2225 - Urethra and Bladder Dosimetry in Patients Treated with Prostate SBRT with and without Intra-Prostatic Boost (IPB)

Sunday, September 29, 2024

4:45 PM – 6:00 PM ET

Location: Hall C

Screen: 32

Presenter(s)

NB

Nisha Bhargava, BS

Beth Israel Deaconess Medical Center
Boston, MA

N. Bhargava, M. Hurwitz, L. Bennett, J. A. Aronovitz, D. R. Schmidt, I. D. Kaplan, and N. Aghdam; Department of Radiation Oncology, Beth Israel Deaconess Medical Center, Boston, MA

Purpose/Objective(s): SBRT is a standard of care treatment for localized prostate cancer. Clinical trials have shown excellent biochemical control in patients who are treated with conventional radiation with a concomitant IPB. Dose escalation to the whole gland may lead to unacceptable toxicity, however the role of IPB to a dominant nodule using SBRT is yet to be determined. In this report, we describe urethral and bladder dosimetric parameters for patients receiving SBRT with IPB and compare them to a cohort of patients treated without IPB. Materials/

Methods: From October 2018 to June 2023, 443 patients were treated with a robotic SBRT platform to the whole gland with fiducial tracking with IPB (cohort A). Nodules were delineated using diagnostic and treatment planning MRI. Patients received the standard dose of 36.25Gy to the prostate and proximal seminal vesicles with isotropic expansion margin of 5mm, except posteriorly towards the rectum (PTV) and 40 Gy to GTV. Urethra was delineated using contrast-enhanced suprapubic urethrogram. Institutional dose constraints were utilized for evaluation of treatment plans. For the comparison cohort (cohort B), 97 patients treated with the same parameters without IPB, from February 2021 to August 2022, were used. Dosimetric data was retrospectively extracted from the treatment planning software and compared between the two cohorts. Using a programming environment, Wilcoxon rank-sum test was used to test the hypothesis of an increase in median value from cohort B to cohort A.
Results: In cohort A, all patients had at least one nodule, while 70 had two, and 13 had three or more, for a total 540 nodules with a median volume of 0.73cc (0.05cc-10.1cc) and median D90% of 42.8 Gy. Prostate volume medians and ranges for cohorts A and B were 32.9cc (12.0cc-142.2cc) and 46.4cc (18.7cc - 122.8cc), respectively. Median values of dosimetric parameters are summarized in the accompanying table. No statistically significant increase in median doses from B to A were observed. When comparing nodule size and urethral dose for cohort A, no correlation was found between intra-prostatic nodule size and urethral D0.03cc (R-value = 0.08).

Conclusion: SBRT is a safe and effective standard of care for localized prostate cancer, however questions remain regarding the role of dose escalation with extreme hypofractionation. Here we report on a large cohort of patients treated with IPB with strict adherence to institutional dose constraints. When comparing key dosimetric parameters, receiving IPB did not lead to higher doses to the urethra and bladder. Long-term follow-up and randomized protocols are needed to confirm the safety and efficacy of this approach. Abstract 2225 – Table 1

A B p-value

D0.03cc Urethra 41.7 Gy 41.9 Gy 0.87

D0.3cc Urethra 40.9 Gy 41.3 Gy 1.0

V40 Gy Urethra 0.81cc 2.5cc 1.0

D0.03cc Bladder 41.2 Gy 41.3 Gy 0.89

D5cc Bladder 34.3 Gy 36.1 Gy 1.0

D10cc Bladder 29.8 Gy 32.5 Gy 1.0

V37 Gy Bladder 2.4cc 3.9cc 1.0