Dosimetric comparison of Acuros^TM BV and AAPM TG-43 formalism for interstitial iridium-192 high-dose-rate brachytherapy

Purpose: The aim of this study was a retrospective dosimetric comparison of iridium-192 (¹⁹²Ir) high-dose-rate (HDR) interstitial brachytherapy plans using model-based dose calculation algorithm (MBDCA) following TG-186 recommendations and TG-43 dosimetry protocol for breast, head-and-neck, and lung patient cohorts, with various treatment concepts and prescriptions.

Material and methods: In this study, 59 interstitial ¹⁹²Ir HDR brachytherapy cases treated in our center (22 breast, 22 head and neck, and 15 lung) were retrospectively selected and re-calculated with TG-43 dosimetry protocol as well as with Acuros^TM BV dose calculation algorithm, with dose to medium option based on computed tomography images. Treatment planning dose volume parameter differences were determined and their significance was assessed.

Results: For the breast planning target volume (PTV), TG-43 formalism calculated higher D_90%, V_95%, V_100%, and V_150% values than Acuros^TM BV, ranging from 2.2% to 5.4% (mean differences), as it did for the head and neck cases, ranging from 2.5% to 4.7% and for the interstitial lung cases, ranging from 2.2% to 4.4%, showing statistical significance (p < 0.001). For the skin D_0.1cm3, D_0.2cm3, and D_1cm3, the values were overestimated by TG-43, with a mean absolute differences of 1.4, 1.8, and 2.0 Gy, respectively for the breast, and 1.0 Gy for all DVH statistics for the head and neck cases compared with Acuros^TM BV (p < 0.001). Ipsilateral lung V_5Gy was also higher in TG-43-calculated plans, with a mean difference of 1.0% and 1.1% in the breast and lung implants, respectively. For the chest wall TG-43, the respective overestimation in D_0.1cm3 and D_1cm3 was 0.8 and 0.8 Gy for the breast, and 0.4 and 0.3 Gy for the interstitial lung cases, respectively.

Conclusions: The TG-43 algorithm significantly overestimates the dose to PTVs and surrounding organs at risk (OARs) for breast, head and neck, and lung interstitial implants. TG-43 overestimation is in accordance with previous findings for breast and head and neck. To our knowledge, this is also exhibited for Acuros^TM BV for the first time in interstitial lung HDR brachytherapy.