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3D Dose Measurements Using the Planned Dose Perturbation Technique (PDP) for the Evaluation of Head and Neck VMAT Treatment


S Koren

S Koren*, R Price, I Veltchev, M Lin, J Fan, C Ma, Fox Chase Cancer Center, Philadelphia, PA

SU-E-T-53 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose:
To evaluate the 3D dose distributions using the PDP technique for VMAT H&N 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 H&N 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.

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