Impact of source position on high-dose-rate skin surface applicator dosimetry

Abstract

Purpose Skin surface dosimetric discrepancies between measured and treatment planning system (TPS) predicted values were traced to source position sag inside the applicator and to source transit time. We quantified their dosimetric impact and propose corrections for clinical use. Material and methods We measured the dose profiles from the Varian Leipzig-style HDR skin applicator, using EBT3 film, photon diode, and optically stimulated luminescence dosimeter for three different GammaMedplus™ HDR afterloaders. The measured dose profiles at several depths were compared with BrachyVision Acuros™ calculated profiles. To assess the impact of the source sag, two different applicator orientations were considered. The dose contribution during source transit was assessed by comparing diode measurements using a HDR timer and an electrometer timer. Results Depth doses measured using the three dosimeters were in good agreement, but were consistently higher than the Acuros dose calculations. Measurements with the applicator face up were significantly (exceeding 10%) lower than those in the face down position, due to source sag inside the applicator. Based on the inverse square law, the effective source sag was evaluated to be about 0.5 mm from the planned position. The additional dose during source transit was evaluated to be about 2.8% for 30 seconds of treatment with a 40700 U (10 Ci) source. Conclusion With a very short source-to-surface distance, the small source sag inside the applicator has a significant dosimetric impact. This effect is unaccounted for in the vendor’s treatment planning template, and should be considered before the clinical use of the applicator. Further investigation of other applicators with large source lumen diameter may be warranted.

Personalized High-Dose-Rate Brachytherapy with Non-Sealed Rhenium-188 in Non-Melanoma Skin Cancer

“…If the lesion to be treated is located in areas with complex geometry such as ears, lips or genitals, the placement of the applicator is cumbersome and dosimetry calculations can become very complicated. They are commonly oversimplified in the planning software [5].…”

Advanced methods for quality assurance and dose distribution improvement for high dose rate brachytherapy

“…This deviation is due to a higher uncertainty on the transit time correction for lower dwell times, and the fact that transit time correction applied for the HDR applicator was measured for a different geometry [134] and may not be correct for the present applicator.…”

“…The resulting dose distribution was measured separately 3 times at 1, 2, 5 and 10 mm depths in a PTW RW3 solid water slab phantom (PTW Freiburg GmbH, Germany) using The dose distribution in the solid water phantom was obtained by simulating the experimental setup using a 0.5 x 0.5 x 0.2 mm 3 voxel grid with a statistical uncertainty 0.3% (Type A ± 1 σ). A transit dose correction described by Jeong et al [134] was applied to the simulations. The correction uses the source activity to calculate the equivalent additional dwell time to the irradiation time, for a Sk of 40700 U (10 Ci) source the resulting contribution is (0.84 ± 0.09) seconds.…”

“…However, the time required to send the dummy sources was the main factor that limited modeling an applicator with more than 9 channels ( Figure 5.10b), having more channels in the applicator would increase the deg of freedom available for the TPS. Another limiting factor to use more channels was the transit dose contribution to the total dose (contribution equivalent to extra dwell time of 0.84 ± 0.09 seconds for the first dwell position using a Sk of 40700 U source) [134]. Increasing 80 the number of channels would reduce the dwell time per channel and the transit dose contribution would be higher, especially for the most distal dwell positions that have no transit time correction in current afterloaders.…”

“…The usual dose delivered in clinical practice for rectal cancer patients using CXB is 30 Gy [132], which requires an average dwell time per channel of 25 seconds to be delivered with the HDR applicator using a Sk of 40700 U source. Therefore, applying the transit dose correction from Jeong et al [134] is precise enough for typical treatment doses, however further investigation of transit dose specific for the HDR applicator is required for low dose treatments with a high activity source, which is the worst case scenario for the transit dose correction.…”