Endocrine Abstracts

Historically, many radionuclide therapy (RNT) procedures practiced in nuclear medicine have employed standardised amounts of a radionuclide for the therapy. This often neglects factors such as patient size, tumour burden, rate of disease progression and intra-lesional heterogeneity. At the opposite end of the spectrum, treatment procedures using external beam radiotherapy (EBRT) have become increasingly individualised to maximise the dose to target whilst minimising radiation dose delivered to normal tissues. Currently, the majority of RNTs still follow the former paradigm. Individualising dosimetry can have two differing aims: (i) to deliver what is believed to be a tumouricidal dose of radiation to the target to attempt to effect local control or (ii) to limit the radiation exposure of dose-limiting organs whilst achieving a measure of tumour control. Dose-limiting organs often include bone marrow and kidneys. Inevitably, treatment planning may try to attempt to achieve both of these sometimes competing aims simultaneously. One limitation in implementing image-based treatment planning for RNT has been the compromised data format when using 2D (planar) whole-body images from the gamma camera (e.g., I-131, Lu-177) to measure the biodistribution over time. These data are typically not quantitative and suffer from significant organ and tissue overlap, such that individual organs and lesions cannot be readily interrogated in isolation. This problem does not exist with PET imaging (e.g., I-124, Cu-64) for treatment planning as the images are inherently 3D and quantitative. Recent advances in gamma camera imaging, driven by the introduction of combined SPET/CT systems and improved iterative reconstruction algorithms, provide the potential for quantitative whole-body (3D) SPECT imaging. This overcomes a number of issues and the remaining challenge for planning RNT with g-emitting radionuclides will be the need to acquire data at a number of time points to study the individual biodistribution. The compromise situation, which may well prevail, is to start a course of RNT with a standard dose and acquire the necessary data to individualise subsequent treatments.