SU‐E‐T‐522: Investigation of Underdosage of Total Body Irradiation with Bilateral Irradiation Scheme

Abstract

Purpose:Patient in‐vivo measurements report lower readings than those predicted from TMR‐based treatment planning on TBI patient knees and ankles where rice was placed to fill the gap between patient's legs. This study is to understand and correct the under dosage of Total Body Irradiation(TBI) with rice tissue equivalent bolus placement at TBI treatment patient setup.Methods:Bilateral TBI scheme was investigated with rice bags bolus placing between patient's two legs acting as missing tissue. In‐house TMR based treatment planning system was commissioned with measurements under TBI condition at 10MV, i.e. source‐to‐reference distance 383.4cm with 40×40cm field size with 1cm thickness Lucite. Predictions of patient specific dose points are reported at different sites with 200cGy prescription at patient umbilicus point. Solid water and rice bag phantoms are used at TBI conditions for the attenuation factor verification and CT scanned to verify the CT number and electron density.Results:We found that the rice bag bolus overall density is 11% lower than the water; however, the attenuation factor of rice bags could become 15% lower than that of water at TBI condition. This overestimate of rice bag electron density could cause the lack of lateral scatter and the lack of backscatter. This could Result in an overestimate of dose at in‐vivo dosimeter measurement points with TMR‐based treatment planning systems. Observations of patient specific optically stimulated luminescent dosimeters(OSLDs) were used to confirm this overestimation. Measurements of setups with increasing the rice bag filled patient leg separation were performed to demonstrate eliminating the overdose issue.Conclusion:Rice bolus has a lower electron density than water does(11%) but results in 15% lower in attenuation factor at TBI condition. This effect was observed in patient delivery with OSLD measurements and can be corrected by increasing the filling rice bolus thickness with 15% longer of separation.