Comparing voxel-based absorbed dosimetry methods in tumors, liver, lung, and at the liver-lung interface for ⁹⁰Y microsphere selective internal radiation therapy

Background: To assess differences between four different voxel-based dosimetry methods (VBDM) for tumor, liver, and lung absorbed doses following ⁹⁰Y microsphere selective internal radiation therapy (SIRT) based on ⁹⁰Y bremsstrahlung SPECT/CT, a secondary objective was to estimate the sensitivity of liver and lung absorbed doses due to differences in organ segmentation near the liver-lung interface. Methods: Investigated VBDM were Monte Carlo (MC), soft-tissue kernel with density correction (SKD), soft-tissue kernel (SK), and local deposition (LD). Seventeen SIRT cases were analyzed. Mean absorbed doses (AD¯ (Formula presented.) were calculated for tumor, non-tumoral liver (NL), and right lung (RL). Simulations with various SPECT spatial resolutions (FHWMs) and multiple lung shunt fractions (LSs) estimated the accuracy of VBDM at the liver-lung interface. Sensitivity of patient RL and NL AD¯(Formula presented.) on segmentation near the interface was assessed by excluding portions near the interface. Results: SKD, SK, and LD were within 5 % of MC for tumor and NL AD¯(Formula presented.). LD and SKD overestimated RL AD¯(Formula presented.) compared to MC on average by 17 and 20 %, respectively; SK underestimated RL AD¯(Formula presented.) on average by −60 %. Simulations (20 mm FWHM, 20 % LS) showed that SKD, LD, and MC were within 10 % of the truth deep (>39 mm) in the lung; SK significantly underestimated the absorbed dose deep in the lung by approximately −70 %. All VBDM were within 10 % of truth deep (>12 mm) in the liver. Excluding 1, 2, and 3 cm of RL near the interface changed the resulting RL AD¯(Formula presented.) by −22, −38, and −48 %, respectively, for all VBDM. An average change of −7 % in the NL AD¯(Formula presented.) was realized when excluding 3 cm of NL from the interface. AD¯(Formula presented.) was realized when excluding 3 cm of NL from the interface. Conclusions: SKD, SK, and LD are equivalent to MC for tumor and NL AD¯(Formula presented.). SK underestimates RL AD¯(Formula presented.) relative to MC whereas LD and SKD overestimate. RL AD¯(Formula presented.) is strongly influenced by the liver-lung interface.