The dosimetric and radiobiological impact of calculation grid size on head and neck IMRT

Small-volume structures usually found in the head and neck may be susceptible to dose-volume averaging, which has not been studied. Here, the impact of calculation grid size on dose distribution for tumor control probability (TCP) and normal tissue complication probability (NTCP) is investigated for head and neck (H&N) intensity modulated radiation therapy (IMRT).

IMRT plans were generated for H&N patients with different grid sizes (1-5 mm) to calculate dose and related TCP and NTCP. Dose parameters such as D_2%, D_50%, D_98%, and the homogeneity and conformity indices were calculated. The dose distributions were also compared with measured dose for all IMRT plans. A 1-tailed pair t test was used to analyze the data.

The mean dose to planning target volume and TCP decreases with increasing grid size, whereas for organs at risk (OARs), mean dose, and NTCP increase with increasing grid size. The average mean dose to planning target volume decreases linearly with grid size, but for OARs such as cochlea, parotid gland, and the spinal cord, mean dose increases with grid size. IMRT dose verification showed that the number of points meeting the gamma criterion of 3%/3 mm increased with decreasing grid sizes. The homogeneity index for the target increased up to 60% and conformity index decreased on average by 3.5% between 1- to 5-mm grid that resulted in decreased TCP and increased NTCP. A 1-tailed pair t test showed significant statistical differences among various grid size calculations compared with 1-mm grids.

Based on our findings, the smallest possible grid size should be used for accurate dose calculation in small-volume structures-especially in H&N planning. A smaller calculation grid provides superior dosimetry with improved TCP as well as reduced NTCP, which is more pronounced for smaller OARs.