Quantitative evaluation of increase in surface dose by
immobilization thermoplastic masks and superficial dosimetry using Gafchromic
EBT film and Monte Carlo calculations
Affiliations: Department of Radiation Oncology, University of
Oklahoma Health Sciences Center, Oklahoma City, OK, USA
Note: [] Corresponding author: Imad Ali, Ph.D., %Assistant Professor,
Medical Physics, Radiation Oncology Department, University of Oklahoma Health
Sciences Center, 825 N.E. 10th Street, OUPB 1430, Oklahoma City, OK 73104, USA.
Tel.: +1 405 271 8290; Fax: +1 405 271 9240; E-mail: iali@ouhsc.edu
Abstract: Purpose: To investigate the increase in surface dose under
immobilization thermoplastic masks by measurements and calculation in the
build-up region using Gafchromic films and Monte Carlo simulation. Materials and methods: Surface doses were measured underneath three
thermoplastic masks in open fields using 6 and 18 MV photon beams. These masks
are used to immobilize patients for head and neck (H&N), pelvis and
thoracic treatment. Gafchromic EBT films were placed on the top of the flat
surface of a phantom partially underneath the mask and exposed in open 10
× 10 cm^2 photon fields. The depth doses were
calculated using BEAMnrc Monte Carlo code for water-equivalent film detectors
with different layers of thickness ranging from 50 μm to 2.5 mm and compared
with film measurements. Results: Surface dose increased by a factor of 3 to 4 underneath the
mask relative to the open areas and 6 MV beam delivers more skin dose than 18
MV. H&N mask increased surface dose by a factor of 3 using 18 MV and a
factor of 4 using 6 MV. In addition, increase in surface dose depended on the
type of the mask, the size of openings, and the amount of stretching performed
during the mask preparation. The measured depth doses were compared with
BEAMnrc Monte Carlo calculation for water-equivalent detectors using different
sizes. The calculated depth dose depended significantly on the thickness of
film detector and varies by more than 15% using layer thickness of 2.5 mm
compared to 50 μm. Surface doses measured by Gafchromic EBT films agreed
within 3% with the Monte Carlo calculations using a small detector layer of 50
μm. Conclusion: Thermoplastic masks used for patient immobilization can
significantly increase skin doses by up to a factor of 4 more than that without
the mask using 6 MV beams. The skin reactions resulting from thermoplastic
masks should be monitored and corrective measures should be taken during
treatment such as partially removing the mask over skin areas with
complications and optimizing the skin dose in IMRT planning. Gafchromic EBT
films provide accurate skin dosimetry which agrees within 3% with Monte Carlo
calculations. Gafchromic EBT film makes an excellent tool for measuring depth
doses in the buildup region and these data can be applied for treatment
planning calculations and IMRT optimization.
Keywords: Skin dose, toxicity, thermoplastic immobilization mask, Gafchromic EBT film, IMRT, Monte Carlo simulation
