ESTRO 2024 - Abstract Book

S3719

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2024

2925

Digital Poster

Urethral dosimetric implications of a multifocal MRI-based microboost in prostate robotic SBRT

Jonathan W. Lischalk 1 , Vianca F. Santos 1 , Meredith Akerman 2 , Christopher Mendez 1 , Astrid Sanchez 1 , James S. Wysock 3 , William C Huang 3 , Moses M. Tam 1 , Anthony Corcoran 4 , Todd J. Carpenter 1 , Matthew R. Witten 1 , Andrew J. Evans 5 , Aaron Katz 4 , Anand Mahadevan 5 , Michael J. Zelefsky 5 , Jonathan A. Haas 1 1 Perlmutter Cancer Center at New York University Langon Hospital - Long Island, Department of Radiation Oncology, New York, USA. 2 NYU Long Island School of Medicine, Division of Health Services Research, Mineola, USA. 3 Perlmutter Cancer Center at New York University School of Medicine, Department of Urology, New York, USA. 4 Perlmutter Cancer Center at New York University Langon Hospital - Long Island, Department of Urology, New York, USA. 5 Perlmutter Cancer Center and New York University Grossman School of Medicine, Department of Radiation Oncology, New York, USA

Purpose/Objective:

As curative radiotherapy for localized prostate cancer has trended towards ultra-hypofractionation, there is renewed interest in previously overlooked organs at risk, specifically the urethra. Recent literature has demonstrated SBRT radiation dose to the prostatic urethra is associated with subsequent genitourinary (GU) toxicity. Apropos, the publication of the FLAME trial has supported the use of MRI-directed focal intra-prostatic lesion ablative microboosts (MIB), which may result in additional dose to the urethra. Here we explore the use of unifocal and multifocal robotic SBRT MIB and their urethral dosimetric implications.

Material/Methods:

An institutional registry of patients undergoing five-fraction robotic prostate SBRT was interrogated to identify those who underwent an SBRT MIB from Feb. 2021 – May 2023. All patients were treated to a total dose of 3500 - 3625 cGy to the prostate and proximal seminal vesicles with or without an MIB. Patients were categorized into those without MIB, with a unifocal MIB, or with a multifocal MIB. The MIB prescription dose ranged from 4000 to 4300 cGy. The urethra was contoured by a single radiation oncologist and separated into two portions: (1) prostatic and (2) membranous. Specific dosimetric parameters were utilized for the prostatic urethra and included: V35 Gy > 95% and V40 Gy < 0.03 cc. A single robotic planning platform was utilized for evaluation of maximum and mean dosimetric parameters. Continuous data was compared using the Mann-Whitney test for two group and the Kruskal-Wallis test for three group comparisons. A result was clinically significant at p < 0.05.

Results:

A total of 119 patients underwent prostate SBRT with the distribution of risk grouping as follows: low - 11.76% (n = 14), intermediate - 81.51% (n = 97), and high - 6.72% (n = 8). The MIB distribution was as follows: none - 72.27% (n = 86), unifocal - 15.97% (n = 19), and multifocal - 11.76% (n = 14). The MIB volume for those with unifocal vs. multifocal boost was nominally lower but not significantly different at 1.11 vs. 2.15 cc (p = 0.196), respectively. The mean MIB dose for unifocal vs. multifocal boost was 4296 vs. 4291 cGy (p = 0.984) and prescription dose coverage was 100% for both groups. Unsurprisingly, similar intraprostatic urethral maximum doses was observed at 3986, 4020, and 3991 cGy (p = 0.55) in the none, unifocal, and multifocal groups, respectively. Interestingly, there was a significantly lower intraprostatic urethral mean dose for those who underwent focal boost vs. no boost (3820 vs.