The dosimetric impact of replacing the TG-43 algorithm by model based dose calculation for liver brachytherapy

To compare treatment plans for interstitial high dose rate (HDR) liver brachytherapy with ¹⁹²Ir calculated according to current-standard TG-43U1 protocol with model-based dose calculation following TG-186 protocol.

We retrospectively evaluated dose volume histogram (DVH) parameters for liver, organs at risk (OARs) and clinical target volumes (CTVs) of 20 patient cases diagnosed with hepatocellular carcinoma (HCC) or metastatic colorectal cancer (mCRC). Dose calculations on a homogeneous water geometry (TG-43U1 surrogate) and on a computed tomography (CT) based geometry (TG-186) were performed using Monte Carlo (MC) simulations. The CTs were segmented based on a combination of assigning TG-186 recommended tissues to fixed Hounsfield Unit (HU) ranges and using organ contours delineated by physicians. For the liver, V_5Gy and V_10Gy were analysed, and for OARs the dose to 1 cubic centimeter (D_1cc). Target coverage was assessed by calculating V₁₅₀, V₁₀₀, V₉₅ and V₉₀ as well as D₉₅ and D₉₀. For every DVH parameter, median, minimum and maximum values of the deviations of TG-186 from TG-43U1 were analysed.

TG-186-calculated dose was found to be on average lower than dose calculated with TG-43U1. The deviation of highest magnitude for liver parameters was -6.2% of the total liver volume. For OARs, the deviations were all smaller than or equal to -0.5 Gy. Target coverage deviations were as high as -1.5% of the total CTV volume and -3.5% of the prescribed dose.

In this study we found that TG-43U1 overestimates dose to liver tissue compared to TG-186. This finding may be of clinical importance for cases where dose to the whole liver is the limiting factor.