Out-of-field dose equivalents delivered by passively scattered therapeutic proton beams for clinically relevant field configurations
Microdosimetric measurements were performed at Massachusetts General Hospital, Boston, MA, to assess the dose equivalent external to passively delivered proton fields for various clinical treatment scenarios.
Methods and Materials
Treatment fields evaluated included a prostate cancer field, cranial and spinal medulloblastoma fields, ocular melanoma field, and a field for an intracranial stereotactic treatment. Measurements were completed with patient-specific configurations of clinically relevant treatment settings using a silicon-on-insulator microdosimeter placed on the surface of and at various depths within a homogeneous Lucite phantom. The dose equivalent and average quality factor were assessed as a function of both lateral displacement from the treatment field edge and distance downstream of the beam's distal edge.
Dose-equivalent value range was 8.3–0.3 mSv/Gy (2.5–60-cm lateral displacement) for a typical prostate cancer field, 10.8–0.58 mSv/Gy (2.5–40-cm lateral displacement) for the cranial medulloblastoma field, 2.5–0.58 mSv/Gy (5–20-cm lateral displacement) for the spinal medulloblastoma field, and 0.5–0.08 mSv/Gy (2.5–10-cm lateral displacement) for the ocular melanoma field. Measurements of external field dose equivalent for the stereotactic field case showed differences as high as 50% depending on the modality of beam collimation. Average quality factors derived from this work ranged from 2–7, with the value dependent on the position within the phantom in relation to the primary beam.
This work provides a valuable and clinically relevant comparison of the external field dose equivalents for various passively scattered proton treatment fields.