Advanced dose calculation algorithm in superficial brachytherapy - the impact of tissue inhomogeneity on treatment plan dosimetry

Purpose: Given tissue inhomogeneity and lack of backscatter media, superficial brachytherapy necessitates more accurate dosimetry than TG-43 formalism. However, the introduction of modern model-based dose calculation algorithms into clinical practice should be carefully evaluated. The aim of this work was to compare dose distributions calculated with TG-43 and advanced collapsed cone engine (ACE) algorithms for individual multi-catheter moulds, and investigate the impact of target size and the lack of bolus to differences between plans.

Material and methods: Eleven treatment plans for individual mould multi-catheter high-dose-rate brachytherapy (IMM HDR) were selected for retrospective analysis. All treatment plans were initially calculated with TG-43 formula and re-calculated using ACE algorithm. Plan re-calculation with ACE was repeated for each plan in order to assess the impact of bolus. To evaluate differences between TG-43 and ACE dose distributions, dose-volume histogram (DVH) parameters for each ROI were compared. D_max (maximal point dose), D_0.1cc, and D_2cc were calculated for each risk's organ (OARs) and for external contour. For clinical target volume (CTV), D₉₈, D₉₀, D₅₀, CTV coverage (CTV-V₁₀₀), and dose delivered to reference point were compared between the plans.

Results: A significantly lower values (p < 0.05) of CTV parameters were observed for treatment plans calculated with ACE algorithm comparing to TG-43. Further analysis showed that differences between CTV-V₁₀₀ for ACE and TG-43 plans depended on CTV volume. Dosimetric parameters for OARs were significantly lower in ACE plans than those of TG-43. Only D_2cc for external and D_0.1cc for both eye lenses in ACE plans were insignificantly different comparing to TG-43 plans.

Conclusions: Results show that differences between dosimetric parameters are statistically significant. However, their clinical relevance is still undetermined. Careful re-evaluation of the clinical results based on long-term research on TG-43 is necessary to safely introduce modern algorithms to clinical practice.