LAUSR

Purpose The aim of this study is to evaluate the spatial sensitivity of the Delta4 diode array phantom (Scandidos, Uppsala, Sweden) investigating how set up errors affect the measurement’s results during head and neck treatment plan verification. Method The Delta4 phantom is used to verify ten IMRT head and neck plans, created on Oncentra v4.5.2 (Nucletron, Elekta) treatment planning system. Patient plans were transferred to the Delta4 geometry and were delivered on a 6MV photon beam SIEMENS Oncor Impression linac. Each reference (no-shift) plan was then delivered on the linac with a ± 3 mm shift of the phantom at lateral, longitudinal and vertical direction. The measurements’ results for all delivered plans were evaluated using the gamma index criteria (3%/3 mm). The application of the treatment plan is considered successful with an over 90% gamma index passing rate. Results The gamma index passing rate was measured at no-shift position (96.1  ±  0.7) %. Mean gamma index for each direction (lateral, longitudinal, vertical) was measured as 85.5% (SD = 2.6), 85.7% (SD = 4.5) and 85.2% (SD = 4.5) respectively. For all shifts, in all directions, the mean gamma index passing rate was found enough below the passing rate limit of 90% and more specifically 10.6% less than the no-shift measured gamma index. Conclusion Delta4 is an efficient QA tool for individual patient treatment plan verification exhibiting high reproducibility of results for γ -index. Shifts of 3 mm reveal significant deviation of measured 3%/3 mm gamma index passing rates, indicating high spatial sensitivity and efficiency of this QA tool. Future work with 3DVH software analysis is needed to report more statistics apart from γ -index values.