The Effect of PTV Size on PTV Dose Homogeneity
The Nomos COR VUS1 System is a clinically-accepted method for IMRT treatment planning. Its dose calculation algorithm uses finite-area pencil beams with typical areas2 of either 1 cm2 (1 X 1), 2 cm2 (1 X 2), or 0.5 cm2 (0.5 X 1). In our particular system, a Siemen’s PRIMART3 linear accelerator using step and shoot static fields, we have selected a pencil beam area of 1 cm2. For our optimization parameters, a pencil beam area of 1 cm2 typically leads to 20–80 treatment segments. A smaller pencil beam area leads to better conformation with the given Planning Target Volume (PTV) and better PTV dose homogeneity, but also results in a larger number of beam segments generated by the inverse treatment plan optimization. The purpose of this investigation is to characterize the limitations of a 1 cm2 pencil beam area by investigating the PTV dose homogeneity as a function of the size (diameter) of a given PTV.