Dose discrepancies in the buildup region and their impact on dose calculations for IMRT fields.
OBJECTIVE
Dose accuracy in the buildup region for radiotherapy treatment planning suffers from challenges in both measurement and calculation. This study investigates the dosimetry in the buildup region at normal and oblique incidences for open and IMRT fields and assesses the quality of the treatment planning calculations.


METHODS
This study was divided into three parts. First, percent depth doses and profiles (for 5 x 5, 10 x 10, 20 x 20, and 30 x 30 cm2 field sizes at 0 degrees, 45 degrees, and 70 degrees incidences) were measured in the buildup region in Solid Water using an Attix parallel plate chamber and Kodak XV film, respectively. Second, the parameters in the empirical contamination (EC) term of the convolution/ superposition (CVSP) calculation algorithm were fitted based on open field measurements. Finally, seven segmental head-and-neck IMRT fields were measured on a flat phantom geometry and compared to calculations using gamma and dose-gradient compensation (C) indices to evaluate the impact of residual discrepancies and to assess the adequacy of the contamination term for IMRT fields.


RESULTS
Local deviations between measurements and calculations for open fields were within 1% and 4% in the buildup region for normal and oblique incidences, respectively. The C index with 5%/1 mm criteria for IMRT fields ranged from 89% to 99% and from 96% to 98% at 2 mm and 10 cm depths, respectively. The quality of agreement in the buildup region for open and IMRT fields is comparable to that in nonbuildup regions.


CONCLUSIONS
The added EC term in CVSP was determined to be adequate for both open and IMRT fields. Due to the dependence of calculation accuracy on (1) EC modeling, (2) internal convolution and density grid sizes, (3) implementation details in the algorithm, and (4) the accuracy of measurements used for treatment planning system commissioning, the authors recommend an evaluation of the accuracy of near-surface dose calculations as a part of treatment planning commissioning.
Department of Radiation Oncology, University of Michigan, Ann Arbor, Michigan 48109, USA. shuihsu@umich.edu
Shu-Hui
SH
Hsu
Jean M
JM
Moran
Yu
Y
Chen
Ravi
R
Kulasekere
Peter L
PL
Roberson
D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated; D015203:Reproducibility of Results; D035501:Uncertainty