Ambient neutron and photon dose equivalent H*(10) around a pencil beam scanning proton therapy facility

Objectives: To measure leakage ambient dose equivalent H*(10) from stray secondary neutron and photon radiation around proton therapy (PT) facility and evaluate adequacy of shielding design. Methods and materials: H*(10) measurement were carried out at 149 locations around cyclotron vault (CV), beam transport system (BTS) and first treatment room (GTR3) of a multiroom PT facility using WENDI-11 and Smartlon survey meter. Measurement were performed under extreme case scenarios wherein maximum secondary neutrons and photons were produced around CV, BTS and GTR3 by stopping 230MeV proton of 300nA on beam degrader, end of BTS and isocenter of GTR3. Weekly time average dose rate (TADR) were calculated from EI*(10) value measured at selective hot spots by irradiating actual treatment plans of mix clinical sites. Results: The maximum total H*(10) were within 2 mu Sy/hr around CV, 5 mu Sv/hr around outer wall of BTS which increases up to 62 mu Sv/hr at the end of inside BTS corridor. Maximum H*(10) of 20.8 mu Sv/hr in treatment control console (P125), 23.4 mu Sv/hr behind the common wall between GTR3 and GTR2 (P132) and 25.7 mu Sv/hr above isocenter (P99) were observed around GTR3. Reduction of beam current from 6 to 3 nA and 1 nA at nozzle exit lead to decrease in total H*(10) at P125 from 20.8 to 11.35 and 4.62 mu Sv/hr. In comparison to extreme case scenario, H*(10) value at P125, P132 and P99 from clinically relevant irradiation parameters were reduce by a factor ranging from 8.6 for high range cube to 46.4 for brain clinical plan. The maximum weekly TADR per fraction was highest for large volume, sacral chordoma patient at 8.5 mu Sv/hr compare to 0.3 mu Sv/hr for brain patient. The calculated weekly TADR for 30 mix clinical cases and 15 fractions of 1 L cube resulted total weekly TADR of 83-84 mu Sv/hr at P125, P132 and P99. The maximum annual dose level at these hot spots were estimated at 4.37 mSv/Yr. Conclusion: We have carried out an extensive measurement of H*(10) under different conditions. The shielding thickness of our PT facility is adequate to limit the dose to occupational worker and general public within the permissible stipulated limit. The data reported here can bridge the knowledge gap in ambient dose around PT facility and can also be used as a reference for any new and existing proton facility for intercomparison and validation. Advances in knowledge: First extensive investigation of neutron and photon H*(10) around PT facility and can bridge the knowledge gap on ambient dose.