title: Impact of SPECT corrections on 3D-dosimetry for liver transarterial radioembolization using the patient relative calibration methodology.
abstract: OBJECTIVE
  Many centers aim to plan liver transarterial radioembolization (TARE) with dosimetry, even without CT-based attenuation correction (AC), or with unoptimized scatter correction (SC) methods. This work investigates the impact of presence vs absence of such corrections, and limited spatial resolution, on 3D dosimetry for TARE.
  
  
  METHODS
  Three voxelized phantoms were derived from CT images of real patients with different body sizes. Simulations of (99m)Tc-SPECT projections were performed with the SIMIND code, assuming three activity distributions in the liver: uniform, inside a "liver's segment," or distributing multiple uptaking nodules ("nonuniform liver"), with a tumoral liver/healthy parenchyma ratio of 5:1. Projection data were reconstructed by a commercial workstation, with OSEM protocol not specifically optimized for dosimetry (spatial resolution of 12.6 mm), with/without SC (optimized, or with parameters predefined by the manufacturer; dual energy window), and with/without AC. Activity in voxels was calculated by a relative calibration, assuming identical microspheres and (99m)Tc-SPECT counts spatial distribution. 3D dose distributions were calculated by convolution with (90)Y voxel S-values, assuming permanent trapping of microspheres. Cumulative dose-volume histograms in lesions and healthy parenchyma from different reconstructions were compared with those obtained from the reference biodistribution (the "gold standard," GS), assessing differences for D95%, D70%, and D50% (i.e., minimum value of the absorbed dose to a percentage of the irradiated volume). γ tool analysis with tolerance of 3%/13 mm was used to evaluate the agreement between GS and simulated cases. The influence of deep-breathing was studied, blurring the reference biodistributions with a 3D anisotropic gaussian kernel, and performing the simulations once again.
  
  
  RESULTS
  Differences of the dosimetric indicators were noticeable in some cases, always negative for lesions and distributed around zero for parenchyma. Application of AC and SC reduced systematically the differences for lesions by 5%-14% for a liver segment, and by 7%-12% for a nonuniform liver. For parenchyma, the data trend was less clear, but the overall range of variability passed from -10%/40% for a liver segment, and -10%/20% for a nonuniform liver, to -13%/6% in both cases. Applying AC, SC with preset parameters gave similar results to optimized SC, as confirmed by γ tool analysis. Moreover, γ analysis confirmed that solely AC and SC are not sufficient to obtain accurate 3D dose distribution. With breathing, the accuracy worsened severely for all dosimetric indicators, above all for lesions: with AC and optimized SC, -38%/-13% in liver's segment, -61%/-40% in the nonuniform liver. For parenchyma, D50% resulted always less sensitive to breathing and sub-optimal correction methods (difference overall range: -7%/13%).
  
  
  CONCLUSIONS
  Reconstruction protocol optimization, AC, SC, PVE and respiratory motion corrections should be implemented to obtain the best possible dosimetric accuracy. On the other side, thanks to the relative calibration, D50% inaccuracy for the healthy parenchyma from absence of AC was less than expected, while the optimization of SC was scarcely influent. The relative calibration therefore allows to perform TARE planning, basing on D50% for the healthy parenchyma, even without AC or with suboptimal corrections, rather than rely on nondosimetric methods.
authors.affiliation: Department of Medical Physics, Azienda Ospedaliera San Camillo Forlanini, Rome 00152, Italy.
authors.forename: Massimiliano
authors.initials: M
authors.lastname: Pacilio
authors.affiliation: Department of Medical Physics, Istituto Europeo di Oncologia, Milan 20141, Italy.
authors.forename: Mahila
authors.initials: M
authors.lastname: Ferrari
authors.affiliation: Department of Nuclear Medicine, Istituto Nazionale Tumori IRCCS Foundation, Milan 20133, Italy.
authors.forename: Carlo
authors.initials: C
authors.lastname: Chiesa
authors.affiliation: Postgraduate School of Medical Physics, "Sapienza" University of Rome, Rome 00185, Italy.
authors.forename: Leda
authors.initials: L
authors.lastname: Lorenzon
authors.affiliation: Post graduate Health Physics School, University of Milan, Milan 20122, Italy.
authors.forename: Marta
authors.initials: M
authors.lastname: Mira
authors.affiliation: Department of Medical Physics, Istituto Europeo di Oncologia, Milan 20141, Italy.
authors.forename: Francesca
authors.initials: F
authors.lastname: Botta
authors.affiliation: Postgraduate School of Medical Physics, "Sapienza" University of Rome, Rome 00185, Italy.
authors.forename: Domenico
authors.initials: D
authors.lastname: Becci
authors.affiliation: Department of Nuclear Medicine, Clinical Research Division of the Center of Isotopes (DIC-CENTIS), Havana 11100, Cuba.
authors.forename: Leonel Alberto
authors.initials: LA
authors.lastname: Torres
authors.affiliation: Department of PET-CT and Nuclear Medicine, Imaging Center Medscan-Concepciòn, Concepciòn 4070061, Chile.
authors.forename: Marco
authors.initials: M
authors.lastname: Coca Perez
authors.affiliation: Department of Nuclear Medicine, Clinical Research Division of the Center of Isotopes (DIC-CENTIS), Havana 11100, Cuba.
authors.forename: Alex
authors.initials: A
authors.lastname: Vergara Gil
authors.affiliation: Department of Medical Physics, Azienda Ospedaliera San Camillo Forlanini, Rome 00152, Italy.
authors.forename: Chiara
authors.initials: C
authors.lastname: Basile
authors.affiliation: Department of Medical Radiation Physics, University of Lund, Lund 22100, Sweden.
authors.forename: Michael
authors.initials: M
authors.lastname: Ljungberg
authors.affiliation: Department of Medico-surgical Sciences and Biotecnologies, "Sapienza" University of Rome, Rome 00185, Italy.
authors.forename: Roberto
authors.initials: R
authors.lastname: Pani
authors.affiliation: Department of Medical Physics, Istituto Europeo di Oncologia, Milan 20141, Italy.
authors.forename: Marta
authors.initials: M
authors.lastname: Cremonesi
mesh_terms: D002138:Calibration; D003198:Computer Simulation; D004307:Dose-Response Relationship, Radiation; D004621:Embolization, Therapeutic / Q000295:instrumentation / Q000379:methods*; D005260:Female; D006801:Humans; D021621:Imaging, Three-Dimensional / Q000295:instrumentation / Q000379:methods*; D008099:Liver / Q000000981:diagnostic imaging / Q000528:radiation effects; D008297:Male; D008953:Models, Anatomic; D009010:Monte Carlo Method; D015609:Organotechnetium Compounds; D019047:Phantoms, Imaging; D011874:Radiometry / Q000295:instrumentation / Q000379:methods*; D019275:Radiopharmaceuticals; D011878:Radiotherapy / Q000295:instrumentation / Q000379:methods*; D012984:Software; D015899:Tomography, Emission-Computed, Single-Photon / Q000295:instrumentation / Q000379:methods*
chemical_list: D015609:Organotechnetium Compounds; D019275:Radiopharmaceuticals