Proton Radiosurgery: A Clinical Transition From Passive Scattering to Pencil Beam Scanning.

Purpose/Objective(s) Our institution developed proton stereotactic radiosurgery (PSRS) techniques and treated patients since 1961. A recent upgrade from passive scattering (PS) delivery to pencil beam scanning (PBS) required validation of this modality for small fields. We describe the adaptation of an existing clinical PSRS service to PBS. We outline potential advantages and disadvantages the modality has to offer. Materials/Methods Patient alignment is determined from 2D/3D x-ray corrections of surgically implanted fiducials. End-to-end tests using a skull phantom were performed to validate a change of treatment planning system (TPS). PSRS PS fields from prior years established applicable parameters for dose, range, modulation, field size and aspect ratio for PBS validation. 18 fields were created to verify agreement with the TPS. A diamond detector was used to measure SOBP doses and film to measure profiles. A QA phantom enabled simultaneous measurement with film and diamond detector. X-rays were used to verify detector alignment prior to measurements. A hybrid 2D/3D γ-analysis was used to assess lateral profiles. Plans were generated for previously treated patients and used to perform end-to-end tests. These included 6 AVM, 3 pituitaries and 1 cavernous sinus lesion. Clinical directives were obtained from the original plans. Results End-to-end phantom tests conformed sub-mm alignment. Distal and proximal depth agreement were 0.10 ± 0.66% and 0.74 ± 0.48 mm. Isocenter and shallow depth dose agreement was -1.7 ± 2.9% and 2.5 ± 5.1%, respectively. Global 2%/1mm/10% γ-analysis resulted in pass rates of 97.2 ± 4.8%. 24 clinical fields were delivered. Dose agreement was -2.8 ± 2.7%. Global 3%/1mm/10% γ- analysis PASS rates were 99.5 ± 1.4%. Conclusion We have revalidated SRS appropriate fiducial-based alignment and defined a small-field QA process using a diamond detector & film to demonstrate PSRS performed with PBS delivery is accurate. Proton delivery systems and TPS modeling can differ significantly. Therefore, validation and plan quality results should not be generalized to all systems. Proton range uncertainties play an important role in determining the appropriateness of PSRS. PBS does not offer distal benefits for small targets compared to passive scattering using range compensators. Larger SRS targets with irregular shapes may benefit from intensity variations and variable modulation offered by PBS. VMAT generally provides slightly superior target coverage for small pituitary lesion, but target coverage for larger pituitary lesions not abutting the optic structures and prescribed to lower doses are more comparable for VMAT and PBS while lowering integral dose. Our validation effort enabled the transition of an active PSRS program from PS to PBS. Author Disclosure M. Bussiere: None. J. Daartz: None. J. Verburg: Employee; Massachusetts General Hospital. Research Grant; National Cancer Institute. N. Depauw: None. H.M. Kooy: None. J.S. Loeffler: Employee; Massachusetts General Hospital. Advisory Board; Mevion. P.H. Chapman: None. H.A. Shih: Employee; Dartmouth Hitchcock. Research Grant; AbbVie, NIH. Honoraria; UpToDate. Consultant; Cleveland Clinic. Speaker's Bureau; prIME Oncology. advisory; The Radiosurgery Society. director of clinical operations; Massachusetts General Hospital. clinical operational leader; Massachusetts General Hospital.