Dose per monitor unit determination for proton therapy treatment portals with and without the range compensator - 013

Proton therapy is particularly advantageous because of its ability to spare normal tissue from unwanted radiation dose. Clinical proton beam treatments require physical devices to conform the dose distribution to the target volume. These devices include patient-specific collimating apertures, which define the field size and shape, and range compensators, which compensate for range perturbations due to air and bone inhomogeneities within the patient. Since beam modification devices are often unique to each field, dose per monitor unit (D/MU) values of proton treatment fields are typically measured prior to the treatment. While the ICRU and IAEA provide dosimetry recommendations for D/MU measurements under reference conditions, methods for measuring D/MU under patient-specific conditions have not been standardized. D/MU values for patient-specific fields can be measured either with or without the patient-specific range compensator in place, yet the literature contains no reports on this fundamental aspect of proton therapy quality assurance. The aim of this study was to quantitatively assess the reliability of two methods for measuring D/MU values for small-field treatment portals: one with the range compensator present and one with the range compensator absent. (authors)