Executive Summary

Abstract

In the context of this Report, stereotactic radiotherapy (SRT) encompasses stereotactic radiosurgery (SRS), stereotactic body radiation therapy (SBRT), and stereotactic ablative body radiation therapy (SABR). SRT involves stereotactic localization techniques (i.e., use of a three-dimensional coordinate system to localize the target) combined with the delivery of multiple small photon fields in a few high-dose fractions. There is wide diversity in the clinical prescriptions for SRS and SBRT, some of which are based on preclinical studies, some based on modeling using the linear-quadratic (LQ) model, and others that result from trial-and-error experiences. The limitations of the LQmodel in scenarios of SRS/SBRTmight suggest the possible existence of new cell-death mechanisms related to stem cells, vascular damage, bystander effects, and immune-mediated effects, or combinations of these. Therefore, there is a need to develop a better rationale for current practice and future hypofractionation clinical trials by incorporating classical and new radiobiology and appropriately upgrading the modeling schemes for high doses to reflect new evidence-based understanding of tumor control and normal-tissue tolerances at higher doses per fraction. The understanding of the radiobiology of hypofractionation has been the subject of renewed intense interest driven by clinical successes and socio-economical benefits of shortened treatment times and potentially improved outcomes. The role of clinical medical physics in radiation therapy is to ensure that the prescription is delivered accurately. Comprehensive quality assurance encompasses all aspects of radiation medicine ranging from SRT device acceptance, commissioning, image guidance, and delivery. Recent incidents in radiation therapy using small fields have indicated that the dosimetry of small fields is complex and prone to errors. The requirements in terms of accurate beam calibration, treatment planning, accuracy of delivery, and quality assurance are more stringent than in other areas of radiation therapy. The active collaboration among all professions in the SRT program is critical for treatment quality and patient safety. Three main features dominate the dosimetry of small beams from accelerators. Firstly, absorbeddose distributions formed by small beams are characterized by a lack of charged-particle equilibrium over a much greater fraction of the treatment volume than for conventional radiotherapies. This has implications in dose measurements as well as treatment-planning dose calculations, especially in the vicinity of tissue heterogeneities. Secondly, in small beams, part of the source is often occluded by the collimation system, leading to beam-penumbra overlap and a drastic reduction in output fluence rate. Overlap of penumbra leads to effective-beam broadening in small beams compared to the geometric beam definition. Thirdly, the measurement of absorbed dose from small beams is highly dependent on the size and construction details of the detector used. Conventional calibration techniques applicable to standard 10 cm × 10 cm radiation beams cannot be applied to small beams without modification and supplementary detector-correction factors must be accurately known. These features have cast considerable dosimetric uncertainty in small-field dosimetry. Three basic criteria, mostly related to the material in the sensitive region of the detector, dictate the suitability of a particular detector for a smallfield absorbed-dose measurement: (1) the sensitive region of the detector is water equivalent in terms of radiation-absorption characteristics; (2) the mass density of the sensitive region is the same as or close to the mass density of water; and (3) the size of the sensitive region can be made small compared to the field size. Physical phenomena that strongly affect detector response in small fields are the effects of volume averaging and fluence perturbation. The latter may be due to the use of materials with a density significantly different from water such as the gas cavity in gas-filled ionization chambers or metals in the wiring of diode detectors. None of the detectors currently available are ideal for small-field dosimetry. In the formalism for reference dosimetry for SRT, a machine-specific reference field (msr) is introduced to account for the fact that modern small-field