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Parameterization of Backscatter Contributions in HDR Brachytherapy with Surface Applicators

X Shen1*, M Bhagwat1, (1) Massachusetts General Hospital, Boston, MA

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PO-GePV-T-48 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: Dose delivered to superficial lesions during high dose rate brachytherapy (HDR-BT) lacks full backscatter conditions included in TG43 based treatment planning systems (TPS). This leads to lower dose delivered to lesion than the prescribed dose. This work quantifies the dependence of backscatter dose contribution on the amount of backscatter material and applicator size.

Methods: Dose were measured using IBA’s StarTrack ion-chamber array with a Freiburg flap applicator centered on top. Plans designed to deliver a uniform dose to 1cm depth with 4x4 to 16x16cm2 applicator sizes were generated in Elekta’s TG-43 based Oncentra TPS using CT images of the setup. Central axis doses were measured without and with 1-10cm of solid water placed on top of the applicator. Backscatter dose contribution was modeled using an exponential model with a rate (cm⁻¹) and a scaling parameter which represents the maximum contribution from backscatter dose.

Results: Doses increased with both increasing backscatter material and applicator sizes. Backscatter dose increased sharply within the first 5cm of added solid water with only moderate increase thereafter. E.g., dose increased by 0.8%, 2.1%, and 2.4% for a 4x4cm2 applicator, compared to 1.3%, 5.1%, and 6.1% for a 10x10cm2 applicator with 1, 5, and 10cm solid water, respectively. The exponential model fit well (R>0.9) for all applicator sizes. The rate and scaling parameters increased linearly with applicator size, from 0.3cm⁻¹ to 0.5cm⁻¹ and 2.5% to 10.8%, respectively.

Conclusion: Addition of backscatter material substantially increases dose at the prescription point. The parameterization of backscatter dose dependence on thickness and applicator size for HDR-BT using surface applicators allows for calculation of delivered dose versus planned dose in these cases. This may inform physicians about the difference between delivered and prescribed dose based on applicator selection and treatment geometry. Future work will investigate backscatter contributions from different applicator geometries.

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