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SU‐E‐T‐554: PTV to Skin Proximity for Head and Neck IMRT Treatment Planning
Author(s) -
Price R,
Koren S,
Hossain M,
Veltchev I,
Galloway T,
Flanagan P,
Ma C
Publication year - 2012
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4735643
Subject(s) - imaging phantom , voxel , nuclear medicine , head and neck , dosimetry , radiation treatment planning , medicine , radiation therapy , radiology , surgery
Purpose: 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. Methods: 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. Results: 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 increasing PTV to skin distance and a CI of unity results for grid sizes of 1–3mm 3 between 4 and 4.5mm. Hot spot decreases as the PTV moves away from the surface and falls below 110% at 4mm. Underdosage worsens as the PTV approaches the skin. Conclusions: For decreasing PTV‐to‐skin distance with this TPS, isodose conformality decreases, ‘hot spot’ increases, and target coverage degrades. Surface dose is accurately predicted for a 2mm 3 voxel size, while choosing a finer or coarser grid results in underestimation or overestimation, respectively. All of the above appear to hold for VMAT.