O2. Verification of an irregular field algorithm of a treatment planning system using a locally designed pelvic phantom: A simple design low cost phantom suitable for research and quality assurance

Abstract

Introduction The accuracy of a treatment planning system (TPS) depends on the mathematical algorithm it uses. One major problem with most radiotherapy centers in Nigeria is the non-availability of QA phantoms for treatment planning system verification. The aim was to design a low cost phantom and to use the designed phantom to verify the accuracy of the Irregular Field Algorithm of a Precise PLAN 2.16 TPS. Materials and Methods The designed pelvic phantom was made of Plexiglas with six tissue equivalent inserts and an ion-chamber port. The pelvic phantom was designed by using elemental compositions as follows: prostate was C:MgO:H2O(56%:21%:23%), bladder was C:MgO:H2O(52%:25.5%:22.5%), adipose was C:MgO:H2O(70%:11%:19%), muscle was C:MgO:H2O(55%:23%:22%), rectum was C:MgO:H2O(56%:26.5%:27.5%) and bone was C:MgO:H2O:Ca (37%:25%:23%:15%). A Hi-Speed CT simulator was used for acquiring images and CT numbers of the designed pelvic phantom, a Precise PLAN TPS was used for the treatment planning. Absorbed dose measurements were carried out at the Elekta-Precise Linear Accelerator using a pre-calibrated NE 2570/1 farmer-type ion-chamber. Data analysis was done using GraphPad Prism 7.0 statistics software. Results The maximum percentage deviation with large field sizes of 22 × 25 cm2 for six different inhomogeneous inserts was −3.95% and bone only homogeneous inserts was 2.38%. The maximum percentage deviation with small field sizes of 5 × 5 cm2 with six different inhomogeneous inserts was −3.57%. The percentage deviation between the solid water phantom and the locally designed pelvic phantom was −3.35%. Conclusion The Irregular Field Algorithm showed an overall accuracy of approximately ± 4% with the locally designed pelvic phantom for both large and small field sizes.