SU-E-T-554: PTV to Skin Proximity for Head and Neck IMRT Treatment Planning

Medical Physics
Robert A PriceC Ma

Abstract

The goal of this work was to evaluate measured vs. calculated surface dose as a function of PTV-to-skin proximity and calculation matrix oxel size, determine effects on plan quality, and provide parameters and levels of uncertainty for clinical use. A right-sided CTV with the lateral border 5mm from the surface was delineated on the CT data of a head and neck phantom. A 5mm PTV was generated except laterally where distances of 0-5mm were used. A 7-field IMRT plan was generated using the Eclipse TPS. Optimization was performed where 95% of the PTV receives the prescription dose using a matrix size of 2mm(3) . Dose calculations were repeated for grid sizes of 1, 3 and 5mm(3) . For each plan nine point dose values were obtained just inside the phantom surface, corresponding to a 2cm(2) grid near the central target region. Nine ultra-thin TLDs were placed on the phantom surface corresponding to the grid. Measured and calculated dose values were compared. Conformality, homogeneity and target coverage were compared. Surface dose is over-estimated by the TPS by 21 and 8% for 5 and 3mm(3) voxels, respectively and accurately predicted for 2mm(3) voxels. A voxel size of 1mm(3) results in underestimation of 13%. Conformality improves with...Continue Reading

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