Development of energy deposition pixel kernel convolution for planar dosimetry in 177Lu therapy

Planar imaging remains widely used for image-based dosimetry in 177Lu therapy due to its simplicity and fast acquisition in clinical practice. The only MIRD method is applicable to planar dosimetry. However, conventional MIRD method cannot provide accurate organ-level absorbed dose and patient-specific dose maps. This study develops a novel planar dosimetry method, the energy deposition pixel kernel (EPK) convolution, to improve accuracy of organ dose estimation and provide patient-specific energy deposition maps for planar dosimetry without a high-performance computer. EPK convolution generated the energy deposition map by convolving a pixel-based time-integrated activity map with a 2D 177Lu energy-deposition kernel incorporating a scaling factor. EPK convolution was validated in simulated two cylindrical water phantoms (containing six hot spheres of varying sizes and two overlapping hot spheres) and two digital human phantoms. EPK convolution was also applied to two open-access 177Lu -DOTATATE patients. All absorbed doses estimated from energy deposition maps incorporating partial volume effect (PVE) correction for small spheres and overlap correction (OC) for partial overlap spheres were compared with MC simulation. In case of human phantoms and patients, organ and lesion absorbed doses from patient-specific energy deposition maps incorporating OC and the conventional MIRD method were compared with MC simulation. Absorbed dose of spheres by EPK convolution showed good agreement with those by MC simulations in both cylindrical phantom studies, with errors within 10% after applying PVE correction or OC. In the digital human phantom and two patient datasets, absorbed doses by EPK convolution were within ± 10% of MC simulation across all organs and lesions, where liver, right kidney and lesions were applied OC. In contrast, the conventional MIRD method showed larger deviations, often exceeding 10% against MC simulation. EPK convolution provides patient-specific energy deposition maps and improved organ or lesion dose accuracy compared with conventional MIRD method. EPK convolution may contribute for supporting personalized 177Lu therapy planning.