Poster - Thur Eve - 59: Dosimetric evaluation on the variation of PTV coverage due to patient size reduction using the prostate dose-volume factor in prostate radiotherapy.

We proposed to use the prostate dose-volume factor (PDVF), derived from the dose-volume dataset of planning target volume (PTV) in prostate radiotherapy to evaluate treatment plans of prostate volumetric modulated arc therapy (VMAT) and intensity modulated radiotherapy (IMRT). To demonstrate plan evaluation using PDVF, VMAT and 7-beam IMRT plans were created in three patients with prostate volumes equal to 32, 48.4 and 86.5 cm . Dose variation of PTV was made by reducing the body contour of the patients with reduced depth equal to 0.5 - 2 cm, mimicking a patient size reduction in the treatment. The Gaussian error function was used to model the cumulative dose-volume histogram of the PTV, and PDVF was calculated as per the parameters of the error function. PDVF = 1 reflects an ideal PTV coverage (i.e. 100% prescribed dose in 100% target volume). We found that for PDVF ranged 0.98 - 1 in prostate VMAT and IMRT without patient size change, reduced depth led to PDVF decreasing 0.03 ± 4.7 × 10 (VMAT) and 0.04 ± 9.7 × 10 (IMRT) per cm for the patients. The variation of PTV coverage on the prostate volume due to the reduced depth was less significant in VMAT plans than IMRT. It is concluded that PDVF was successfully used to evaluate the variation of PTV coverage due to the weight loss of patient in prostate VMAT and IMRT. Degradation of PTV coverage in prostate VMAT regarding patient size reduction is less significant than that in IMRT.