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Effects of immobilization mask material on surface dose
Scott W Hadley1, Robin Kelly, Kwok Lam
1Department of Radiation Oncology Physics, The University of Michigan, Box 0010, 1500 E. Medical Center Drive, Ann Arbor, Michigan 48109, USA. swhadley@.umich.edu
Journal of Applied Clinical Medical Physics
|March 17, 2005
Summary
Thermoplastic masks used in radiation therapy can increase surface dose, reducing the skin-sparing effect. This study quantifies the dose increase from different mask hole sizes, finding significant variations.
Area of Science:
- Medical Physics
- Radiation Oncology
- Radiotherapy Physics
Background:
- Thermoplastic masks are crucial for patient immobilization in radiotherapy, ensuring accurate treatment delivery.
- The presence of mask material can compromise the skin-sparing effect of megavoltage X-ray beams.
- Masks with perforations are designed to mitigate this effect, with varying hole sizes available.
Purpose of the Study:
- To investigate and quantify the increase in surface dose caused by thermoplastic masks.
- To compare the impact of different mask perforation sizes on surface dose.
- To assess the reduction in the skin-sparing effect due to mask material.
Main Methods:
- Measurements of tissue maximum ratios (TMRs) were performed using an Attix parallel plate chamber and solid water phantoms.
- Experiments were conducted with and without thermoplastic mask materials for 6-MV and 15-MV X-ray beams.
- The effective water-equivalent thickness of the masks was determined from TMR curves to estimate surface dose increases.
Main Results:
- The buildup effect was found to be equivalent to 0.6 mm to 2.2 mm of water, depending on mask stretching.
- For 6-MV beams, surface dose increased from 16% to 27%-61%.
- For 15-MV beams, surface dose increased from 12% to 18%-40%.
Conclusions:
- Thermoplastic masks significantly increase surface dose in radiotherapy, impacting the skin-sparing effect.
- The degree of surface dose increase varies with mask perforation size and beam energy.
- Careful consideration of mask properties is necessary to optimize dose delivery and minimize skin reactions.