Dose variations with varying calculation grid size in head and neck IMRT.

Ever since the advent and development of treatment planning systems, the uncertainty associated with calculation grid size has been an issue. Even to this day, with highly sophisticated 3D conformal and intensity-modulated radiation therapy (IMRT) treatment planning systems (TPS), dose uncertainty due to grid size is still a concern. A phantom simulating head and neck treatment was prepared from two semi-cylindrical solid water slabs and a radiochromic film was inserted between the two slabs for measurement. Plans were generated for a 5,400 cGy prescribed dose using Philips Pinnacle(3) TPS for two targets, one shallow ( approximately 0.5 cm depth) and one deep ( approximately 6 cm depth). Calculation grid sizes of 1.5, 2, 3 and 4 mm were considered. Three clinical cases were also evaluated. The dose differences for the varying grid sizes (2 mm, 3 mm and 4 mm from 1.5 mm) in the phantom study were 126 cGy (2.3% of the 5,400 cGy dose prescription), 248.2 cGy (4.6% of the 5,400 cGy dose prescription) and 301.8 cGy (5.6% of the 5,400 cGy dose prescription), respectively for the shallow target case. It was found that the dose could be varied to about 100 cGy (1.9% of the 5,400 cGy dose prescription), 148.9 cGy (2.8% of the 5,400 cGy dose prescription) and 202.9 cGy (3.8% of the 5,400 cGy dose prescription) for 2 mm, 3 mm and 4 mm grid sizes, respectively, simply by shifting the calculation grid origin. Dose difference with a different range of the relative dose gradient was evaluated and we found that the relative dose difference increased with an increase in the range of the relative dose gradient. When comparing varying calculation grid sizes and measurements, the variation of the dose difference histogram was insignificant, but a local effect was observed in the dose difference map. Similar results were observed in the case of the deep target and the three clinical cases also showed results comparable to those from the phantom study.