Quantitative evaluation of increase in surface dose by immobilization thermoplastic masks and superficial dosimetry using Gafchromic EBT film and Monte Carlo calculations

Imad Ali, Chance Matthiesen, Ozer Algan, Spencer Thompson, Carl Bogardus, Terence Herman, Salahuddin Ahmad

Research output: Contribution to journalArticlepeer-review

6 Scopus citations

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 cm2 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.

Original languageEnglish (US)
Pages (from-to)319-326
Number of pages8
JournalJournal of X-Ray Science and Technology
Volume18
Issue number3
DOIs
StatePublished - 2010
Externally publishedYes

Keywords

  • Gafchromic EBT film
  • IMRT
  • Monte Carlo simulation
  • Skin dose
  • thermoplastic immobilization mask
  • toxicity

ASJC Scopus subject areas

  • Radiation
  • Instrumentation
  • Radiology Nuclear Medicine and imaging
  • Condensed Matter Physics
  • Electrical and Electronic Engineering

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