Room: Exhibit Hall | Forum 4
Purpose: The purpose of this study is to determine the dosimetric effects of varying dwell position step size during Ir-192 skin lesion treatment utilizing a flap applicator.
Methods: Utilizing the Elekta Flexitron HDR afterloader and a four-channel Freiburg Flap applicator, treatment plans for a hypothetical 3 cmÂ² treatment area were created using a 1mm, 2mm, 3mm, 5mm, and 10mm step size, prescribing uniform dose to a depth of 5 mm from the skin surface. The dwell positions were located five mm above skin surface. The plans were delivered to solid water with EBT3 film placed at the surface and at several depths along the central axis. Two sets of measurements were acquired; one with four cm of bolus on top of the applicator to replicate the dose calculation conditions of an infinite water medium, and another with the clinical setup of applicator only. Film measurements were compared with the corresponding point calculations from the TPS.
Results: For clinical setup, smallest step size has lowest surface maximum dose, which increases with increasing step size. Skin surface maximum doses were 18.12, 19.28, 20.07, 20.21, and 21.41 Gy for an 8 Gy prescription. Measured maximum surface dose is approximately 10% higher than predicted by the TPS. Without bolus, the prescription plane mean dose is 5-7% lower than predicted by the TPS. Surface heterogeneity increases with step size. Adding bolus on top of applicator marginally improves prescription plane dose agreement with TPS while increasing surface maximum dose and inhomogeneity.
Conclusion: For lowest surface maximum dose and best surface dose homogeneity, we recommend using step sizes of two to three millimeters. Increasing step size only serves to increase surface maximum dose with no improvement in prescription plane dose accuracy or uniformity. To ensure desired prescription plane dose is achieved, four cm bolus should be added.