On the spatial sensitivity of a 3D dosimetric phantom

Abstract

Purpose The aim of this study is to evaluate the spatial sensitivity of the 3D dosimetric phantom Delta4 (Scandidos, Uppsala, Sweden) investigating whether phantom set up errors affect or not the measurement’s results during plan verification. Materials and methods The Delta4 phantom is used to verify a test IMRT prostate plan that was created on Oncentra v4.3 (Nucletron, Electa) treatment planning system. The patient plan was transferred to the Delta4 geometry and was delivered on a 6 MV photon beam SIEMENS Oncor Impression linac. The measurements’ results are evaluated using the gamma index criteria (3%/3 mm) and the application of the plan is considered successful with an over 90% gamma index passing rate. The reference (no-shift) plan was delivered on the linac with a ±3 mm shift of the phantom at lateral, longitudinal and vertical direction. Results The gamma index passing rate was measured at no-shift position 99.7%. For shifts in every direction gamma index was measured lower than 99.7% for each other position, but still over the passing rate limit 90%. The maximum and minimum gamma index passing rate for lateral direction was measured 96.7% (at +3 mm shift) and 89.6% (at −3 mm shift) respectively, for longitudinal direction 99.4% (at −3 mm shift) and 84.3% (at −3 mm shift) respectively and for vertical direction 96.2% (at −3 mm shift) and 88.7% (at −3 mm shift) respectively. Conclusion In this study, the testing of the spatial sensitivity of Delta4 phantom indicates that for 3%/3 mm gamma-index criteria the phantom lacks sensitivity for shifts less than 3 mm.