Urethra-sparing stereotactic body radiotherapy for prostate cancer: how much can the rectal wall dose be reduced with or without an endorectal balloon?
Abstract:BackgroundThis is a dosimetric comparative study intended to establish appropriate low-to-intermediate dose-constraints for the rectal wall (Rwall) in the context of a randomized phase-II trial on urethra-sparing stereotactic body radiotherapy (SBRT) for prostate cancer. The effect of plan optimization on low-to-intermediate Rwall dose and the potential benefit of an endorectal balloon (ERB) are investigated.MethodsTen prostate cancer patients, simulated with and without an ERB, were planned to receive 36.25Gy… Show more
“…Indeed, measurements from real-time electromagnetic beacon data have shown that in the absence of intra-fractional corrections, a posterior margin of 3 mm would result in unacceptably low (93%) CTV coverage (32). If we assume 1-2 mm margins are necessary for systematic errors related to contouring and machine performance (33)(34)(35), then the minimum PTV margins required in the absence of frequent intra-fractional monitoring and correction would be 2.9-3.9 mm LR, 3.7-4.7 mm SI, and 4.1-5.1 mm AP. The fact that a greater posterior margin is necessary is practically relevant, as many contemporary SBRT protocols allow for smaller posterior margins (5,(36)(37)(38)(39).…”
Purpose: To assess the optimal planning target volume (PTV) margins for stereotactic body radiotherapy (SBRT) of prostate cancer based on inter-and intra-fractional prostate motion determined from daily image guidance. Methods and Materials: Two hundred and five patients who were enrolled on two prospective studies of SBRT (8 Gy × 5 fractions) for localized prostate cancer treated at a single institution between 2012 and 2017 had complete inter-and intra-fractional shift data available. All patients had scheduled kilovoltage planar imaging during SBRT with rigid registration to intraprostatic fiducials prior to each of four half-arcs delivered per fraction, as well as cone beam CT verification of anatomy prior to each fraction. Inter-and intra-fractional shift data were obtained to estimate the required PTV margins based on the classic van Herk formula. Inter-and intra-fractional motion were compared between patients with and without severe toxicities using the independent two-sample Wilcoxon test. Results: The margins required to account for inter-fractional motion were estimated to be 0.99, 1.52, and 1.45 cm in lateral (LR), longitudinal (SI), and vertical (AP) directions, respectively. The margins required to account for intra-fractional motion were estimated to be 0.19, 0.27, and 0.31 cm in LR, SI and AP directions, respectively. Large intra-fractional shifts were mostly observed in the SI and AP directions, with 2.0 and 5.4% of patients experiencing average intra-fractional motion >3 mm in the SI and AP directions, respectively, compared with none experiencing mean shifts >3 mm in the LR direction. Six patients experienced grade 3 gastrointestinal or genitourinary toxicity. There were no significant differences in mean inter-or intra-fractional motion in any of the cardinal directions compared to patients without severe toxicity (inter-fractional p = 0.46-0.99, intra-fractional p = 0.10-0.84). Levin-Epstein et al. Prostate Displacement and SBRT Margins Conclusion: The inter-and intra-fractional margins estimated from this study are in line with prior reported values. Intra-fractional prostate motion was generally small with larger margins required for the SI and AP directions, notably just slightly exceeding the commonly used 3 mm posterior PTV margin even with realignment between half-arcs. Development of severe toxicity was not significantly associated with the degree of inter-or intra-fractional motion.
“…Indeed, measurements from real-time electromagnetic beacon data have shown that in the absence of intra-fractional corrections, a posterior margin of 3 mm would result in unacceptably low (93%) CTV coverage (32). If we assume 1-2 mm margins are necessary for systematic errors related to contouring and machine performance (33)(34)(35), then the minimum PTV margins required in the absence of frequent intra-fractional monitoring and correction would be 2.9-3.9 mm LR, 3.7-4.7 mm SI, and 4.1-5.1 mm AP. The fact that a greater posterior margin is necessary is practically relevant, as many contemporary SBRT protocols allow for smaller posterior margins (5,(36)(37)(38)(39).…”
Purpose: To assess the optimal planning target volume (PTV) margins for stereotactic body radiotherapy (SBRT) of prostate cancer based on inter-and intra-fractional prostate motion determined from daily image guidance. Methods and Materials: Two hundred and five patients who were enrolled on two prospective studies of SBRT (8 Gy × 5 fractions) for localized prostate cancer treated at a single institution between 2012 and 2017 had complete inter-and intra-fractional shift data available. All patients had scheduled kilovoltage planar imaging during SBRT with rigid registration to intraprostatic fiducials prior to each of four half-arcs delivered per fraction, as well as cone beam CT verification of anatomy prior to each fraction. Inter-and intra-fractional shift data were obtained to estimate the required PTV margins based on the classic van Herk formula. Inter-and intra-fractional motion were compared between patients with and without severe toxicities using the independent two-sample Wilcoxon test. Results: The margins required to account for inter-fractional motion were estimated to be 0.99, 1.52, and 1.45 cm in lateral (LR), longitudinal (SI), and vertical (AP) directions, respectively. The margins required to account for intra-fractional motion were estimated to be 0.19, 0.27, and 0.31 cm in LR, SI and AP directions, respectively. Large intra-fractional shifts were mostly observed in the SI and AP directions, with 2.0 and 5.4% of patients experiencing average intra-fractional motion >3 mm in the SI and AP directions, respectively, compared with none experiencing mean shifts >3 mm in the LR direction. Six patients experienced grade 3 gastrointestinal or genitourinary toxicity. There were no significant differences in mean inter-or intra-fractional motion in any of the cardinal directions compared to patients without severe toxicity (inter-fractional p = 0.46-0.99, intra-fractional p = 0.10-0.84). Levin-Epstein et al. Prostate Displacement and SBRT Margins Conclusion: The inter-and intra-fractional margins estimated from this study are in line with prior reported values. Intra-fractional prostate motion was generally small with larger margins required for the SI and AP directions, notably just slightly exceeding the commonly used 3 mm posterior PTV margin even with realignment between half-arcs. Development of severe toxicity was not significantly associated with the degree of inter-or intra-fractional motion.
“…The use of hydrogel spacers could serve to move the rectal wall out of the high‐dose region, meaning that the relative dose changes we observe around unplanned gas cavities would result in lower absolute dose increases. Furthermore, using an air‐ or a liquid‐filled EndoRectal Balloon (ERB) to create a planned cavity could potentially avoid large unplanned air cavities forming . Another option could be to employ a new gas‐release balloon, originally designed for proton treatment, which has been seen to reduce the frequency of trapped air …”
Section: Discussionmentioning
confidence: 99%
“…Furthermore, using an air-or a liquid-filled EndoRectal Balloon (ERB) to create a planned cavity could potentially avoid large unplanned air cavities forming. 16,[41][42][43] Another option could be to employ a new gas-release balloon, originally designed for proton treatment, which has been seen to reduce the frequency of trapped air. 44…”
Section: A Implications Of Unplanned Gas In the Rectal Wallmentioning
Purpose
It has been proposed that beam modulation and opposing beam configurations can cancel effects of the Electron Return Effect (ERE) during MR‐guided radiotherapy (MRgRT). However, this may not always be the case for unplanned gas cavities outside of the target in the pelvic region. We evaluate dosimetric effects, including effects in the rectal wall, due to unplanned spherical air cavities during MRgRT.
Methods
Nine virtual cuboid water phantoms containing spherical air cavities (0.5–7.5 cm diameter) and a reference phantom without air were created. Monte Carlo dose calculations of 7 MV photons under the influence of a 1.5 T transverse magnetic field were produced using Monaco 5.19.02 Treatment Planning System (TPS) (Elekta AB, Stockholm, Sweden). Cavities in the path of a single and multiple beam plans were considered. Dose distributions of phantoms with and without air cavities were compared (ΔD%) using a spherical coordinate system originating in the center of the cavity. Effects in the rectal wall were quantified by comparing dose volume histogram (DVH) parameters for solid and gaseous filling from simulated rectal wall structures.
Results
Max(ΔD%) of ~70% and 20% were observed around large cavities in the path of a single and multiple beam plans, respectively. Approximately 45 cm3 of phantom surrounding the largest cavity in a single beam received dose changes of >10%. Dmean in the rectal wall was unchanged when comparing gaseous and solid filling in the path of a single beam; however, D1cc and Dmax increased by up to ~45% and ~63%, respectively.
Conclusions
Unplanned gas cavities in the path of a single beam during pelvic MRgRT with a 1.5 T transverse magnetic field cause dose changes which may impact toxicity in the rectal wall, depending on local dose and fractionation. Effects are reduced but not eliminated with a five‐beam plan.
“…1 using a threshold limit for Calypso ® of ±3 mm with geometric check limits set to 2 mm (geometrical residual) and rotations of 10° (default values). Table 1 summarizes the dose constraints for the OAR [ 24 ]. Fig.…”
BackgroundStereotactic body radiotherapy (SBRT) is an emerging treatment alternative for patients with localized prostate cancer. Promising results in terms of disease control and toxicity have been reported with 4 to 5 SBRT fractions. However, question of how far can the number of fractions with SBRT be reduced is a challenging research matter. As already explored by some authors in the context of brachytherapy, monotherapy appears to be feasible with an acceptable toxicity profile and a promising outcome. The aim of this multicenter phase I/II prospective trialis to demonstrate early evidence of safety and efficacy of a single-fraction SBRT approach for the treatment of localized disease.MethodsPatients with low- and intermediate-risk localized prostate cancer without significant tumor in the transitional zone will be treated with a single SBRT fraction of 19 Gy to the whole prostate gland with urethra-sparing (17 Gy). Intrafractional motion will be monitored with intraprostatic electromagnetic transponders. The primary endpoint of the phase I part of the study will be safety as assessed by CTCAE 4.03 grading scale, while biochemical relapse-free survival will be the endpoint for the phase II. The secondary endpoints include acute and late toxicity, quality of life, progression-free survival, and prostate-cancer specific survival.DiscussionThis is the first multicenter phase I/II trial assessing the efficacy and safety of a single-dose SBRT treatment for patients with localized prostate cancer. If positive, results of ONE SHOT may help to design subsequent phase III trials exploring the role of SBRT monotherapy in the exclusive radiotherapy treatment of localized disease.Trial registrationClinicaltrials.gov identifier: NCT03294889; Registered 27 September 2017.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
