SU‐E‐T‐350: Investigation About Source of Errors in Treatment Planning of HDR Brachytherapy by Using a New Phantom Design Combined with Gafchromic Films and TG‐43 Calculation

Purpose: GZP6 machine is used for high dose rate (HDR) brachytherapy treatment. In this project, treatment planning of GYN brachytherapy of this machine has been evaluated using a newly fabricated phantom along Gafchromic films and TG‐43 calculation. Methods: A new QA phantom has been designed and fabricated from 90 slabs of Perspex to accommodate ovoids assembly of GYN brachytherapy treatment. Two large oviods with 3 cm diameter was used for this assembly. EBT Gafchoromic films were exactly sandwiched between slabs and completely fitted to shape of ovoids for verification of GZP6 treatment planning. Plan designed to deliver 2.5 Gy dose to surface of oviods. The measured doses values at several points of interest were evaluated. The measured data were compared with the calculated values by the GZP6 treatment planning software. With seeing large difference between film dosimetry and planning, Air kerma strength (Sk) was obtained from Monte Carlo simulation and considered for TG‐43 calculation to reduce error. Results: The Result of these investigations have indicated differences of up to ± 23.4 % between the treatment planning and measured film dosimetry data at different points in GYN implant with GZP6 60Co HDR system. Between air kerma strength used in planning and Sk from simulation, 26% difference was calculated. TG‐43 calculation with simulated Sk can reduces difference between measurements to around 9%. Conclusion: The new phantom could be utilized for the QA procedure of the GZP6 60Co HDR system or the Ir‐192 HDR system to confirmation the accuracy of treatment planning in GYN implants and to find source of systematic errors.