<sup>18</sup>F-FDG PET and PET-CT as Methods of Diagnosis and Efficacy Estimation of Antitumor Treatment

Background. Primary bone lymphoma (PBL) is a rare malignant tumor. Initial examination aimed at detecting all primary lesions is an indispensable prerequisite for the choice of optimal antitumor treatment. Standard methods of diagnosis (X-ray, CT, and MRI) are not always adequate to measure the real tumor mass. Another well-known characteristic feature of PBL is a challenge in evaluating the effect of its treatment because of residual changes in the bones of most patients. However, the data on using ¹⁸F-FDG PET, another method of metabolic imaging, in PBL are rather rare in accessible literature. Aim. To study the specific use of PET with ¹⁸F-FDG at initial examination and efficacy estimation of PBL treatment. Materials & Methods. The trial included 21 PBL patients who received PET with ¹⁸F-FDG at initial examination and a month after the end of treatment. The results of ¹⁸F-FDG PET imaging were compared with the data obtained by means of structural diagnostic methods (CT and MRI) and the analysis of biopsy samples with pathologic lesions. Results. Intensive uptake of ¹⁸F-FDG(SUV_max 8.6-40.1, mean SUV_max 23.5), according to PET data, was reported in all patients in those tumor lesions which were identified by the structural diagnostic methods and confirmed by biopsies. Besides, each of 21 cases showed pathologic infiltration of adjacent soft tissues with high metabolic activity. In PET-CT with ¹⁸F-FDG 13 further tumor localizations were revealed in 8 (38 %) patients. On completing the therapy, according to CT and MRI data, residual changes were observed in all (n = 21, 100 %) patients. The residual metabolic activity in the involved bones was identified in 13 (62 %) patients (SUV_max 2.91-8.7, mean SUV 4.2). In 4 of them the residual lesions were subjected to biopsy. None of 4 cases was reported to show tumors. Only in 1 out of 13 patients with residual metabolic changes a tumor relapse was detected. Overall 10-year survival in the groups of patients with and without FDG+ residual changes was 91 % and 100 %, respectively, with insignificant differences (p = 0.39). Conclusion. PET-CT with ¹⁸F-FDG is a highly sensitive technique for evaluating the primary lesion volumes in PBL patients. In 100 % of bone and soft tissue lesions an intensive uptake of ¹⁸F-FDG was observed. At the same time our study showed persistent metabolic activity on completing antitumor treatment in more than a half of patients, and in most of them it was not caused by tumor. Therefore, in our view, ongoing residual metabolic activity in PBL cannot always be regarded as an indication for continued treatment or consolidation radiotherapy.