Validation of Calculated Tissue Maximum Ratio (TMR) Obtained from Measured Percentage Depth Dose (PPD) Data for High Energy Photon Beam (6 MV and 15 MV)

Abstract

During external beam radiotherapy treatments, high doses are delivered to the cancerous cell. Accuracy and precision of dose delivery are primary requirements for effective and efficiency in treatment. This leads to the consideration of treatment parameters such as percentage depth dose (PDD), tissue air ratio (TAR) and tissue phantom ratio (TPR), which show the dose distribution in the patient. Nevertheless, tissue air ratio (TAR) for treatment time calculation, calls for the need to measure in-air-dose rate. For lower energies, measurement is not a problem but for higher energies, in-air measurement is not attainable due to the large build-up material required for the measurement. Tissue maximum ratio (TMR) is the quantity required to replace tissue air ratio (TAR) for high energy photon beam. It is known that tissue maximum ratio (TMR) is an important dosimetric function in radiotherapy treatment. As the calculation methods used to determine tissue maximum ratio (TMR) from percentage depth dose (PDD) were derived by considering the differences between TMR and PDD such as geometry and field size, where phantom scatter or peak scatter factors are used to correct dosimetric variation due to field size difference. The purpose of this study is to examine the accuracy of calculated tissue maximum ratio (TMR) data with measured TMR values for 6 MV and 15 MV photon beam at Sweden Ghana Medical Centre. With the help of the Blue motorize water phantom and the Omnipro-Accept software, PDD values from which TMRs are calculated were measured at 100 cm source-to-surface distance (SSD) for various square field sizes from 5x5 cm to 40x40 cm and depth of 1.5 cm to 25 cm for 6 MV and 15 MV x-ray beam. With the same field sizes, depths and energies, the TMR values were measured. The validity of the calculated data was determined by making a comparison with values measured experimentally at some selected field sizes and depths. The results show that; the reference depth of maximum dose (dmax) were 1.5 cm for both PDD and TMR at 6 MV and 2.5 cm and 2.6 cm for PDD and TMR at 15 MV respectively. The minimum relative percentage differences recorded were 1.6 % for 40x40 cm field size and 1.2 % for 5x5 cm field size at 6 MV and 15 MV respectively, while the maximum relative percentage differences recorded were 2.0 % for 10x10 cm field size and 1.4 % for 40x40 cm at 6 MV and 15 MV x-ray beam respectively. The relative differences between the calculated and the measured TMRs increases with depth at all field sizes. It can be concluded that; although based on the results the general agreement between the calculated and measured TMRs is good but for clinically high energy photon beam, it is required that TMR should be measured directly especially when treating deep internal tumor. However it is recommended that comparison of the measured TMR and PDD with that obtained during commission, should be included in the annual quality assurance (QAs) of the machine at SGMC.