A twelve-year-old boy suffering from chronical immunologic thrombocytopenic purpura (ITP) was referred to the nuclear medicine department of Montpellier University Hospital to evaluate platelet survival and sequestration, to discuss splenectomy. The patient was refractory to therapy (corticosteroids, immunosuppressive agents, and immunoglobulins) and his platelet count consistently remained below 10 G.L− 1. Below this threshold, the number of platelets required for correct radiolabeling cannot be obtained with a sample volume that is tolerable for his morphology. Two autologous platelets tests in two different hospitals both failed due to low platelet counts leading us to perform a scintigraphy exam using homologous platelets. To confirm our results, radiolabeled autologous platelet exam was performed remotely thereafter, with a platelet count around 30 G.L− 1 after a thrombopoietin receptor-agonists (TPO-RA) treatment intensification. A summary of the patient's history is shown in Fig. 1. For information, he was diagnosed with ITP in august 2020. He has moderate hemorrhagic syndrome, with an impact on his quality of life, and a psychological impact on him and his parents. Anti-platelet antibodies search reveals anti-glycoprotein IIb-IIIa and Ib-IX autoantibodies. Our patient has A RH1 (D+) blood group phenotype, had anti-HLA class I antibodies, and no irregular antibodies.
The radiolabeling process follows the French good preparation practices [8]. Platelets are isolated from blood after sampling for autologous process, or concentrated for homologous process into a pellet. [111In]In-oxinate solution is used for platelet radiolabeling. [111In]In3+ has good radiopharmaceutical characteristics such as medium gamma energies (171–245 keV), and a radioactive period of 2.8 days that allows a several days monitoring [9]. As a metal cation in the + 3 oxidation state, indium(III) oxine can easily coordinate with species that are abundant in electrons. The present radiolabeling process is simple, not specific and has been validated long time ago: after buffering to pH 6, the complex [111In]In-oxinate can pass through cell membranes. Then it dissociates and [111In]In3+ forms a bond with intracellular proteins [10, 7].
Platelet concentrate (PC) used in homologous method must be from apheresis, irradiated and be strictly ABO/RH1 grouped and HLA compatible. Donor’s platelets were collected, processed by filtration, quantified, and tagged the day before radiolabeling by the French blood establishment (EFS). Conformity and security controls upon receipt of the PC and pre-transfusion controls are carried out by the hemovigilance department. The use of PC for this technique is subject to an authorization issued by the French national agency for the safety of medicines and health products (ANSM). This agency recommends that the PC used be untreated with amotosalen that could impact viability results [11]. Radiolabeling tests were carried out beforehand by the radiopharmacists to validate the process and set up a procedure. Good transfusion practices were followed, and an infusion set with a 200 µm filter was used to block any small aggregates that may be present in the PC as required for classic platelet transfusion [12].
Radiolabeled platelets were injected by a catheter at the elbow crease, 15 MBq and 6 MBq, for homologous and autologous radiolabeled platelet procedures respectively. We halved the dose for the second exam to reduce the child's radiation exposure, given the good results of the first exam. Clinical parameters of the patient were continuously monitored. This patient benefits a particular hemovigilance and pharmacovigilance follow-up for any adverse event.
The same data collection and data analysis procedures were carried out for radiolabeled platelet examinations in both homologous and autologous settings. Blood samplings were performed from the contralateral arm up at 0.5, 1, 3, 24, 48, 72 hours post-injection (additional 96 hours point for autologous exam). Radioactivity measures were performed with a gamma counter (Wizard 1480®, Perkin Elmer) to evaluate platelet lifespan. The platelet half-life was determined graphically. Single-photon emission computerized tomography (SPECT) acquisitions of the thoraco-abdominal region were performed 0.75, 1, 4, 24, 48, 72 hours post-injection using a dual-head NM/CT 870-DR gamma-camera equipped with middle-energy-general-purpose parallel-hole collimators (GE Healthcare, Tirat Carmel, Israel) (additional 96 hours image for autologous exam). Anterior and posterior images were acquired simultaneously and geometrically averaged. Platelet sequestration was determined from a radioactivity count in a region of interest (ROI) traced around an organ, normalized by the activity measured in that organ 45 minutes after administration of labelled platelets. Late sequestration was defined as splenic or hepatic if the ratio exceeded 1.2 [13].