3276 - MRI PI-RADS Lesion Location in Transitional vs. Peripheral Zone and Impact on Acute GU Toxicity Following Prostate SBRT Microboost
Presenter(s)
J. W. Lischalk1,2, V. Santos2, B. Vizcaino3, C. Mendez4, A. Sanchez4, N. Aghdam5, T. J. Carpenter4, A. Corcoran6, W. Huang7, S. Niglio8,9, H. Lepor7, A. Katz6, J. E. Leeman10, and J. Haas4; 1Department of Radiation Medicine, Georgetown University Hospital, Washington, DC, 2Department of Radiation Oncology, Perlmutter Cancer Center at New York University Langone Hospital - Long Island, New York, NY, 3Department of Radiation Oncology, Perlmutter Cancer Center at New York University Grossman School of Medicine,, New York, NY, 4Department of Radiation Oncology, Perlmutter Cancer Center at New York University Langone Hospital - Long Island, Mineola, NY, 5Department of Radiation Oncology, Beth Israel Deaconess Medical Center, Boston, MA, 6Department of Urology, Perlmutter Cancer Center at New York University Langone Hospital - Long Island, Mineola, NY, 7Department of Urology, Perlmutter Cancer Center at New York University Grossman School of Medicine, New York, NY, 8Department of Medicine, Perlmutter Cancer Center at NYU Langone Medical Center, New York, NY, 9Memorial Sloan Kettering Cancer Center, New York, NY, 10Department of Radiation Oncology, Dana-Farber Cancer Institute/Brigham and Women’s Hospital, Harvard Medical School, Boston, MA
Purpose/Objective(s):
The FLAME trial provides evidence for the oncologic efficacy of intraprostatic MRI-based microboost. Several ongoing prospective trials are investigating a similar approach with 5 fraction SBRT. Understanding the relationship between the lesion location and acute toxicity may enhance our understanding of therapeutic outcomes. This study aims to investigate the impact of the anatomical location of MRI-identified PI-RADS lesions on acute toxicity following SBRT prostate microboost.Materials/Methods:
An institutional registry was reviewed to identify patients who completed 5-fraction robotic prostate SBRT from Feb 2021 to May 2024, receiving a prescription dose of 3500-3625 cGy. Patients with pelvic nodal involvement or missing MRI data were excluded. The SIB dose ranged from 4000 to 4500 cGy and prostatic urethra dose constraint was max < 4200 cGy. Patients were categorized by peripheral zone (PZ) or transitional zone (TZ) tumors, with those having multiple lesions classified by the highest PI-RADS lesion. Toxicity was assessed using CTCAE v5.0. Statistical analyses, including chi-square tests, evaluated the correlation between lesion location and acute toxicity outcomes.Results:
A total of 134 patients underwent prostate SBRT with a risk group distribution of: 8% low, 81% intermediate, and 10% high. The majority (76%) had PZ tumors. Almost 50% of patients (n=66) received a microboost. There was no significant difference in categorization of PI-RADS lesions between PZ and TZ tumors (p = 0.21). Uni- and multifocal PI-RADS lesions were evenly distributed between PZ and TZ (p = 0.42). Overall, lesions localized to the TZ had higher prostatic urethra maximum doses (p = 0.005), though below dose constraints, which remained true when isolating for those who had a microboost (p = 0.02). Delivery of a microboost was not different between PZ and TZ lesions (p = 0.60). With a median follow up of 14 months, overall GU toxicity was not different between PZ and TZ tumors (p = 0.06). However, tumors displayed slightly higher low grade side effects when a microboost was delivered (p = 0.01) and no difference in the absence of a microboost (p = 0.92).Conclusion:
There was no significant difference in PI-RADS categorization or lesion multifocality between PZ and TZ tumors. Although all standard dose constraints were achieved, prostatic urethra maximum doses were higher in the TZ cohort regardless of microboost delivery. With early follow up, there were slightly more GU side effects in patients with underlying TZ tumors who received a microboost, specifically in the grade 2 domain. Future prospective research should explore intraprostatic lesional location impact on longer term GU function.