733Background: Definitive chemoradiation (CRT) with concurrent 5-fluorouracil (5-FU)/mitomycin C (MMC) for anal cancer yields high rates of long term survival. However, CRT has significant acute and long term toxicity. Pencil beam scanning proton beam (PBS-PT) may potentially reduce this toxicity. This is a pilot study evaluating the feasibility of definitive concurrent CRT with PBS-PT with 5-FU/MMC for anal cancer. Methods: Pts were enrolled on an NCI sponsored, prospective, multi-institutional, pilot study (NCT 01858025). Key eligibility were ECOG 0-2, age 18+, invasive SCC of the anal canal, clinically staged T1-4, N0-3. Pts were treated per RTOG 0529 RT schema; for T1-2, N0: 50.4 Gy to primary CTV, 42 Gy to elective LNs; for T3+ or N+: 54 Gy to primary CTV, 50.4-54 Gy to involved LN, and 45 Gy to elective LNs. Pts received 5-FU/MMC on day 1 and 29. The primary objective of this study was to determine feasibility of PBS-PT with 5-FU/MMC, determined if grade 3+ dermatologic toxicity is < 48% (grade 3+ d...