3220 - Dose-Escalated Intensity Modulated Proton Therapy (IMPT) for Node-Positive Prostate Cancer - First Report of Clinical Outcomes

12:45pm - 02:00pm PT

Hall F

Screen: 11

POSTER

Presenter(s)

Marcio Fagundes, MD - Miami Cancer Institute, Miami, FL

M. A. Fagundes¹, M. M. Zerey¹, O. Gal¹, J. Wohl², M. A. M. Rodrigues¹, A. Kaiser³, R. Garje¹, A. Muina¹, A. Gutierrez⁴, A. Wroe¹, Z. Fellows⁵, and M. P. Mehta⁵; ¹Miami Cancer Institute Baptist Health South Florida, Miami, FL, ²Miami Cancer Institute: Baptist Health South Florida, Miami, FL, United States, ³Miami Cancer Institute, Miami, FL, ⁴Florida International University, Herbert Wertheim College of Medicine, Miami, FL, ⁵Department of Radiation Oncology, Miami Cancer Institute, Baptist Health South Florida, Miami, FL

Purpose/Objective(s): IMPT employs intrinsic proton stopping properties to achieve dose sparing of normal organs with the ability to dose escalate (DE) with potentially greater safety, compared to photon radiotherapy. To our knowledge, this is the first study to report outcomes of node-positive (N1) prostate cancer patients treated with dose escalated IMPT.

Materials/Methods: Data were reviewed for consecutive patients treated between 9/2018-9/2024 with DE-IMPT for imaging detected node positive PCa. Prostate and seminal vesicles were treated to 78 Gy/39 Gy fractions; elective nodal chains received 46-48.6 Gy in 1.8-2 Gy/fraction, while positive nodes were DE to a mean equivalent dose at 2 Gy/fraction (EQD2) of 67.4 Gy (54-77.1Gy) at 1.8-2.5 Gy/fraction. Acute toxicities were assessed at baseline, weekly during treatment, and up to 3 months post-IMPT and late toxicities were assessed at 6 month intervals, using Common Terminology Criteria for Adverse Events (CTCAE version 5).

Results:

The study included 60 patients presenting with positive nodes (median = 2, range 1-12), detected by PET (81.7%), MRI (15.0%), CT (3.3%). Median follow up is 24.6 months (range 3-67 months). Median age was 73 years (range 49-86). Majority of patients had Gleason 8-10 (66.6%), MRI evidenced T3a-b (77%). Twelve (20%) had PSA > 50. All pts received androgen deprivation therapy (ADT). Androgen receptor pathway inhibitors were added in 28 pts (46.6%). Endorectal balloon (ERB) was employed in 51 (85%) and rectal spacer (RS) in 9 (15%). Mean rectal V70, V60 and V50 Gy (ERB/RS) was 7.4%/3.2%, 11.9%/6.9% and 17.4%/10.8%, respectively. Bladder V70, V60 and V50 Gy was 11.6%, 16.8% and 23.6%, respectively. Mean small bowel D0.03cc was 52.1Gy (range 47-61.8Gy). Three and 5-year freedom from biochemical recurrence (FFBR) was 93% and 65%, respectively. Patients with PSA < 50 had significantly better 3-year FFBR, when compared to those with PSA > 50, 100% vs 67% (p=0.006). The 5-year FFBR for patients with PSA <50 was 86% compared to 0% for PSA > 50. Worst acute and late gastrointestinal Grade > 2 toxicities were 5% and 11.6%, respectively. One patient (1.6%) developed acute transient Grade 3 enteritis. There was no late grade 3 or higher GI toxicities. Late Grade 2 proctitis rate was 13.7% with ERB and 0% with RS. Worst acute and late genitourinary Grade 2 toxicities were 30% and 21.6%, respectively. There were no acute or late GU Grade 3 or higher toxicities.

Conclusion: Dose escalated IMPT is a safe treatment option for node-positive PCa with acceptable rates of acute and late toxicities. Acute grade 3 GI toxicity is rare, occurring in only one patient in this cohort. Preliminary disease control is encouraging but longer follow up is needed.