Small Field Dosimetry is a serious matter

With the introduction of novel techniques in radiotherapy such as Image Guided Radiotherapy (IGRT), Intensity Modulated Radiotherapy (IMRT) (Laub and Wong, 2003), Volumetric Modulated Arc Therapy (VMAT) (Wolfs et al., 2018), Stereotactic radiotherapy (SRT) (Beddar et al., 2006), Stereotactic Radiosurgery (SRS) (Pettiet al., 2021), and Stereotactic Body Radiotherapy (SBRT) (Benedict et al., 2010), that can be made the treatment radiation field into small segments and narrow fields to deliver a high radiation dose to the target volumes with limiting damage to the normal tissues for this purpose, there have been many developments in treatment machines. In contrast, this technique has increased the uncertainty of clinical dosimetry and weakened its traceability to reference dosimetry; Conventional Codes of Practice (COPs) such as Technical Reports Series (TRS-398) (Musolino, 2001), the American Association of Physicists in Medicine (AAPM) publication titled AAPM’s TG-51 Protocol (Almond et al., 1999).

In radiotherapy, we can define the small field as that field with dimensions smaller than the lateral range of the electrons that contribute to the dose, the Multi-leaf Collimators (MLC) roughly can be made in narrow field sizes up to 1cm x 1cm or less, so at least one of the following three physical conditions are generally considered to determine if an external photon beam can be designated small: (a) Lack of charged particle (Loss of lateral charged particle equilibrium) LCPE. (b) There is partial occlusion of the primary photon source by the collimating devices on the beam axis. (c) The size of the detector like or larger than the cross-sectional beam dimensions at the depth of measurement (Palmans et al., 2018).

The selection of an appropriate detector for dosimetry in small fields is challenging, and it is necessary to choose a suitable detector with the best performance, and not all detections of ionization chambers are sensitive enough to radiation dosimetry (Zhu et al., 2009), also, there is no common agreement among researchers on the use of specific detector types. Some studies have investigated the effect of the construction and size of detectors in small radiation fields (Scott et al., 2012; Underwood et al., 2013). Researchers have frequently evaluated the effectiveness of various detectors at dose distribution measurements, but most of them focused only on the stereotactic radiation field created by radiosurgery devices and circular cones, a few studies focused on small fields used in beamlets of linear accelerators.

IMRT and VMAT fields use small segments shaped by MLCs of Linac for dose delivery, so a consensus has been that suitable detectors must be considerably smaller than the field size as it impacts detector readings when charged particle equilibrium (CPE) breaks down around the sensitive volumes of detectors (Das et al., 2008a), which could lead to significant uncertainty in the accuracy of clinical dosimetry (Bich, 2014) compared to traditional radiotherapy, as we pointed out that accurately measuring absolute dose or output factors at an absorbed dose to the water at sub-centimeter fields was very difficult, so the IAEA recommended appropriate detector systems and measurement methodologies at TRS-483 (Palmans et al., 2018). An overview of the issue of LCPE and the changes in photon beam perturbations with decreasing field size were provided. The dosimetry and commissioning of traditional large field sizes starting from 5cm x 5cm and more up to 40cm x 40cm used high photon energy in radiotherapy has been addressed in several reports of the TG-106 (Das et al., 2008b). These and other dosimetry protocols are based on measurements using an ionization chamber of absorbed dose to water, traceable to International Standards of units dosimetry laboratory (PSDL) at reference conditions, such as a conventional field size of 10cm x 10cm (Allisy et al., 2009) read more https://pmc.ncbi.nlm.nih.gov/articles/PMC10685229/