ESTRO 37 Abstract book

S156

ESTRO 37

OC-0301 NTCP-model based patient selection for hypofractionated prostate treatment - A computer simulation R. Bijman 1 , A.W. Sharfo 1 , W. Schillemans 1 , W. Heemsbergen 1 , M. Witte 2 , F. Pos 2 , L. Incrocci 1 , B. Heijmen 1 1 Erasmus MC Cancer Center, Radiation Oncology, Rotterdam, The Netherlands 2 Antoni van Leeuwenhoek Hospital, Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective External beam radiotherapy (EBRT) is one of the primary treatment modalities for prostate cancer. In the last years, hypofractionated EBRT has gained increasing popularity for prostate cancer treatment, largely related to reduced hospital visits for the patients and lower overall costs. In case hypofractionation has a (slightly) enhanced risk for late toxicity compared to conventional fractionation, this has to be balanced against the advantages. To this purpose, automated planning could be used to generate for each individual patient a plan for conventional fractionation and a plan for hypofractionation. Patient selection could then be based on differences in calculated NTCPs (similar to model- based patient selection for proton therapy). In this work, we have performed computer simulations to establish percentages of patients that could be treated with hypofractionation. These percentages were determined as a function of accepted thresholds for increases in calculated normal tissue complication probabilities (NTCP) relative to conventional fractionation. Material and Methods Data from the randomized phase 3 HYPRO trial were used in this study. In this trial, 820 patients were treated with either 39x2 Gy or 19x3.4 Gy. Overall, reported incidences of grade ≥ 2 genitourinary (GU) and gastrointestinal (GI) toxicity (RTOG/EORTC criteria) at 3 years were 40.2% and 19.8%, respectively. For 724 patients, reported late toxicities and the clinically delivered dose distributions and clinical baseline parameters were available and used to derive NTCP models for these toxicities, using logistic regression. For this study, fully automated treatment plan generation was then used to generate for all patients a VMAT plan for conventional fractionation and a plan for hypofractionation with similar planning constraints relative to the prescribed dose. When the differences in GU and GI NTCP (ΔNTCP) between the two fractionation schemes were below pre-defined thresholds, the patient was assumed eligible for hypofractionation. Percentages of patients being selected for hypofractionation were established as a function of ΔNTCP thresholds for GI and GU complications. Results Differences in PTV coverage between the two arms were not significant (ΔV 95% = 0.03%, p = .83). Figure 1 shows that with accepted NTCP increases of 4.7% and 7.0% for GU and GI toxicity, respectively, 95% of the patients could be treated with hypofractionation. Accepted GU and GI NTCP increases of 4.3% and 0% would result in only 5% of patients treated with hypofractionation. For acceptable GU and GI NTCP increases of 4.6% and 2.4%, 50% of patients could be treated with hypofractionation.

Conclusion NTCP-model based selection of prostate patients for hypofractionation is feasible. The number of patients eligible for hypofractionation depends on applied thresholds for NTCP increases. Model-based selection could be used in shared decision making. OC-0302 Endorectal balloon for prostate SBRT: dosimetric impact on erectile function preservation? G. Lamanna 1 , A. Dubouloz 1 , M. Rouzaud 1 , R. Miralbell 1 , T. Zilli 1 1 HUG, Radiotherapy, Genève, Switzerland Purpose or Objective One of the most common side effects from radiotherapy treatment of localized prostate cancer remains erectile dysfunction. Vessel-sparing radiotherapy has shown promising results on preserving erectile function when compared to historical series. We hypothesized that the use of an endorectal balloon (ERB) for prostate stereotactic body radiotherapy (SBRT) might improve erectile function preservation by decreasing the dose to the internal pudendal arteries (IPA). The aim of this dosimetric study was to compare the IPA dose in 10 prostate cancer patients simulated with and without ERB before SBRT of the prostate. Material and Methods Ten prostate cancer patients simulated with and without ERB were planned to receive 36.25Gy (5x7.25Gy) to the planning treatment volume (PTV) while the dose to urethra (urethral planning risk volume, uPRV) was reduced to 32.5 Gy. IPAs were delineated on both planning CT scans with MRI registration. Plans with and without ERB were optimized using a volumetric modulated arc (VMAT) technique within well defined dose constraints. No optimization was undertaken on IPA. SBRT doses to IPA (D max and D mean ) were compared between groups using non-parametric tests. Results IPA volumes were similar in the two CT datasets, with a median volume of 4.7 cc (range, 3.8-7.2) versus 5.1 cc (range, 3.8-7) in patients with and without ERB (p=NS), respectively. No differences in target coverage, rectal, and bladder walls doses were observed between the two groups. The dose delivered to IPA was significantly lower in patients simulated with ERB compared to patients planned without. Median IPA D max decreased from 26.8 Gy (range, 16.2-37.7) to 21.1 Gy (range, 13.5-36.9) with ERB (p=0.013). Similarly, the median IPA D mean decreased from 15.4 Gy (range, 8.8-23.0) to 11.5 Gy (range, 8.7-17.1) with ERB (p=0.009). Thus, with ERB the mean decrease in IPA dose was a 16% and 22% for D max and D mean , respectively, compared to patients planned without ERB. Conclusion The use of ERB to treat prostate cancer patients with SBRT showed a significant sparing of IPA compared to plans generated with an empty rectum. As no specific dose constraints are available for vessel-sparing radiotherapy, maximal reduction on IPA doses should be attempted in SBRT treatments to maximize erectile function preservation.