2575 - Patient Outcomes Following Palliative Hypofractionated Fast Neutron Therapy

A. A. Menon¹, A. B. Barbour², R. D. Stewart², J. J. Liao², G. E. Laramore², C. P. Rodriguez³, and U. Parvathaneni⁴; ¹University of Washington, Seattle, WA, ²Department of Radiation Oncology, University of Washington - Fred Hutchinson Cancer Center, Seattle, WA, ³Division of Oncology, Department of Medicine, University of Washington/ Fred Hutchinson Cancer Center, Seattle, WA, ⁴Department of Radiation Oncology, University of Washington/ Fred Hutchinson Cancer Center, Seattle, WA

Purpose/Objective(s): Fast Neutron Radiotherapy (NRT) is a high linear energy transfer modality that can overcome tumor radioresistance to conventional radiotherapy (RT). This may be particularly beneficial in the palliative recurrent/metastatic setting. An ideal dose-fractionation in this setting is unknown. This study evaluates clinical and early toxicity outcomes of short course (2-4 fraction) palliative hypofractionated NRT (hNRT). Materials/

Methods: Clinical characteristics, oncologic treatment history, clinical (tumor shrinkage and/or symptom response) and radiographic responses were reviewed in a single-institution, IRB-approved retrospective review of patients who received at least one palliative treatment course with 3D conformal hNRT from 11/2016 to 12/2022.
Results: Twenty-seven patients with Stage IV cancer received hNRT at median age of 79yr (range 47–100) with median follow-up of 5.4mo (range 0.1-75.6). Histology included squamous (n = 7), urothelial (n = 6), Merkel (n = 5), adenocarcinoma (n = 3), renal cell (n = 2), and other (n = 4). Median hNRT dose was 6 Gy in 3 fractions (range 3.45-10 Gy, 2-4 fractions), equivalent to around 18-30 Gy of x-rays. There were 48 hNRT treatment courses across 32 unique anatomic sites, which included head-and-neck (HN) (n = 14), bone (n=5), genitourinary (n = 4), non-HN lymph nodes (n = 7), and non-HN skin (n = 2). Twenty-three unique treatment sites (across 19 patients) received a single hNRT course, whereas nine sites (across 9 patients) received 2-4 successive courses (median 37d between courses). 16 patients (60%) were on concurrent systemic therapy and 8 sites (25%) were previously irradiated with photon RT. Median overall survival time from the end of the first hNRT course was 400d (95% CI 221-not reached). Of treatment sites receiving one hNRT course, 18 (78%) of 23 had clinical response, and 11 of 13 (85%) sites with radiographic follow-up had radiographic response. Of sites receiving 2 or more successive courses, 9 (100%) of 9 had clinical response and 6 of 8 (75%) sites with radiographic follow-up had radiographic response. Of 8 sites previously irradiated with photon RT, 6 (75%) had a clinical response of which 4 (50%) had a concurrent radiographic response. Of 20 sites receiving hNRT for pain relief/bleeding, 3 (15%) had symptom stability and 17 (85%) had partial-to-full clinical response, 11 of which had symptom progression at a median of 145d post-response. Of 12 sites receiving hNRT to slow disease progression and/or stimulate an immune response (n = 11 on concurrent immunotherapy or ADT), 10 (83%) had partial-to-full clinical response with radiographically stable-to-improved disease and 1 had radiographic progression. Overall, RTOG Grade 2 (n=1) or 3 (n=2) side effects were uncommon. No patients experienced pain flares.
Conclusion: Most patients treated with hNRT had symptom relief and radiographic response. None had pain flares and high-grade side effects were rare. Single or repeat-course hNRT may be a safe and effective method of palliation.