Angular Distributions of Secondary Neutron and Photon Ambient Dose Equivalent in Proton Therapy: A Systematic PHITS 3.35 Study at 70–250 MeV

Monte Carlo simulation of secondary neutron and photon dosimetry -specifically the ambient dose equivalent H*(10) -is central to radiation protection in pencil-beam scanning (PBS) proton therapy. We report Particle and Heavy Ion Transport code System (PHITS) version 3.35 calculations of H*(10) for proton beams from 70 to 250 MeV incident on a water phantom representative of the Ion Beam Applications (IBA) Blue Phantom, scored at 11 emission angles at 80 cm using International Commission on Radiological Protection (ICRP) Publication 74 fluence-to-dose conversion coefficients. Results are normalised to peak absorbed dose, yielding the quantity H*(10)/D [nSv Gy-1] relevant to facility shielding andpatient out-of-field dose estimation. The distal Bragg-peak range R80 was benchmarked against independent Geant4/Gate calculations, agreeing within -0.56% to -3.61% and meeting the ±3% clinical criterion for energies ≥ 100 MeV. Enabling the low-energy neutron eventgenerator mode changed secondary-neutron yields by less than 0.2%, indicating that default settings are adequate for these radiation-protection estimates. The H*(10)/D angular distribution showed a forward maximum near 40° and a backward maximum at 180°; the forward-to-lateral ratio H*(40°)/H*(90°) rose from 1.1 at 70 MeV to 2.5 at 250 MeV -the forward peak becoming the global maximum above 150 MeV -while neutrons exceeded photons at 90° by factors of 22.9-56.2. A representative PBS nozzle model was added to quantify the effect of beam-line components; the simulated forward energy dependence and neutron spectral composition agree with published pencil-beam-scanning measurements, while absolute magnitudes remain lower owing to the single-pencil bare-phantom geometry. For a 150 MeV spread-out Bragg peak (SOBP), H*(10)/D was 4.8-6.2× higher than the monoenergetic beam, reflecting the larger number of protons required per unit dose. The dosenormalised dataset provides a systematic, practical reference for PHITS users in proton therapy radiation protection.