Purpose: To evaluate the 3D dose distributions using the PDP technique for VMAT HN treatments. Methods: The novel PDP algorithm uses the patient structures, TPS dose calculation and plan as a base line, then applies the ARC delivery time dependent ArcCheck (Sun Nuclear, Inc.) measurement with the TPS phantom dose to derive patient dose. Five VMAT HN plans were generated on a Rando phantom with PTV‐to‐skin distances of 0,1,2,3, and 5 mm, using the Eclipse TPS (Varian, Inc.). Treatments were then delivered on a Varian iX linear accelerator. We compared the measured to calculated data by using 3D gamma analysis, and examined the mean and maximum dose of the PTV DVH. Results: By using a recommended 2 mm3 calculation voxel the 3D gamma analysis passed 99.6 to 99.9% for a 3% global dose difference and 3mm DTA with a 5% dose threshold. The PTV organ analysis hot‐to‐cold dose failing point ratio was about 33.8, 21.7 and 22.2, for the 5, 3, and 2 mm PTV‐to‐surface distance respectively. For the 1 mm distance case, the ratio was about 0.45 and for the 0 mm distance the ratio was found to be 0.37. With a PTV‐to‐surface distance decrease, the hot spot was found to increase, and the target coverage and homogeneity were degraded. Conclusions: For the recommended 5 mm PTV‐to‐surface distance the DVH analysis indicated a lower measured target coverage and homogeneity than the planned. This indication is more pronounced as the PTV‐to‐surface distance decreases. The failing points grew colder as the PTV moves closer to the skin, indicating a TPS over estimation of the surface dose, which agrees with TLD skin measurement published data.
ASJC Scopus subject areas
- Radiology Nuclear Medicine and imaging