Urethra-sparing stereotactic body radiotherapy for prostate cancer: how much can the rectal wall dose be reduced with or without an endorectal balloon?

Predictors of radiation-induced late rectal toxicity in prostate cancer treatment: a volumetric and dosimetric analysis

Introduction

Prostate cancer (PCa) is a prevalent malignancy in European men, often treated with radiotherapy (RT) for localized disease. While modern RT achieves high success rates, concerns about late gastrointestinal (GI) toxicities persist. This retrospective study aims to identify predictors for late GI toxicities following definitive conventionally fractionated external beam RT (EBRT) for PCa, specifically exploring the dose to the rectal wall.

Materials and methods

A cohort of 96 intermediate- to high-risk PCa patients underwent EBRT between 2008 and 2016. Rectum and rectum wall contours were delineated, and 3D dose matrices were extracted. Volumetric and dosimetric indices were computed, and statistical analyses were performed to identify predictors using the Mann-Whitney U-rank test, logistic regression, and recursive feature elimination.

Results

In our cohort, 15 out of 96 patients experienced grade II late proctitis. Our analysis reveals distinct optimal predictors for rectum and rectum wall (RW) structures varying with α/β values (3.0 and 2.3 Gy) across prescribed doses of 68 to 76 Gy. Despite variability, RW predictors demonstrate greater consistency, notably V68Gy[%] to V74Gy[%] for α/β 3.0 Gy, and V68Gy[%] to V70Gy[%] for α/β 2.3 Gy. The model with α/β 2.3 Gy, featuring RW volume receiving 70 Gy (V70Gy[%]), stands out with a BIC value of 62.92, indicating its superior predictive effectiveness. Finally, focusing solely on the rectum structure, the V74Gy[%] emerges the best predictor for α/β 3.0 Gy, with a BIC value of 66.73.

Conclusion

This investigation highlights the critical role of V70Gy[%] in the rectum wall as a robust predictor for grade II late gastrointestinal (GI) toxicity following external beam radiation therapy (EBRT) for prostate cancer (PCa). Furthermore, our findings suggest that focusing on the rectum wall specifically, rather than the entire rectum, may offer improved accuracy in assessing proctitis development. A V70Gy (in EQD2 with α/β 2.3 Gy) of ≤5% and if possible ≤1% for the rectal wall should be achieved to minimize the risk of late grade II proctitis.

Every-Other-Day Versus Once-a-Week Urethra-Sparing Prostate Stereotactic Body Radiation Therapy: 5-Year Results of a Randomized Phase 2 Trial

PURPOSE

The objective of this study was to present the 5-year results from a prospective, multicenter, phase 2 randomized trial of every-other-day (EOD) versus once-a-week (QW) urethra-sparing stereotactic body radiation therapy for localized prostate cancer.

METHODS AND MATERIALS

Between 2012 and 2015, 170 patients with cT1c-3aN0M0 prostate cancer from 9 European institutions were randomized to 36.25 Gy in 5 fractions (6.5 Gy/fraction to the urethra) delivered either EOD (arm A, n = 84) or QW (arm B, n = 86). The median follow-up was 78 months (interquartile range, 66-89 months) and 77 months (interquartile range, 66-82 months) for arms A and B, respectively.

RESULTS

Among the 165 patients treated and retained for the final analysis (arm A, n = 82; arm B, n = 83), acute toxicity (National Cancer Institute Common Terminology Criteria for Adverse Events version 4.03 scale) was mild or absent, with no differences between arms. The 5-year grade 2 or greater genitourinary toxicity-free survival was 75.9% and 76.1% for arms A and B, respectively (P = .945), whereas the 5-year grade 2 or greater gastrointestinal toxicity-free survival was 89% and 92% for arms A and B, respectively (P = .596). No changes in European Organisation for Research and Treatment of Cancer QLQ-PR25 scores were observed in both arms for genitourinary, gastrointestinal, and sexual domains at 5-year follow-up compared with baseline. At the last follow-up, biochemical failure was observed in 14 patients in the EOD arm and in 7 patients in the QW arm, with a 5-year biochemical relapse-free survival rate of 92.2% and 93% for arms A and B, respectively (P = .13).

CONCLUSIONS

Stereotactic body radiation therapy for prostate cancer with a 10% dose reduction to urethra was associated with a minimal effect on urinary function and quality of life regardless of an EOD or QW fractionation schedule. Biochemical control so far has been encouraging and much alike in both study arms, although longer follow-up is probably needed to assess the true value of overall treatment time on disease outcome.

Surgical Treatments of Benign Prostatic Hyperplasia and Prostate Cancer Stereotactic Radiotherapy: Impact on Long-Term Genitourinary Toxicity

AIMS

Although the results on acute and late toxicity of ultrahypofractionation are encouraging, data on safety in prostate cancer patients with a medical history of transurethral resection of the prostate (TURP) or adenomectomy remain scarce, especially in cases of repeated procedures. The aim of the present study was to report on long-term toxicities after stereotactic body radiotherapy (SBRT) of prostate cancer patients with previous surgical treatment of benign prostatic hyperplasia.

MATERIALS AND METHODS

Among 150 patients treated with SBRT (median dose 36.25 Gy in five fractions) realised from 2014 to 2019 in a single-centre institution, data of 24 men with a history of TURP (n = 19) or adenomectomy (n = 5) were analysed. Repeated TURP was carried out in three patients, with a median time between surgery and SBRT of 54 months. Genitourinary toxicity was assessed using the Common Terminology Criteria for Adverse Events v4.0 grading scale.

RESULTS

With a median follow-up of 45 months, 10 of 24 (42%) patients experienced at least one episode of transient haematuria. One patient (4%) with three previous TURP presented a grade 3 acute non-infective cystitis. Late grade 2 and 3 genitourinary toxicities were observed in eight (33%) and four patients (17%) (two treated with adenomectomy, one with multiple TURP and one with a 140 cm³ prostate size), respectively, with no grade ≥4 adverse events. A complete recovery of grade 3 genitourinary toxicities was observed for all patients after hyperbaric oxygen therapy.

CONCLUSION

Prostate SBRT is feasible and well-tolerated in patients with a medical history of surgical treatments of benign hyperplasia. Patients with prior adenomectomy or multiple TURP are at higher risk of developing severe genitourinary toxicity and should be carefully evaluated before SBRT treatments.

Dataset for predicting single-spot proton ranges in proton therapy of prostate cancer

The number of radiotherapy patients treated with protons has increased from less than 60,000 in 2007 to more than 220,000 in 2019. However, the considerable uncertainty in the positioning of the Bragg peak deeper in the patient raised new challenges in the proton therapy of prostate cancer (PCPT). Here, we describe and share a dataset where 43 single-spot anterior beams with defined proton energies were delivered to a prostate phantom with an inserted endorectal balloon (ERB) filled either with water only or with a silicon-water mixture. The nuclear reactions between the protons and the silicon yield a distinct prompt gamma energy line of 1.78 MeV. Such energy peak could be identified by means of prompt gamma spectroscopy (PGS) for the protons hitting the ERB with a three-sigma threshold. The application of a background-suppression technique showed an increased rejection capability for protons hitting the prostate and the ERB with water only. We describe each dataset, document the full processing chain, and provide the scripts for the statistical analysis.

Extreme Hypofractionation with SBRT in Localized Prostate Cancer

Prostate cancer is the most commonly diagnosed cancer among men around the world. Radiotherapy is a standard of care treatment option for men with localized prostate cancer. Over the years, radiation delivery modalities have contributed to increased precision of treatment, employing radiobiological insights to shorten the overall treatment time, improving the control of the disease without increasing toxicities. Stereotactic body radiation therapy (SBRT) represents an extreme form of hypofractionated radiotherapy in which treatment is usually delivered in 1-5 fractions. This review assesses the main efficacy and toxicity data of SBRT in non-metastatic prostate cancer and discusses the potential to implement this scheme in routine clinical practice.

Towards real-time PGS range monitoring in proton therapy of prostate cancer

Proton therapy of prostate cancer (PCPT) was linked with increased levels of gastrointestinal toxicity in its early use compared to intensity-modulated radiation therapy (IMRT). The higher radiation dose to the rectum by proton beams is mainly due to anatomical variations. Here, we demonstrate an approach to monitor rectal radiation exposure in PCPT based on prompt gamma spectroscopy (PGS). Endorectal balloons (ERBs) are used to stabilize prostate movement during radiotherapy. These ERBs are usually filled with water. However, other water solutions containing elements with higher atomic numbers, such as silicon, may enable the use of PGS to monitor the radiation exposure of the rectum. Protons hitting silicon atoms emit prompt gamma rays with a specific energy of 1.78 MeV, which can be used to monitor whether the ERB is being hit. In a binary approach, we search the silicon energy peaks for every irradiated prostate region. We demonstrate this technique for both single-spot irradiation and real treatment plans. Real-time feedback based on the ERB being hit column-wise is feasible and would allow clinicians to decide whether to adapt or continue treatment. This technique may be extended to other cancer types and organs at risk, such as the oesophagus.

Reproducibility and accuracy of a target motion mitigation technique for dose-escalated prostate stereotactic body radiotherapy

BACKGROUND AND PURPOSE

To quantitate the accuracy, reproducibility and prostate motion mitigation efficacy rendered by a target immobilization method used in an intermediate-risk prostate cancer dose-escalated 5×9Gy SBRT study.

MATERIAL AND METHODS

An air-inflated (150 cm³) endorectal balloon and Foley catheter with three electromagnetic beacon transponders (EBT) were used to mitigate and track intra-fractional target motion. A 2 mm margin was used for PTV expansion, reduced to 0 mm at the interface with critical OARs. EBT-detected ≥ 2 mm/5 s motions mandated treatment interruption and target realignment prior to completion of planned dose delivery. Geometrical uncertainties were measured with an in-house ESAPI script.

RESULTS

Quantitative data were obtained in 886 sessions from 189 patients. Mean PTV dose was 45.8 ± 0.4 Gy (D95 = 40.5 ± 0.4 Gy). A mean of 3.7 ± 1.7 CBCTs were acquired to reach reference position. Mean treatment time was 19.5 ± 12 min, 14.1 ± 11 and 5.4 ± 5.9 min for preparation and treatment delivery, respectively. Target motion of 0, 1-2 and >2 mm/10 min were observed in 59%, 30% and 11% of sessions, respectively. Temporary beam-on hold occurred in 7.4% of sessions, while in 6% a new reference CBCT was required to correct deviations. Hence, all sessions were completed with application of the planned dose. Treatment preparation time > 15 min was significantly associated with the need of a second reference CBCT. Overall systematic and random geometrical errors were in the order of 1 mm.

CONCLUSION

The prostate immobilization technique explored here affords excellent accuracy and reproducibility, enabling normal tissue dose sculpting with tight PTV margins.

Visualizing the urethra by magnetic resonance imaging without usage of a catheter for radiotherapy of prostate cancer

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Post urination MRI is useful for urethra-sparing radiotherapy treatment planning.

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This prospective clinical trial included 11 prostate cancer patients.

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Post urination MRI is the identification method of prostatic urinary tract in non-invasive manner.

The urethra position may shift due to the presence/absence of the catheter. Our proposed post-urination-magnetic resonance imaging (PU-MRI) technique is possible to identify the urethra without catheter. We aimed to verify the inter-operator difference in contouring the urethra by PU-MRI. The mean values of the evaluation indices of dice similarity coefficient, mean slice-wise Hausdorff distance, and center coordinates were 0.93, 0.17 mm, and 0.36 mm for computed tomography, and 0.75, 0.44 mm, and 1.00 mm for PU-MRI. Therefore, PU-MRI might be useful for identifying the prostatic urinary tract without using a urethral catheter.

Clinical trial registration: Hokkaido University Hospital for Clinical Research (018-0221).

Urethra-Sparing Stereotactic Body Radiation Therapy for Prostate Cancer: Quality Assurance of a Randomized Phase 2 Trial

PURPOSE

To present the radiation therapy quality assurance results from a prospective multicenter phase 2 randomized trial of short versus protracted urethra-sparing stereotactic body radiation therapy (SBRT) for localized prostate cancer.

METHODS AND MATERIALS

Between 2012 and 2015, 165 patients with prostate cancer from 9 centers were randomized and treated with SBRT delivered either every other day (arm A, n = 82) or once a week (arm B, n = 83); 36.25 Gy in 5 fractions were prescribed to the prostate with (n = 92) or without (n = 73) inclusion of the seminal vesicles (SV), and the urethra planning-risk volume received 32.5 Gy. Patients were treated either with volumetric modulated arc therapy (VMAT; n = 112) or with intensity modulated radiation therapy (IMRT; n = 53). Deviations from protocol dose constraints, planning target volume (PTV) homogeneity index, PTV Dice similarity coefficient, and number of monitor units for each treatment plan were retrospectively analyzed. Dosimetric results of VMAT versus IMRT and treatment plans with versus without inclusion of SV were compared.

RESULTS

At least 1 major protocol deviation occurred in 51 patients (31%), whereas none was observed in 41. Protocol violations were more frequent in the IMRT group (P < .001). Furthermore, the use of VMAT yielded better dosimetric results than IMRT for urethra planning-risk volume D_98% (31.1 vs 30.8 Gy, P < .0001), PTV D_2% (37.9 vs 38.7 Gy, P < .0001), homogeneity index (0.09 vs 0.10, P < .0001), Dice similarity coefficient (0.83 vs 0.80, P < .0001), and bladder wall V_50% (24.5% vs 33.5%, P = .0001). To achieve its goals volumetric modulated arc therapy required fewer monitor units than IMRT (2275 vs 3378, P <.0001). The inclusion of SV in the PTV negatively affected the rectal wall V_90% (9.1% vs 10.4%, P = .0003) and V_80% (13.2% vs 15.7%, P = .0003).

CONCLUSIONS

Protocol deviations with potential impact on tumor control or toxicity occurred in 31% of patients in this prospective clinical trial. Protocol deviations were more frequent with IMRT. Prospective radiation therapy quality assurance protocols should be strongly recommended for SBRT trials to minimize potential protocol deviations.

Clinical Assessment of Prostate Displacement and Planning Target Volume Margins for Stereotactic Body Radiotherapy of Prostate Cancer

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). 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.

Electromagnetic Transponder Localization and Real-Time Tracking for Prostate Cancer Radiation Therapy: Clinical Impact of Metallic Hip Prostheses

PURPOSE

Our purpose was to assess the ability of electromagnetic transponders (EMTs) to localize and track movements in patients with prostate cancer (PCa) with metallic hip prostheses (MHPs) treated with curative radiation therapy (RT).

METHODS AND MATERIALS

Data sets of 8 PCa patients with MHPs (3 bilateral and 5 unilateral) treated between 2016 and 2018 with RT and EMT tracking were retrospectively assessed. The distances between the 3 EMTs (apex to left, left to right, right to apex) and the isocenter were calculated both on planning computed tomography (CT) and cone beam CT (CBCT) at the first treatment fraction and compared with data reported by Calypso. EMT position and treatment interruptions triggered by Calypso were analyzed for all evaluable treatment fractions (n = 120). Localization accuracy was quantified by recording the geometric residual value (expected limit ≤0.2 cm) at the RT setup.

RESULTS

The Calypso system was able to localize and track prostate position without any detectable interference from MHP. For every treatment fraction, the agreement between the CBCT images and Calypso guidance was optimal, with EMTs always within the defined tolerance (ie, CT-Calypso or CBCT-Calypso measured differences in inter-EMT distances within 0.3 cm). EMT to isocenter distances measured by Calypso reproduced CT data and were confirmed on CBCT scans. During RT, the EMT centroid exceeded the threshold 24 times (20% of all fractions): 5 times in the left-right, 15 times in the anteroposterior, and 4 times in the superoinferior directions. The largest motions recorded were in the anteroposterior axis: 0.6 cm anteriorly and 0.5 cm posteriorly in patients with unilateral and bilateral MHP, respectively.

CONCLUSIONS

Our study represents the first clinical experience assessing the localization and tracking accuracy of Calypso EMTs during curative RT of patients with PCa with unilateral or bilateral MHP.

Once-a-week or every-other-day urethra-sparing prostate cancer stereotactic body radiotherapy, a randomized phase II trial: 18 months follow-up results

Background

To present the 18 months results from a prospective multicenter phase II randomized trial of short vs protracted urethra‐sparing stereotactic body radiotherapy (SBRT) for localized prostate cancer (PCa).

Methods

Between 2012 and 2015, a total of 170 PCa patients were randomized to 36.25 Gy in 5 fractions (6.5 Gy × 5 to the urethra) delivered either every other day (EOD, arm A, n = 84) or once a week (QW, arm B, n = 86). Genitourinary (GU) and gastrointestinal (GI) toxicity (CTCAE v4.0 scale), IPSS, and QoL scores were assessed at baseline, at the 5th fraction (5fx), 12th weeks (12W), and every 6 months after SBRT. The primary endpoint was biochemical control at 18 months and grade ≥ 3 toxicity (including grade ≥ 2 for urinary obstruction/retention) during the first 3 months.

Results

Among the 165 patients analyzed, the toxicity stopping rule was never activated during the acute phase. Maximum acute grade 2 GU toxicity rates at 5fx were 17% and 19% for arms A and B, respectively, with only 2 cases of grade 2 GI toxicity at 5fx in arm A. At month 18, grade ≥ 2 GU and GI toxicity decreased below 5% and 2% for both arms. No changes in EORTC QLQ‐PR25 scores for GU, GI, and sexual domains were observed in both arms between baseline and month 18. Four biochemical failures were observed, 2 in each arm, rejecting the null hypothesis of an unfavorable response rate ≤ 85% in favor of an acceptable ≥ 95% rate.

Conclusions

At 18 months, urethra‐sparing SBRT showed a low toxicity profile, with minimal impact on QoL and favorable biochemical control rates, regardless of overall treatment time (EOD vs QW).

Assessing localized dosimetric effects due to unplanned gas cavities during pelvic MR-guided radiotherapy using Monte Carlo simulations

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.

ONE SHOT - single shot radiotherapy for localized prostate cancer: study protocol of a single arm, multicenter phase I/II trial

Background

Stereotactic 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.

Methods

Patients 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.

Discussion

This 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 registration

Clinicaltrials.gov identifier: NCT03294889; Registered 27 September 2017.