Minimum phantom size for megavoltage photon beam reference dosimetry

• Published in: Medical physics (Lancaster) Vol. 51; no. 8; pp. 5663 - 5671
• Main Author: Rogers, D. W. O.
• Format: Journal Article
• Language: English
• Published: United States 01.08.2024
• Subjects: Monte Carlo; Monte Carlo Method; Phantoms, Imaging; photon beams; Photons; Radiometry - instrumentation; reference dosimetry; Reference Standards; TG51; TRS398; Water; water phantoms; reference dosimetry; TG51; photon beams; Monte Carlo; TRS398; water phantoms
• ISSN: 0094-2405; 2473-4209; 2473-4209
• DOI: 10.1002/mp.17099

Background Water phantoms are required to perform reference dosimetry and beam quality measurements but there are no published studies about the size requirements for such phantoms. Purpose To investigate, using Monte Carlo techniques, the size requirements for water phantoms used in reference dosimetry and/or to measure the beam quality specifiers %dd(10)x$\%dd(10)_{\sf x}$ and TPR1020$TPR^{20}_{10}$. Methods The EGSnrc application DOSXYZnrc is used to calculate D(10)$D(10)$, the dose per incident fluence at 10 cm depth in a water phantom irradiated by incident 10×10cm2$10\,\times \,10 \, {\rm {cm}}^{2}$  beams of 60Co$^{60}{\rm {Co}}$  or 6 MV photons. The water phantom dimensions are varied from 30×30×40cm3$30 \,\times \, 30 \,\times \, 40 \, {\rm {cm}}^3$ to 15×15×22cm3$15 \,\times \, 15 \,\times \, 22 \, {\rm {cm}}^3$ and occasionally smaller. The %dd(10)x$\%dd(10)_{\sf x}$ and TPR1020$TPR^{20}_{10}$ values are also calculated with care being taken to distinguish TPR1020$TPR^{20}_{10}$ results when using Method A (changing depth of water in phantom) and Method B (moving entire phantom). Typical statistical uncertainties are 0.03%. Results Phantom dimensions have only minor effects for phantoms larger than 20×20×25cm3$20 \,\times \, 20 \,\times \, 25 \, {\rm {cm}}^3$. A table of corrections to the dose at 10 cm depth in 10×10cm2$10 \,\times \, 10 \, {\rm {cm}}^{2}$  beams of 60Co$^{60}{\rm {Co}}$  or 6 MV photons are provided and range from no correction to 0.75% for a 60Co$^{60}{\rm {Co}}$ beam incident on a 20×20×15cm3$20 \,\times \, 20 \,\times \, 15 \, {\rm {cm}}^3$ phantom. There can be distinct differences in the TPR1020$TPR^{20}_{10}$ values measured using Method A or Method B, especially for smaller phantoms. It is explicitly demonstrated that, within ±$\pm$ 0.15%, TPR1020$TPR^{20}_{10}$ values for a 30×30×30cm3$30 \,\times \, 30 \,\times \, 30 \, {\rm {cm}}^3$ phantom measured using Method A or B are independent of source detector distance between 40 and 200 cm. Conclusions The phantom sizes recommended in the TG‐51 and IAEA TRS‐398 reference dosimetry protocols are adequate for accurate reference dosimetry and in some cases are even conservative. Correction factors are necessary for accurate measurement of the dose at 10 cm depth in smaller phantoms and these factors are provided. Very accurate beam quality specifiers are not required for reference dosimetry itself, but for specifying beam stability and characteristics it is important to specify phantom sizes and also the method used for TPR1020$TPR^{20}_{10}$ measurements.