ESTRO 2024 - Abstract Book

S2380

Clinical - Urology

ESTRO 2024

Material/Methods:

Patients with intermediate or high-risk prostate adenocarcinoma, with an identifiable DIL in mpMRI and Ga68-PSMA PETCT, were enrolled in this prospective IRB approved trial (IEC 900917; CTRI/2022/09/045258). Key exclusion criteria were previous androgen deprivation therapy (ADT) and ineligibility for SBRT (urethral stricture, IPSS>20, prostate>80 cc, TURP within 3 months). Ga68-planning PSMA-PETCT was done in radiotherapy treatment position with urinary catheter in-situ, and images co-registered with planning mpMRI sequences (T2 weighted, diffusion weighted and dynamic contrast-enhanced). DIL was delineated on mpMRI (DIL-MRI) with a radiologist blinded to the PSMA-PET findings. DIL volumes corresponding to multiple threshold percentages of the maximum standardized uptake value (%SUVmax) on PSMA-PET were autogenerated at 2% increments from 20% to 90% and were labelled as DILx%. Volumetric and spatial concordance between DIL-MRI and each DILx% was compared using absolute volumes, Dice Similarity Coefficient (DSC) and Jaccard Index (JI). The DILx% corresponding best with DIL-MRI was labelled DIL-PET. Union and overlap of DIL-MRI and DIL-PET were used to create GTVunion and GTVoverlap, respectively. The co-primary objectives of the study were to measure the concordance between DIL-MRI and DIL PET, and, to determine a suitable PSMA-PET DIL auto-contouring SUV threshold. Doses prescribed were 36.25 Gy to the prostate, 42.5 Gy to GTVoverlap, 40 Gy to GTVunion and 25 Gy to the elective pelvic nodal volume (if indicated), delivered in five fractions every other day. While an isotropic margin of 5 mm was used for prostate and pelvic planning target volume(s) (PTV), no expansion was given for GTVunion and GTVoverlap. Contours were modified from urethral and bladder uptake as necessary. Patients were treated with Volumetric Modulated Arc Therapy (VMAT) technique with daily cone-beam CT (CBCT). All patients were planned for ADT of 6 months duration commencing with SBRT. Follow-up was done with biochemical response (PSA) assessment every 3-monthly and metabolic response (Ga68-PSMAPET) evaluation at SBRT conclusion and at 3 months after SBRT. Acute urinary and gastrointestinal (GI) toxicity was scored using CTCAE version 5.0 at baseline, at treatment conclusion and at 3 months after treatment.

Results:

Twenty patients were included (IR=10, HR=10). Median PSA at diagnosis was 16.2 ng/ml (IQR 9.4-19.5). The median baseline DIL SUVmax on PSMA-PET was 13.8 (IQR, 9.17-32.50). Median volumes of DIL-MRI, DIL-PET, GTVunion and GTVoverlap were 1.95 cc (IQR, 1.35-3.17), 2.25 cc (IQR, 1.70-3.75), 2.9 cc (IQR, 2.1-5.07) and 1.1 cc (IQR, 0.82-1.95), respectively (Figure 1) .