Planning target volume-to-skin proximity for head-and-neck intensity modulated radiation therapy treatment planning

Abstract

Purpose: The goal of this work is to evaluate planning target volume (PTV)-to-skin proximity versus plan quality as well as the effects of calculation voxel size on dose uncertainty in the surface region.

Methods and materials: A right-sided clinical target volume with the lateral border 5 mm from the surface was delineated on the computed tomographic data of a head-and-neck phantom. A 5-mm PTV expansion was generated except laterally where distances of 0-5 mm were used. A 7-field intensity modulated radiation therapy plan was generated using the Eclipse treatment planning system. Optimization was performed where 95% of the PTV receives the prescription dose using a voxel size of 2 mm(3). Dose calculations were repeated for voxel sizes of 1, 3, and 5 mm(3). For each plan, 9 point dose values were obtained just inside the phantom surface, corresponding to a 2 cm × 2 cm grid near the central target region. Nine ultrathin thermoluminescent dosimeters were placed on the phantom surface corresponding to the grid. Measured and calculated dose values were compared. Conformality, homogeneity, and target coverage were compared as well. This process was repeated for volumetric modulated arc therapy (VMAT) calculated with a 2-mm(3) voxel size.

Results: Surface dose is overestimated by the treatment planning system (TPS) by approximately 21% and 9.5% for 5- and 3-mm(3) voxels, respectively, and is accurately predicted for 2-mm(3) voxels. A voxel size of 1 mm(3) results in underestimation by 11%. Conformality improves with increasing PTV-to-skin distance and a conformality index of unity is obtained for grid sizes between 1 and 3 mm(3) and PTV-to-skin distances of 4-4.5 mm. Hot spot also improves and falls below 110% at 4-mm PTV-to-skin distance. Underdosage worsens as the PTV approaches the skin. All of the above appear to hold for volumetric modulated arc therapy.

Conclusions: For decreasing PTV-to-skin distance with this TPS, isodose conformality decreases, "hot spot" increases, and target coverage degrades. Surface dose is overestimated when voxel sizes greater than 2 mm(3) are chosen, and underestimated for smaller voxels.