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Heterogeneous Dose Increases Estimated for Intraluminal High Dose Rate Brachytherapy
L Young1,2 , T Smith2 , N Cao1,2*, A Kalet1,2 , C Dempsey3 , F Yang4, J Meyer1,2 , S Patel1 , G Laramore1 , W Koh1,2 , (1) Univ Washington, Seattle, WA, (2) Seattle Cancer Care Alliance, Seattle, WA, (3) University of Newcastle, Newcastle, Australia, (4) University of Miami, Miami, FL,
Presentations
TU-C3-GePD-T-3 (Tuesday, August 1, 2017) 10:30 AM - 11:00 AM Room: Therapy ePoster Lounge
Purpose: : The AAPM Task Group (TG) 43 formalism used by commercial brachytherapy treatment planning systems assumes the irradiated media is homogeneous. This study evaluates the change in tissue dose influenced by the air gap between the applicator and treatment site for intraluminal brachytherapy using film dosimetry.
Methods: GafChromic EBT3 film was irradiated with an 192Ir high dose rate (HDR) brachytherapy source in a gel phantom. Brachyvision treatment plans were generated from cone beam CT images of the gel phantom. The prescribed dose of 5 Gy was normalized to a reference line 2-mm from the phantom surface. Films were acquired in water and with 7.3 and 10.9 mm (radii of cylinders tested)air gaps created with the catheter centered in a hollow cylinder. The red channel was separated from the scanned film profiles using ImageJ software. The percent depth dose (PDD) profiles were normalized to the point of maximum dose prior to comparison and analysis.
Results: On average, the differences in relative dose between an air gap of 7.3 mm and water ranged between 1.8% and 3.9% for distances from 7.3 mm to 12 mm. At distances beyond 20 mm from the phantom center, the dose discrepancies increased to 6.2%. Slightly larger dose discrepancies were found when the air gap was increased to 10.9 mm. For the same distances (7.3 mm to 12 mm), the average dose differences were 3.6% to 5.6%. Variations in the measured dose discrepancies were observed depending on whether profiles were measured between or centered on a source dwell position.
Conclusion: The dose discrepancies caused by the intraluminal applicator air gap are relatively small in close proximity to the prescription point and are within the normal margin of overall treatment uncertainty. Treatment planning with heterogeneous corrections is ideal, but not essential for small air gap diameters.
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