Development of an optimum photon beam model for head and-neck intensity-modulated radiotherapy.

Intensity-modulated radiotherapy (IMRT) for complex sites such as tumors of the head and neck requires a level of accuracy in dose calculation beyond that currently used for conformal treatment planning. Recent advances in treatment planning systems have aimed to improve the dose calculation accuracy by improving the modeling of machine characteristics such as interleaf leakage, tongue and groove, and rounded multileaf collimator (MLC) leaf ends. What is uncertain is the extent to which these model parameters improve the agreement between dose calculation and measurements for IMRT treatments. We used Pinnacle version 7.4f (Philips Medical Systems, Andover, MA) to carry out optimization of additional photon-beam model parameters for both an Elekta Precise (Elekta, Stockholm, Sweden) and a Varian (Varian Medical Systems, Palo Alto, CA) linear accelerator (LINAC). One additional parameter was added to the beam models in turn, and associated models were commissioned to investigate the dosimetric impact of each model parameter on 5 clinical head-and-neck IMRT plans. The magnitude and location of differences between the models was determined from gamma analysis of the calculated planar dose maps. A final model that incorporated all of the changes was then commissioned. For the Elekta Precise, the impact of all the changes was determined using a gamma analysis as compared with measured films. Cumulative differences of up to more than 3%/3 mm were observed when dose distributions with and without all of the model changes were compared. Individually, for both LINACs, the addition of modeling for the rounded MLC leaf ends caused the most dramatic change to the calculation of the dose distribution, generating a difference of 3%/3 mm in up to 5% of pixels for the 5 patient plans sampled. The effect of tongue-and-groove modeling was more significant for the Varian LINAC (at 1%/1 mm, mean of 25% of pixels as compared with 5% of pixels with the Elekta Precise LINAC). The combined changes to the Elekta model were found to improve agreement with measurement. Current commercially available treatment planning systems offer accuracy sufficient for clinical implementation of head-and-neck IMRT. For this treatment site, the ability to accurately model the rounded MLC leaf ends has the greatest affect on the similarity of the calculated dose distribution to measurements. In addition, for the Varian LINAC, modeling of the tongue-and-groove effect was also advantageous.