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Limitations and Clinical Implications of the TG-43 Formalism for High-Dose-Rate Skin Brachytherapy

D Granero¹*, J Perez-Calatayud², J Vijande³, F Ballester³, C Candela-Juan², M Rivard⁴, (1) ERESA,Hospital General Universitario, Valencia, Spain,(2) La Fe University and Polytechnic Hospital, Valencia, Spain,(3) University of Valencia, Burjassot, Spain,(4) Tufts University School of Medicine, Boston, MA

SU-E-T-516 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: In high-dose-rate (HDR) skin brachytherapy, sources are located outside, in contact with, or implanted at some depth below the skin surface. Nowadays, most TPS use the TG-43 formalism that is based on sources within an infinite water medium, without accounting for the scatter defect due to surrounding air. The purpose of the present study is to evaluate the limitations of the TG-43 formalism in HDR skin brachytherapy.

Methods: This study examined two HDR sources (Ir-192 and Co-60) together with a hypothetical Yb-169 source. Dose distributions were obtained using Monte Carlo methods (i.e., GEANT4). The following configurations were investigated:
1)A typical clinical case consisting of a 5x5 cm2 superficial mould, using an Ir-192 HDR source, embedded within an infinite water medium (i.e., TG-43 conditions),
2)The same mould positioned over the skin in a semi-infinite water medium, and
3)A single source, instead of a mould, with the same configuration as in 1), but with 0, 1, or 2 cm of bolus.

Results: Dose rate distribution differences between cases 1) and 2) ranged from 1.5% to 3% at the skin surface. At the typical prescription depth of 0.5 cm, differences were 2.5% to 3%. For case 3) using a Ir-192 source without bolus, dose rate differences were within 2% compared with TG-43 data for depths < 0.5 cm. When 1 cm or 2 cm of bolus was added, the differences became negligible. For Co-60 with no bolus, the differences approached 15% near the surface. From a clinical perspective, these differences were negligible considering the dose gradients at the typical prescription depth (0.5 cm).

Conclusion: Although dose differences exist amongst the different scatter configurations, they are not clinically significant.

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