1. Introduction

Abstract

A small photon field is usually characterized by the following limitations to accurate dosimetry: (1) lack of lateral electron equilibrium; (2) obscuring of the primary photon source by the collimation system; and (3) the limitations of conventional broadfield detectors to accurately measure absorbed dose under small-field conditions. Small photon fields are used both for intensity-modulated radiation therapy (IMRT) (ICRU, 2010) and for stereotactic radiation therapy (SRT), which is discussed in the present report. Stereotaxy historically refers to the use of a 3dimensional (3D) coordinate system to localize intracranial targets. As of the time of publication of this report the concept of stereotactic treatment has been expanded to refer to the accurate and reproducible localization of the clinical target in space (i.e., at the mm level) and in time. SRT involves stereotactic localization techniques combined with delivery of multiple small photon fields in a few high-dose fractions, leading to a highly conformal dose delivery with steep dose gradients. Stereotactic localization techniques may include the use of relocatable rigid frames, image-guidance techniques, and other positioning tools. Stereotactic radiosurgery (SRS) is intracranial SRT delivered in a single fraction. Stereotactic body radiation therapy (SBRT) is the use of SRT for extracranial treatments. Other nomenclature such as SABR, which stands for stereotactic ablative (body) radiotherapy, is also used, but is not recommended by ICRU. In SRT techniques, a high dose per fraction is a prescribed absorbed dose to the target per treatment fraction. The dose is higher than what would be delivered using conventional radiation therapy in a similar situation. A “few fractions” refers to a number of fractions that is significantly lower than the number of fractions that would be delivered using conventional radiation therapy in the identical clinical situation (i.e., typically 25 fractions or more) in a non-palliative scenario. Fractionation protocols can range from a single fraction of over 180 Gy (Kooshkabadi et al., 2013) for functional indications to 12 fractions of 4.3 Gy (Ritter et al., 2009) in early prostate cancer. The therapeutic ratio is enhanced through delivery of highly conformal dose distributions with steep dose fall-off ensuring optimal dose in the target volume with minimal normal tissue irradiation. The scope of this report is to discuss spatially small photon beam delivery, including dosimetry, commissioning, and quality assurance as well as to recommend a framework for prescribing, recording, and reporting stereotactic radiotherapy. This report covers all pathologies eligible for stereotactic delivery (malignant and non-malignant). Note that other radiation therapy techniques may deliver a high dose per fraction in a well-defined target volume, such as, intra-operative radiation therapy or high dose rate brachytherapy (HDR-BT). These techniques are not discussed.