Po14

Purpose

Leipzig surface applicators are used for skin cancer treatment in high-dose-rate (HDR) brachytherapy. In clinical settings, physicians prescribe dose to a point at a depth between 3 to 5 mm for treatment planning. Therefore, it is important to accurately measure this point dose (related to the applicator's output). A parallel plate ionization chamber is a favorable option because it has a flat detection surface that fits the geometry of the applicator's treatment plane. This facilitates an accurate source-to-surface distance (SSD) setup, which is extremely important for the dose measurement because the applicator has a very short SSD. In this study, we quantified the volume averaging effect for using a Roos parallel plate ionization chamber to measure outputs of the Leipzig applicators.

Materials and Methods

The applicators (H10, H20 and H30) have three sizes (10, 20 and 30 mm). For each size, we measured the applicator's profiles with radiochromic films that were placed at depths of 0, 3 and 5 mm in solid water phantom. An Elekta Flexitron HDR ¹⁹²Ir source was used with the applicators to deliver radiation to the films. We digitized the films with a flatbed scanner and measured the intensities along the crossline profile at two locations: at radiation center and at 7.5 mm away from the center. We then calculated the ratios of the two intensities. The key element behind this design is the Roos chamber has an active, circular detecting area of 7.5 mm radius. Thus, the two points were selected at the center and the edge of the Roos chamber. In this way, the ratio quantifies the volume averaging effect (1 being no volume averaging).

Results

The ratios for H10 were significantly higher than those for H20 and H30. At depths of 0 and 3 mm, H10 had ratios of 1.46 and 1.49, respectively, and H20 and H30 had an averaged ratio of 1.08, indicating a 38.0% to 41.0% difference in heterogeneity. This evidence suggests the Roos chamber has significantly higher volume averaging effect for H10. The heterogeneity difference decreased at 5 mm depth. However, H10 still had approximately 10.0% higher ratio than H20 and H30, which had ratios closer to 1 at all three depths.

Conclusions

We found the Roos parallel plate ionization chamber had overall low volume averaging effect for output measurement of Leipzig surface applicators. However, it should not be used with the smallest applicator.