Abstract
The purpose of this study is to examine the accuracy of calculated tissue phantom ratio (TPR) data with measured TPR values of a 6MV photon beam. TPR was calculated from the measured percent depth dose (PDD) values using 2 methods – with and without correcting for the differences in peak scatter fraction (PSF). Mean error less than 1% was observed between the measured and calculated TPR values with the PSF correction, for all clinically relevant field sizes and depths. When not accounting for the PSF correction, mean difference between the measured and calculated TPR values was larger than 1% for square field sizes ranging from 3 cm to 10 cm.
Highlights
There are several different radiation dosimetry quantities that are in use – percentage depth dose (PDD), tissue-phantom ratio (TPR), tissue maximum ratio (TMR), tissue air ratio (TAR), backscatter factor (BSF).[1]
TPR values were measured along the central axis (CAX) of 6MV photon beam of 7 field sizes mentioned earlier up to a maximum depth of 22 cm
A comparison study was performed between the 3 methods - (a) measured TPR data; (b) TPR values calculated from PDD without peak scatter factor (PSF) correction; and (c) TPR values calculated from PDD with PSF correction
Summary
There are several different radiation dosimetry quantities that are in use – percentage depth dose (PDD), tissue-phantom ratio (TPR), tissue maximum ratio (TMR), tissue air ratio (TAR), backscatter factor (BSF).[1] Even though all these quantities can be determined empirically, most of the tabulated data have been calculated from the measured PDD of open field central axis (CAX).[2] While it is easier to measure radiation beam data in the form of PDD, it is often convenient to calculate dose per monitor unit (MU) using isocentric beam data based on TPR values. The PDD values are measured at a fixed source to surface distance (SSD) along the CAX and TPR at a fixed source to axis distance (SAD). Calculation of TPR values from the measured PDD data involves inverse square factor and possibly, the peak scatter factor (PSF) correction. PSF or phantom scatter factor (denoted as Sp) is defined as the ratio of absorbed dose to water at the depth of dose maximum to the absorbed dose in free air at the same location for a given radiation beam
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