This supports the implementation of (90)Y PET in selective internal radiation therapy of the liver.

The decay of 90 Y has a minor branch to the 0 + excited state [1], followed by an internal e + e − creation which happens in 32 out of one million decays [2]. Consequently, 90 Y PET scan was proposed in order to assess the biodistribution [3] of 90 Y-labelled therapeutic agents. A 61-year-old woman was referred for treatment of chemorefractory colorectal liver metastasis. Based on the pretreatment evaluation (including a diagnostic FDG PET/CT scan on day 1, and a prophylactic embolization of the right gastric and gastroduodenal arteries followed by a 99m Tc-MAA SPECT/CT scan on day 8), 1.3 GBq of 90 Y-labelled SIR-Spheres were administered by sequential catheterization of both liver lobes (day 15). Subsequently, a 30-min 90 Y TOF PET/CT scan was performed using a Philips GEMINI TF camera. In order to prevent saturation of the detectors, a copper ring of 2.5 mm thickness was inserted into the gantry to absorb the bremsstrahlung x-rays. The TOF data were reconstructed with attenuation and scatter correction using Philips RAMLA software (eight iterations, three subsets). An additional 20 min bremsstrahlung 90 Y-SPECT was acquired using a Trionix XLT20 triple head camera (medium energy collimator, 30% window centred on 90 keV). Data were reconstructed using OSEM (four iterations, six subsets). As illustrated, despite the differences in their respective uptake mechanism, 90 Y-PET better reflects the tumour heterogeneity assessed by FDG PET/CT (a necrotic core surrounded by active tumour margins) than traditional bremsstrahlung 90 Y-SPECT. This gain in resolution should therefore contribute to increasing the accuracy of the dose distribution into the tumours and their surrounding healthy tissues. A new internal pair production branching ratio of 90Y: the development of a non-destructive assay for 90Y and 90Sr. Appl

90 Y resin and glass microsphere liver radioembolizations delivering lobar doses of 70 and 120 Gy, respectively, display hepatic toxicity similar to 40-Gy fractionated external-beam radiotherapy. We investigated how the lower number of glass microspheres could induce a sufficiently nonuniform dose distribution explaining this paradox. Methods: Microscale dosimetry was assessed in the realistic liver model developed by Gulec et al. but using the Russell's dose deposition kernel. A lattice of hexagonal prisms represented the hepatic lobules. Two hepatic arterial tree models-that is, a fixed-length and a variable-branches length-were used for the microsphere transport. Equal or asymmetric microsphere relative-spreading probability between 2 daughter vessels was assumed. Several 120-Gy liver simulations were performed: periodic simulations, where 1 or 6 glass microspheres were trapped in all and in only 1 of 6 portal tracts, respectively, and random simulations, where glass microsphere trapping assumed an equal probability for all the portal tracts or a variable probability depending on the successions of artery connections leading to the portal tract, both for the 2 arterial tree models. Results: For the 2 uniform simulations, all hepatic structures received at least 100 Gy. The fast decrease of the 90 Y kernel as the inverse of the square of the distance r is counterbalanced by the number of contributing lobules containing microspheres that increases as r 2 . The random simulation with equal-spreading probability gave for the less irradiated tissue a lobule dose distribution centered around 103 Gy (full width at half maximum, 20 Gy). The distribution became significantly asymmetric with the 60%-40% relative-spreading probability, with a shift of the maximum from 103 down to 50 Gy, and about 17% of the lobules got a dose lower than 40 Gy to their different structures. Conclusion: The large nonuniform trapping produced by the microsphere transport in the arterial tree jointly with the low number of injected glass microspheres begins to explain their lower hepatic toxicity per Gray. In addition, the nonuniform trapping supports the fact that the granular aspect of 90 Y PET imaging observed in patients could represent some reality and not only statistical noise.