- 3.4 mm, maximum 10.8 mm) increased the coverage of the prostate by the 95% isodose by up to 12.7%-p (boost plan), while the mean rectum dose was reduced by up to -8.9%-p (PTV plan). For the complete treatment a reduction of complication probability of the rectum of approximately 5%-p was calculated. CONCLUSION: The use of an image guidance system with linear translational correction can improve radiation treatment accuracy for prostate cancer, if geometric changes are within certain limits.     

A Prospective Three–Dimensional Analysis about the Impact of Differences in the Clinical Target Volume in Prostate Cancer Irradiation on Normal–Tissue Exposure

BACKGROUND AND PURPOSE: Rectal toxicity following external-beam irradiation of prostate cancer correlates with the exposed percentage of rectal volume. Recently, it has been recommended to reduce the volume of the seminal vesicles that should be included in the clinical target volume (CTV). The purpose of this study was to quantitatively assess the impact of this CTV reduction on the expected rectal and bladder dose sparing. PATIENTS AND METHODS: 14 patients with localized prostate cancer undergoing external-beam radiotherapy were investigated. The prostate, the prostate + entire seminal vesicles, or the prostate + proximal seminal vesicles were delineated as CTV. Treatment plans were generated and compared concerning rectum and bladder dose-volume histograms (DVHs). RESULTS: The exposure of rectum and bladder volume was significantly lower in case of irradiation of the prostate only compared to inclusion of the proximal or entire seminal vesicles into the CTV. The reduction of the CTV from prostate + entire seminal vesicles to prostate + proximal seminal vesicles led to a significant reduction of the rectal and bladder dose exposure. CONCLUSION: Reduction of the CTV to the prostate only, or to the prostate + proximal seminal vesicles led to significant rectal and bladder dose sparing compared to irradiation of the prostate + entire seminal vesicles. In patients with a higher risk for seminal vesicles involvement, irradiation of the prostate + proximal seminal vesicles should be preferred. In case of a need for irradiation of the entire seminal vesicles, patients should be informed about a higher risk for chronic rectal toxicity and, possibly, for bladder complications.     

Multifield optimization intensity-modulated proton therapy (MFO-IMPT) for prostate cancer: Robustness analysis through simulation of rotational and translational alignment errors

To evaluate the dosimetric consequences of rotational and translational alignment errors in patients receiving intensity-modulated proton therapy with multifield optimization (MFO-IMPT) for prostate cancer. Ten control patients with localized prostate cancer underwent treatment planning for MFO-IMPT. Rotational and translation errors were simulated along each of 3 axes: anterior-posterior (A-P), superior-inferior (S-I), and left-right. Clinical target-volume (CTV) coverage remained high with all alignment errors simulated. Rotational errors did not result in significant rectum or bladder dose perturbations. Translational errors resulted in larger dose perturbations to the bladder and rectum. Perturbations in rectum and bladder doses were minimal for rotational errors and larger for translational errors. Rectum V45 and V70 increased most with A-P misalignment, whereas bladder V45 and V70 changed most with S-I misalignment. The bladder and rectum V45 and V70 remained acceptable even with extreme alignment errors. Even with S-I and A-P translational errors of up to 5 mm, the dosimetric profile of MFO-IMPT remained favorable. MFO-IMPT for localized prostate cancer results in robust coverage of the CTV without clinically meaningful dose perturbations to normal tissue despite extreme rotational and translational alignment errors.     

Direct Segment Aperture and Weight Optimization for Intensity-Modulated Radiotherapy of Prostate Cancer

BACKGROUND AND PURPOSE: To describe the implementation and to evaluate the results of direct segment aperture optimization using the segment outline and weight adapting tool (SOWAT) in intensity-modulated radiotherapy (IMRT) for prostate cancer. PATIENTS AND METHODS: 14 consecutive, unselected patients with localized prostate cancer were entered in a planning study comparing IMRT without and with the use of SOWAT. The clinical target volume (CTV) consisted of the prostate and seminal vesicles in all cases. To create the planning target volume (PTV), a three-dimensional anisotropic margin (10 mm in craniocaudal direction, 7 mm in both other directions) was used. To compare both plans, physical as well as biological endpoints were considered. RESULTS: Considering the CTV, SOWAT resulted in a significantly higher minimal dose together with a higher dose to 95% (D(95)) and 90% (D(90)) of the CTV volume (p < 0.05; Figure 2). Target dose homogeneity was significantly improved (p < 0.001). Tumor control probability (TCP) was significantly increased (p < 0.05). Considering the PTV, D(90) was significantly increased (p < 0.05). Target dose homogeneity was significantly improved (p < 0.05; Figure 1). For rectum, the volumes receiving 50 Gy (R(vol50)), 60 Gy (R(vol60)), or 65 Gy (R(vol65)) as well as the mean dose were significantly lowered after SOWAT (p = 0.0001; Figure 3). Rectal normal tissue complication probability (NTCP) was significantly lower after SOWAT (p = 0.005). Probability of uncomplicated local control (P+) was significantly higher after SOWAT (p < 0.0001). CONCLUSION: SOWAT is a powerful planning tool to increase the therapeutic ratio of IMRT for prostate cancer. It leaves the delivery time unchanged, so that treatments can still be delivered within a time slot of 8 min.     

Comparative evaluation of CT-based and respiratory-gated PET/CT-based planning target volume (PTV) in the definition of radiation treatment planning in lung cancer: preliminary results

Purpose

The aim of this study was to compare planning target volume (PTV) defined on respiratory-gated positron emission tomography (PET)/CT (RG-PET/CT) to PTV based on ungated free-breathing CT and to evaluate if RG-PET/CT can be useful to personalize PTV by tailoring the target volume to the lesion motion in lung cancer patients.

Methods

Thirteen lung cancer patients (six men, mean age 70.0 years, 1 small cell lung cancer, 12 non-small cell lung cancer) who were candidates for radiation therapy were prospectively enrolled and submitted to RG-PET/CT. Ungated free-breathing CT images obtained during a PET/CT study were visually contoured by the radiation oncologist to define standard clinical target volumes (CTV1). Standard PTV (PTV1) resulted from CTV1 with the addition of 1-cm expansion of margins in all directions. RG-PET/CT images were contoured by the nuclear medicine physician and radiation oncologist according to a standardized institutional protocol for contouring gated images. Each CT and PET image of the patient’s respiratory cycle phases was contoured to obtain the RG-CT-based CTV (CTV2) and the RG-PET/CT-based CTV (CTV3), respectively. RG-CT-based and RG-PET/CT-based PTV (PTV2 and PTV3, respectively) were then derived from gated CTVs with a margin expansion of 7–8 mm in head to feet direction and 5 mm in anterior to posterior and left to right direction. The portions of gated PTV2 and PTV3 geometrically not encompassed in PTV1 (PTV2 out PTV1 and PTV3 out PTV1) were also calculated.

Results

Mean ± SD CTV1, CTV2 and CTV3 were 30.5?±?33.2, 43.1?±?43.2 and 44.8?±?45.2 ml, respectively. CTV1 was significantly smaller than CTV2 and CTV3 (p?=?0.017 and 0.009 with Student’s t test, respectively). No significant difference was found between CTV2 and CTV3. Mean ± SD of PTV1, PTV2 and PTV3 were 118.7?±?94.1, 93.8?±?80.2 and 97.0?±?83.9 ml, respectively. PTV1 was significantly larger than PTV2 and PTV3 (p?=?0.038 and 0.043 with Student’s t test, respectively). No significant difference was found between PTV2 and PTV3. Mean ± SD values of PTV2 out PTV1 and PTV3 out PTV1 were 12.8?±?25.4 and 14.3?±?25.9 ml, respectively. The percentage values of PTV2 out PTV1 and PTV3 out PTV1 were not lower than 10 % of PTV1 in 6/13 cases (46.2 %) and than 20 % in 3/13 cases (23.1 %).

Conclusion

Our preliminary data showed that RG-PET/CT in lung cancer can affect not only the volume of PTV but also its shape, as demonstrated by the assessment of gated PTVs outside standard PTV. The use of a gating technique is thus crucial for better delineating PTV by tailoring the target volume to the lesion motion in lung cancer patients.     

膀胱充盈状态对宫颈癌调强放疗子宫及危及器官的影响

目的 通过研究膀胱充盈状态对子宫、宫颈的移位、靶区和危及器官（OAR）体积的影响,为宫颈癌调强放疗（IMRT）个体化内靶区（ITV）及临床靶区（CTV）到计划靶区（PTV）界定提供理论基础。方法 获取宁夏医科大学总医院27例局部进展期初治宫颈癌患者膀胱排空、膀胱充盈1.0 h、充盈1.5 h定位CT图像,分别勾画宫颈、宫体、OAR,以及膀胱充盈1.0 h的CTV、PTV,分析不同膀胱充盈间宫颈、宫体的移位,不同充盈状态下子宫、直肠、小肠、PTV内小肠、PTV内膀胱、PTV内直肠体积的差异;分析膀胱充盈与子宫移位及OAR体积的相关性。分析膀胱排空与充盈1.5 h靶区脱出PTV的体积。结果 膀胱充盈状态个体差异较大,子宫颈和子宫体随膀胱充盈状态变化引起的最大移动范围分别为0~3.04 cm、0~4.31 cm。不同充盈间子宫体在前方移位差异有统计学意义（F=7.818,P<0.05）;不同充盈状态下膀胱体积、PTV内膀胱及小肠体积差异有统计学意义（F=46.197、44.609、29.546,P<0.05）;膀胱的充盈状态与子宫体前缘的移位、小肠体积、PTV内膀胱体积、PTV内小肠体积间有相关性（r=-0.232、-0.298、0.915、-0.336,P<0.05）。在膀胱排空及充盈1.5 h时,宫颈、宫体脱出PTV的体积差异均具有统计学意义（t=-1.326、-1.559,P<0.05）。结论 膀胱充盈状态具有较大的个体差异,膀胱充盈状态对子宫前缘的影响较大,扩大CTV-PTV前方外放范围可能降低靶区的漏照,控制膀胱充盈状态的一致性对宫颈癌精确调强放疗是必要的。     

CT-MRI image fusion for delineation of volumes in three-dimensional conformal radiation therapy in the treatment of localized prostate cancer

The objective of this study was to assess the utility of CT-MRI image fusion software and compare both prostate volume and localization with CT and MRI studies. We evaluated the differences in clinical volumes in patients undergoing three-dimensional conformal radiation therapy for localized prostate cancer. After several tests performed to ensure the quality of image fusion software, eight patients suffering from prostate adenocarcinoma were submitted to CT and MRI studies in the treatment position within an immobilization device before the start of radiotherapy. The clinical target volume (CTV) (prostate plus seminal vesicles) was delineated on CT and MRI studies and image fusion was obtained from the superimposition of anatomical fiducial markers. A comparison of dose-volume histograms relative to CTV, rectum, bladder and femoral heads was performed for both studies. Image fusion showed a mean overestimation of CTV of 34% with CT compared with MRI. Along the anterior-posterior and superior-inferior direction, CTV was a mean 5 mm larger with CT study compared with MRI. The dose-volume histograms resulting from CT and MRI comparison showed that it is possible to spare a mean 10% of rectal volume and approximately 5% of bladder and femoral heads, respectively. This study confirmed an overestimation of CTV with CT images compared with MRI. Because this finding only allows a minimal sparing of organs at risk, considering the organ motion during each radiotherapy session and the excellent outcomes of prostate cancer treatment with CT based target identification, we are still reluctant to reduce the CTV to that identified by MRI.     

Optimal selection of optimization bolus thickness in planning of VMAT breast radiotherapy treatments

The aim of this study was to find an optimal optimization skin flash thickness in volumetric modulated arc radiotherapy of the breast in consideration of soft tissue deformations during the treatment course. Ten breast radiotherapy patients with axillary lymph node inclusion were retrospectively planned with volumetric modulated arc radiotherapy technique. The plans were optimized with the planning target volume (PTV) extending outside the skin contour by 0, 5, 7, and 10 mm; and with optimization boluses of 3 or 5 mm on the extended PTV. The final dose was calculated without the bolus. The plans were compared in terms of PTV homogeneity and conformity, and dose minima and maxima. The doses to organs at risk were also evaluated. The doses were recalculated in real patient geometries based on cone beam computed tomography (CBCT) images captured 3 to 6 times during each patient's treatment course. The optimization to the PTV without the PTV extension resulted in the best CTV coverage in the original plans (V95% = 98.0% ± 1.2%). However, when these plans were studied in real CBCT-based patient geometries, the CTV V95% was compromised (94.6% ± 8.3%). In addition, for the surface (4 mm slap inside the PTV 4 mm below the body contour) dose V95% was reduced from the planned 74.7% ± 7.5% to the recalculated 65.5% ± 11.5%. Optimization with an 8-mm bolus to a PTV with 5-mm extension was the most robust choice to ensure the CTV and surface dose coverage (recalculated V95% was 95.2% ± 6.4% and 74.6% ± 8.4%, respectively). In cases with the largest observed deformations, even a 10-mm PTV extension did not suffice to cover the target. Optimization with a 5-mm PTV extension and an 8-mm optimization bolus improved the surface dose and slightly improved the CTV dose when compared to no extension plans. For deformations over 1 cm, no benefit was seen in PTV extensions and replanning is recommended. Frequent tangential and CBCT imaging should be used during treatment course to detect potential large anatomical changes.     

Distortion-corrected T2 weighted MRI: a novel approach to prostate radiotherapy planning

The purpose of this study was to evaluate distortion-corrected MRI as a radiotherapy planning tool for prostate cancer and the resultant implications for dose sparing of organs at risk. 11 men who were to be treated with radical conformal radiotherapy for localized prostate cancer had an MRI scan under radiotherapy planning conditions, which was corrected for geometric distortion. Radiotherapy plans were created for planning target volumes derived from the MRI- and CT-defined prostate. Dose volume histograms were produced for the rectum, bladder and penile bulb. The mean volume of the prostate as defined on CT and MRI was 41 cm3 and 36 cm3, respectively (p = 0.009). The predicted percentage of the rectum treated to dose levels of 45-65 Gy was significantly lower for plans delineating the prostate with MRI than for those with CT. The rectal-sparing effect was confined to the lowermost 4 cm of the rectum (anal canal). There were no differences between the predicted doses to bladder or penile bulb (as defined using MRI) between plans. In conclusion, prostate radiotherapy planning based on distortion-corrected MRI is feasible and results in a smaller target volume than does CT. This leads to a lower predicted proportion of the rectum, in particular the lower rectum (anal canal), treated to a given dose than with CT.