LAUSR

Radioembolization is nowadays dominated by Tc MAA for pretreatment study of biodistribution and dosimetry, and by Y microspheres for therapy Physical properties of such radionuclide are: 64 h half-life, beta emission maximal energy 2280 keV, beta abundance 99.9%, no gamma, and 32 positron emissions per million of transformations. A new kind of medical device available for therapy recently became commercially available also for planning: polylactic acid (PLLA) microspheres labeled with Ho. Such radionuclide has a 26.8 h half-life, dual beta emission with maximal energy of 1770 keV (49%)–1850 keV (50%) and, new aspect, it is paramagnetic and it emits gamma photons at 81 keV, with low abundance (6.7%). Gamma photons allow SPECT/CT imaging and dosimetry (some days after therapy to avoid gammacamera saturation) [1], while paramagnetism gives the additional possibility of post-therapy MRI evaluation. Publications about the use of Ho in therapy are available [2, 3], but this application is not under discussion here. We are rather focusing on the just published work by the Utrecht centre [4], which demonstrated higher accuracy of intra-liver dosimetric prediction using a tracer injection of Homicrospheres (the so-called Ho scout dose) with respect to Tc MAA. Scout dose consists in the angiography-guided administration of 250 MBq of Ho microspheres (about 3 million of particles) instead of some hundred thousand of Tc MAA particles. The activity was fixed on one side in order to be safe even in case of lung shunt during simulation [5], and on the other side to be sufficiently high to allow ordinary gamma camera imaging (planar + SPECT/CT scan), despite the low photon abundance. Medium energy collimators are needed to reduce the remarkable amount of bremsstrahlung photons impinging on the detector. Readers used to simulations with MAAmight argue which advantages could be provided by Ho scout dose. Several authors investigated the problem of accuracy of prediction with Tc MAA. This is obviously a crucial factor for an accurate dosimetric treatment planning. The first evidence of differences between MAA prediction and the actual microspheres biodistribution were published studying Y resin particles with bremsstrahlung SPECT [6, 7]. Those papers indicated the first reasons for discrepancy: bad catheter tip repositioning, above all in proximity of an arterial bifurcation. The Utrecht group confirmed this, but remarked the importance of the difference in number and size betweenMAA and Y resin spheres [8]. They further investigated the problem with a key work comparing prediction of lung absorbed dose with MAA versus Ho scout dose, having as gold standard the true lung absorbed dose obtained with Ho SPECT/CTafter therapeutic infusion [9]. The lung dose overestimation with MAA was systematic, huge with planar imaging, but still large even if evaluated on fully corrected 3D Tc SPECT/CT images. Median lung absorbed dose was 0.2 Gy evaluated both pretherapy (scout dose) and post-therapy with Ho SPECT/CT, while it was overestimated to 2.5 Gy according to TcMAA SPECT/CT. A major reason for such a large discrepancy is the difference in size and material between the simulation and therapy particles. MAA are not solid, and, above all, their size distribution extends from 10 μm to more than 100 μm [10]. The problem of the large lung shunt overestimation with MAA probably derives from the portion of MAA with small size (10 μm). These have higher penetrability in capillaries with respect any kind of therapeutic microspheres (resin, glass, and PLLA), which are more than twice larger in diameter. MAA can therefore more easily pass through arterious to venous capillaries, and reach lungs. As a first note about methodology found in publications, correlation and concordance tests are not the best statistical This article is part of the Topical Collection on Editorial