SU-E-T-345: Dose Comparison of TG-43 and Acuros Algorithm to Account Lateral Scatter in Dose Distribution.
PURPOSE
To study Effect of lateral scatter in dose distribution in Ir192 HDRBTMethods: Brachytherapy phantom was fabricated using tissue-equivalent material E.D.1.045gm/cm3 Phantom has provision to place plastic catheters at every 5mm and TLD and Gafchromic film at every 10 mm distance along vertical direction.Phantom CT scan was performed, images were transferred to Brachyvision TPS, which has option of analytical dose calculation algorithm Acuros.Acuros is Grid Based Boltzmann Solver code which solves Linear Boltzmann equation. We reconstructed single catheter at 5 mm from edge of phantom with three source dwell positions, 10mm apart. Dose was prescribed to single point at 10 mm from central source along transverse axis.Dose calculation was done using TG43 and Acuros algorithm. Dose was measured at 10, 20and 30 mm depth using TLDs and GafchromicEBT3 films. Same procedure was repeated by placing catheter at 10mm, 15mm, 20mm and 30mm from the edge of phantom.


RESULTS
For prescription point 5mm away from edge of phantom, where scatter contribution to total dose is minimum, we found TG43 overestimates dose by 4.5% compared to Acuros. As we move away from edge of phantom lateral scatter contribution increases, dose calculation variation between two algorithms decreases and is within 1% at 20mm from edge of phantom.At greater depths, catheter close to phantom edge we observed TG43 overestimates dose by 7% at 20mm and 10 % at 30mm depth. For catheter at 20mm from phantom edge, TG43 overestimates dose by 3% at 20mm and 4 % at 30mm depth. These results were confirmed by measurements of TLD and Gafchromic filmsConclusion: Non consideration of lateral scatter contribution by TG43 can leads to dose variation up to 5% to target which is nearby skin. Overestimation of dose must be considered while deciding dose to critical structures which are at greater depths.