3349 - PSMA Based Dose Escalation to Dominant Intraprostatic Lesion (DIL) in Localized Carcinoma Prostate Treated with Stereotactic Body Radiotherapy

Purpose/Objective(s): Dose escalation has been proven effective in prostate cancer due to low ?/?? ratio. Extreme hypofractionation with SBRT to the whole prostate is a well established treatment modality for localized prostate cancer. The role of SBRT in high-risk groups is also gaining popularity in recent days. Nevertheless, local recurrences are common in about 30% of cases, mostly in the dominant nodule, which mandates dose escalation to that region. However, due to dose limiting toxicities of surrounding critical structures, simultaneous boost to the nodule is not commonly followed during SBRT. Here we analysed the feasibility of dose escalation to PSMA avid dominant nodules(DIL) in localized prostate cancer treated with SBRT with regards to local control and toxicity profiles.

Materials/Methods: In our prospective single arm interventional study, we included 31 patients with biopsy proven localized carcinoma prostate treated with SBRT using a frameless robotic radiosurgery system from September 2009 to November 2024. All patients underwent gold fiducial placement to the prostate (TRUS guided) followed by PSMA PET CT and MRI based planning. Entire prostate is contoured as Gross Tumor Volume (GTV-PROS) and PSMA avid nodule correlating with MRT2 was marked as GTV PET-DIL. Patients with DIL in proximity to rectum were excluded. Dose delivered to GTV-PROS and GTV PET-DIL was 35 Gy and 45 Gy respectively, for 5 fractions treated on alternative days using real time fiducial based tracking. All patients were assessed at regular intervals and monitored with Serum PSA, PSMAPET CT and MRI Pelvis once every 3 months. All patients were assessed for toxicities with Quality of Life(QOL).

Results: 31 patients were analyzed. Median age was 68 years (Range:52 to 94 years). Mean volume for GTVPET and GTVPROS were 44cc and 6cc respectively. Median prescription isodose was 82% . Mean Rectal and bladder max dose was 36.42Gy and 37.74Gy respectively. All patients tolerated the treatment well with Grade 1/2 toxicities. After a median followup of 44 months (range 3 to 156 months), local control was 100%. 2 patients who belong to the high risk category and denied ADT had distant progression after 4 years from treatment. None of the patients had significant late toxicities.

Conclusion: From our study we conclude that dose escalation with SIB during SBRT using real time tracking has improved local control when compared to standard extreme hypofractionation schedules. PSMA PET CT when combined with MRI not only helps in accurate delineation and guide for precise dose escalation strategies, but also aids in response evaluation post SBRT. This variable dose painting can also lead to a hypothesis of relative reduction of dose to microscopic disease (entire prostate there by decreasing the toxicities.