IPEM report 103: Small field MV photon dosimetry

Abstract

Full text: The use of small photon fields has been an established practice in stereotactic radiotherapy/radiosurgery (SRT/SRS) for many years and a lot of work has been published on their use. Nevertheless, when small fields have been included in the delivery of significant patient doses, there have been serious dosimetric errors reported in patient treatment. Recent technological advances in conventional linear accelerators have improved their mechanical accuracy, stability, dosimetric control and there has been an increasing availability in the clinic of standard-, mini- and micro- multi-leaf collimators (MLCs). Small fields are nowadays being used on treatment equipment (machines and treatment planning systems (TPS)) not originally designed for their delivery and modelling. Furthermore specialised systems specifically designed for intensity modulated radiotherapy (IMRT) are in use for patient treatment (e.g. TomoTherapy and Cyberknife). The main aim of this report is to assist the medical physicist in understanding the potential for error, identifying areas to focus on to minimise these, enable the clinic to identify the field size limit where treatment planning and delivery systems can be commissioned with confidence. An MV photon field is defined as 'small' when either of the following conditions is met: 1. The field size is not large enough to provide CPE at the position of measurement. That is, when the field dimensions are comparable or less than the lateral range of secondary electrons for the beam energy under investigation; 2. The collimating device obstructs part of the direct beam source, as viewed from the point of measurement. The first challenge in narrow field dosimetry is the definition of field size because the conventional approach of classifying fields based on the full width half maximum (FWHM) breaks down at small collimator settings. The second challenge is that most commonly used detectors are simply too big for the determination of dosimetric parameters in a small field. In addition to this, detector size and perturbation effects can be substantial for small fields as opposed to larger fields where they are in general small and manageable. The third challenge is ensuring accurate dose modelling by the TPS. Lastly, changes in the beam spectrum as the field size decreases and the deviation from the conventional reference field size mean that there is no established protocol for absolute or reference dosimetry in small fields and this has implications in the measurement of reference dose based on existing dosimetry protocols. The large amount of published information on the subject of small field dosimetry is presented in twelve chapters. Attention is drawn to relevant aspects of quality assurance for the treatment machine and collimating jaw. The characteristics of commercially available detectors for small field applications are summarised. The majority of the report presents established or newly proposed methodologies on the determination of dosimetric parameters (profiles, depth functions and output factors) for single narrow collimated fields, because it is data on these that are needed to configure fluence and dose calculation models in TPSs and independent monitor unit (MU) verification software. The challenges in absolute, reference and relative dosimetry are addressed in detail. Although dose calculations for treatment planning are not the main focus of this report, a discussion is included on necessary elements in fluence and dose calculation methods that are needed to model the drop in beam output and narrowing of the penumbra at small collimator settings in order to achieve the requirement of a computational accuracy of at least 3%. The report provides, where possible, recommendations of good working practice on all aspects of small field MV photon dosimetry with the hope that these would be consulted and used alongside the clinical experience, scientific judgement and existing expertise. The areas where solutions are not yet established or would benefit from supporting work to validate existing publications are identified. The report ends suggesting future directions along with future work and research efforts that are needed to reduce uncertainty in the determination of dose in small MV photon fields. This publication shows that these need to be treated as a new modality as far as commissioning is concerned and, that previous assumptions on all aspects of planning, dosimetry, commissioning, delivery and validation should be questioned