3223 - Rectal Spacer Impact on MRI-Directed SBRT Prostate Microboost in Concert with Pelvic Nodal Irradiation

J. W. Lischalk¹, V. Santos¹, M. Akerman², A. Sanchez³, C. Mendez⁴, T. J. Carpenter¹, S. Niglio⁵, D. R. Wise⁵, T. B. Daniels⁶, H. Marans⁷, M. OKeeffe⁸, S. Taneja⁹, W. Huang⁹, A. Corcoran¹⁰, A. Katz¹¹, M. J. Zelefsky¹², and J. A. Haas¹; ¹Department of Radiation Oncology, Perlmutter Cancer Center at New York University Langone Hospital - Long Island, Mineola, NY, ²Biostatistics Unit, Office of Academic Affairs, Northwell Health, Lake Success, NY, ³NYU Langone Health - Long Island, Mineola, NY, United States, ⁴Department of Radiation Oncology, Perlmutter Cancer Center, NYU Long Island, Mineola, NY, ⁵Department of Medicine, Perlmutter Cancer Center at NYU Langone Medical Center, New York, NY, ⁶NYU Langone Health, New York, NY, ⁷NYU Langone, NY, NY, ⁸Department of Medicine, Perlmutter Cancer Center at NYU Langone Medical Center, Mineola, NY, ⁹Department of Urology, Perlmutter Cancer Center at New York University Grossman School of Medicine, New York, NY, ¹⁰Department of Urology, New York University Langone Hospital - Long Island, Mineola, NY, ¹¹Department of Urology, Perlmutter Cancer Center at New York University Langone Hospital - Long Island, Mineola, NY, ¹²Department of Radiation Oncology, Perlmutter Cancer Center at New York University Langone Hospital, New York, NY

Purpose/Objective(s): The use of intraprostatic MRI based microboost has level one evidence supporting its oncologic efficacy based on the results of the FLAME trial. A number of ongoing prospective trials explore a similar strategy utilizing prostate SBRT. Artificial rectal spacers have been demonstrated to yield dosimetrically superior plans and reduce the risk of gastrointestinal toxicity during prostate radiotherapy. Little is known regarding the impact of rectal spacers on MRI directed SBRT prostate microboosts. Herein, we explore the impact of rectal spacers on SBRT prostate microboost used in concert with pelvic nodal irradiation for aggressive prostate cancer. Materials/

Methods: An institutional registry was interrogated from April 2021 – March 2023 to identify patients treated with 3-fraction SBRT boost to a total dose of 1950 – 2100 cGy in concert with conventionally fractionated pelvic nodal irradiation. Rectal spacers were placed in patients without evidence of clinical or radiological posterior extracapsular extension. MRI-directed microboost was delivered to a prescription dose of 2100 – 2400 cGy to a visible PI-RADS 3+ lesion(s). Patients who did and did not receive spacers were compared using the chi-square test or Fisher’s exact test for categorical variables, and the Mann-Whitney test for continuous variables. A multivariable linear regression model was performed for rectal mean dose, adjusting for possible confounders. A result was significant at p<0.05.
Results: A total of 40 patients with a median age of 73 years underwent 3-fraction prostate SBRT boost with the distribution of risk grouping as follows: intermediate - 13% (n = 5), high - 70% (n = 28), and regional - 18% (n = 7). Rectal spacer placement was evenly distributed within the cohort with 53% (n = 21) of patients undergoing spacer placement. Unsurprisingly, ECE was significantly more common in the no spacer cohort (59% vs. 14%, p = 0.004). Delivery of a microboost was performed in nearly half (43%) of the overall cohort (n = 17), most commonly to a total dose of 2300 cGy (71%), and was evenly distributed between those who did and did not have a rectal spacer (p = 0.96). In those who underwent a microboost with spacer present, median microboost mean dose was higher (2408 vs. 2350 cGy, p = 0.036) and median rectal mean dose was lower (912 cs. 966, p = 0.015) than the no spacer comparative cohort. On multivariable analysis, presence of rectal spacer significantly reduced rectal mean dose when controlling for prostate volume and delivery of a microboost (902 vs. 976 cGy, p = 0.017).
Conclusion: Delivery of a 3-fraction prostate SBRT with an MRI-directed microboost in concert with pelvic nodal irradiation is feasible in eligible patients without posterior ECE, and results in elevated dose to the intraprostatic lesion and simultaneously lower rectal mean doses. Rectal spacers allow dose escalation to intraprostatic lesions sans excess rectal dose.