Minimum phantom size for megavoltage photon beam reference dosimetry

Abstract

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 $\%dd{\left(10\right)}_x$ and $TP{R}_{10}^{20}$ .

Methods: The EGSnrc application DOSXYZnrc is used to calculate $D\left(10\right)$ , the dose per incident fluence at 10 cm depth in a water phantom irradiated by incident $10\times 10{\mathrm{cm}}^2$ beams of ${}^{60}\mathrm{Co}$ or 6 MV photons. The water phantom dimensions are varied from $30\times 30\times 40{\mathrm{cm}}^3$ to $15\times 15\times 22{\mathrm{cm}}^3$ and occasionally smaller. The $\%dd{\left(10\right)}_x$ and $TP{R}_{10}^{20}$ values are also calculated with care being taken to distinguish $TP{R}_{10}^{20}$ 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 $20\times 20\times 25{\mathrm{cm}}^3$ . A table of corrections to the dose at 10 cm depth in $10\times 10{\mathrm{cm}}^2$ beams of ${}^{60}\mathrm{Co}$ or 6 MV photons are provided and range from no correction to 0.75% for a ${}^{60}\mathrm{Co}$ beam incident on a $20\times 20\times 15{\mathrm{cm}}^3$ phantom. There can be distinct differences in the $TP{R}_{10}^{20}$ values measured using Method A or Method B, especially for smaller phantoms. It is explicitly demonstrated that, within $\pm$ 0.15%, $TP{R}_{10}^{20}$ values for a $30\times 30\times 30{\mathrm{cm}}^3$ 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 $TP{R}_{10}^{20}$ measurements.

Keywords: Monte Carlo; TG51; TRS398; photon beams; reference dosimetry; water phantoms.