2. Small Field Dosimetry

Abstract

The role of clinical medical physics in radiation therapy is to ensure that the prescription is delivered accurately. This means that there is a need for a comprehensive quality assurance program, which encompasses all aspects of radiation medicine ranging from therapy machine acceptance, commissioning and calibration, image guidance and delivery, and that all procedures and workflows are in place to deliver the prescription accurately. The dosimetric portion of the quality assurance program starts with accurate calibration of the beam and the measurement of output factors and other dosimetric functions. In this context, the requirements in terms of accurate beam calibration, treatment planning, delivery, and quality assurance of stereotactic radiation therapy are as stringent, if not more stringent, as in conventional radiation therapy (Thwaites, 2013). Recent incidents in radiation therapy (Ford and Evans, 2014; IAEA, 2014) using small fields have indicated that dosimetry of small fields is complex and prone to errors. In general, absorbed dose determination in small fields requires multiple levels of redundancy including the use of different detectors with appropriate detector-dependent correction factors for the measurements, a critical analysis of measured data on equipment of the same type in comparison with peer centers, corroboration of data with manufacturer “golden beam data” (i.e., reference data), etc. As will be clear from the present section, simple pooling of relative reading data from different detector types is not the same as the relative dose in small fields; accurate depth and field-size dependent correction factors are required to ensure that the detector signal is faithfully converted into absorbed dose. In addition, the execution of an independent third-party dosimetry end-to-end review (e.g., Imaging and Radiation Oncology Core, IROC, MD Anderson Houston) is strongly suggested. Because of this complexity, an institution starting a new small-field radiation therapy program should consider training programs for physicists, radiation therapy planning personnel and radiation oncologists that includes a review of the basis of small field dosimetry. The active collaboration between all professions in the SRT program is critical for treatment quality and patient safety. In 3D conformal radiation therapy, higher energies (defined for the purpose of this discussion as photon beams created from accelerating potentials of larger than 10MV) are regularly used to improve coverage in the case of deep-seated tumors. The secondary electron path for a 15MV beam may be of the order of 3 cm or more thereby significantly affecting penumbra width. This leads to problems in the application of SRT using higher energies especially for small targets in regions involving lower density, such as lung. In these conditions, the accuracy requirements imposed on the dose calculation algorithm used for treatment planning are more difficult to meet for higher energies (>10MV). Secondly, collimation systems, such as MLCs, typically block 6MV photons better than they block 15MV or higher energies. This and other effects lead to a more significant dose beyond the penumbra in higher (>10MV) compared to lower (≤10MV) photon energies, ultimately affecting offaxis ratios (OAR) sparing. Finally, higher-energy (>10MV) photon beams give rise to neutron production through the (γ , n) reaction. At 18MV, the cross-section for neutron production by collimation is two orders of magnitude greater than for 10MV photons (Maglieri et al., 2015). This may lead to activation of linac components as well as unwanted out-of-field patient exposure although the dosimetric impact of this needs further investigation (Horst et al., 2015). For all of these reasons, the present Report recommends the use of lowerenergy photon beams (≤10MV) for the clinical implementation of SRT programs. This recommendation is consistent with ICRU Report 83, which states that the use of higher-energy beams is not justified for IMRT (ICRU, 2010). In the present section, the specific physics aspects of dosimetry in small beam radiation therapy are summarized. The nomenclature introduced covers small beams, for which defining conditions will be presented. For the discussion of reference dosimetry and output factors, this section follows the recommendations of the IAEA-AAPM code of practice