Dosimetric comparison of AP/PA and bilateral geometries for total body irradiation treatment

Abstract

Total body irradiation (TBI) is an external radiotherapy technique. Its aim is to deliver a therapeutic dose uniformly within ± 10% of the absorbed dose to the prescription point. In the present study, the TBI technique was implemented in anterior/posterior (AP/PA), and bilateral geometry with photons from a 6 $\mathrm{MV}$ and 18 $\mathrm{MV}$ accelerator. The TBI technique was implemented on an Alderson Rando phantom at 312 $\mathrm{cm}$ source surface distance. During bilateral fraction, rice bags were applied as tissue compensators. To reduce the lung’s absorbed dose to the acceptance level, in AP/PA geometry lung blocks made of Cerrobend were used. The required monitor unit (MU) for each fraction was calculated regarding depending on the prescribed dose and beam output. Gafchromic EBT₃ films were used for dosimetry between the phantom layers in eight selected points. It is demonstrated that dose uniformity for AP/PA geometry with 6 $\mathrm{MV}$ and 18 $\mathrm{MV}$ photons was within ± 10%. In contrast, for the bilateral geometry the dose uniformity was not acceptable for both studied energies; However, the results for 18 $\mathrm{MV}$ were better than those for 6 $\mathrm{MV}$. Dose accuracy for all measurements was within ± 5 of the prescribed dose. The absorbed dose to the lungs was successfully reduced using the lung blocks. By combining different therapeutic geometries and energies over six fractions, the results of uniformity and accuracy of dose delivery could be improved. It is concluded that the introduced TBI method achieved good dose accuracy and acceptable dose uniformity. Lungs absorbed dose was lower than 10 $\mathrm{Gy}$ using the lungs blocks. Based on these results, the TBI technique can now be implemented in radiotherapy at Tehran’s Imam Hospital. The approach developed in the present study can be used and adapted to match with the conditions at other hospitals.