Spectral distribution of particle fluence in small field detectors and its implication on small field dosimetry

Abstract

The present work improves the understanding of small-field effects by relating output correction factors to spectral fluence perturbations in small field detectors. It is shown that the main reasons for the well-known small-field effects in silicon diodes are the high-Z and density of the "extracameral" detector components and the high I-value of silicon relative to that of water and diamond. Compared to these parameters, the density and atomic number of the radiation sensitive volume material play a less significant role.

Dosimetry of small static fields used in external photon beam radiotherapy: Summary of TRS‐483, the IAEA–AAPM international Code of Practice for reference and relative dose determination

“…Additionally, the dimensions of most detectors commonly used in conventional fields become large compared to the small‐field size. This has physical repercussions on dosimetry, which have in general been well described in the literature . In addition, fields used for reference dosimetry in an increasing number of special devices do not conform to the reference conditions prescribed in conventional CoPs and dosimetry protocols.…”

“…This has physical repercussions on dosimetry, which have in general been well described in the literature. 12,13,[21][22][23][24][25] In addition, fields used for reference dosimetry in an increasing number of special devices do not conform to the reference conditions prescribed in conventional CoPs and dosimetry protocols. This section describes the physical conditions that determine if a field should be considered small and how its size should be defined; it also discusses deviations from conventional reference dosimetry that are at the basis of the recommendations in IAEA TRS-483, such as the measurement of beam quality and the response of detectors in small fields.…”

Small‐field dosimetry of TrueBeamTM flattened and flattening filter‐free beams: A multi‐institutional analysis

“…Although photon beams with small field sizes are often used for such treatments, accurate small‐field dosimetry remains challenging . Beam‐related causes of variations in beam characteristics include lateral charged‐particle disequilibrium, partial occlusion of the direct beam source, and change to the energy spectrum of photons, whereas detector‐related causes include volume‐averaging effects as well as detector and shielding materials affecting the perturbation of the charged‐particle fluence and the mass electronic stopping power . Many detectors for small‐field dosimetry, such as shielded and unshielded diodes, diamond detectors, and plastic scintillators, have different characteristics, such as sensitive volumes, shielding materials, and detector materials affecting the perturbations and stopping power ratio …”

Comment on ‘Origins of the changing detector response in small megavoltage photon radiation fields’

“…it is a circular argument. The impact of the differences for silicon or other materials on the conclusions by Fenwick et al could only be evaluated with simulations using the correct input dataset for each material, as was done by Benmakhlouf and Andreo (2017) when they changed ρ and I-values for the different materials in a fully simulated diode (see their figures 6 and 9). It then appears that the two assumed corollaries and conclusions by Fenwick et al for ' Z →H 2 O' and 'density→ 1' fictitious material detectors having k factors of one and the same k values as for real substances, respectively, based on the approach of using equal stopping powers, remain to be proven.…”