Utility of FDG-PET in Post Transplant Lymphoproliferative Disorders: An Update from a Single Center Experience

Backgrounds Positron emission tomography using ¹⁸F-fluorodeoxyglucose (FDG-PET) is a well known imaging technique that has been proven being useful in staging and response to treatment assessment of Hodgkin and non-Hodgkin lymphomas, providing information about the metabolic behaviour of tissues independently of morphological criteria. At the moment its role in Post Transplant Lymphoproliferative Disorders (PTLDs) remains unclear, because large series of patients evaluated with FDG-PET and compared with conventional imaging still lack.

Aims To establish the usefulness of FDG-PET in staging, remission assesment and follow-up of PTLD in comparison to the conventional imaging (CT) findings in terms of sensitivity, specificity, accuracy, positive predictive value (PPV) and negative predictive value (NPV).

Methods Between February 2003 and July 2008, 79 FDG-PET were performed in 31 patients who developed PTLD after solid organ transplantation. According to WHO classifications, patients could be divided as follow: 1 Plasmacytic Hyperplasia, 9 Polymorphic Lymphoproliferative Disorder, 20 Malignant Lymphoma; in one case histological diagnosis was not obtained. We retrospectively compared results from 46/79 FDG-PET to findings obtained from conventional imaging performed at the same time. Thirty-three/79 more FDG-PET scans were performed as follow up assesment and results were compared according to disease status as defined on the basis of clinical signs and symptoms, biochemical markers and imaging technique.

Results When compared to conventional imaging (46 scans), FDG-PET showed the same results as CT in 34/46 cases, while discordant results were found in 12/46 cases (data are reported in the table). No differences in FDG-PET sensitivity were observed in the different histological subtypes of PTLD. According to disease status FDG-PET showed good sensitivity, specificity and accuracy; when compared to CT, FDG-PET had lower sensitivity (81% vs 97%), but higher specificity (96% vs 78%) and accuracy (92% vs 85%). Moreover, this technique provided a high PPV (90%) and NPV (93%)

Conclusions: Our data support the role of FDG-PET as diagnostic tool in PTLD as well as other FDG-avid lymphomas. The histological PTLD subtype doesn’t seem to have any impact on FDG avidity, even though in our analysis ML were the prevalent forms. In the majority of cases FDG-PET can well identify all disease areas seen at standard imaging modalities. Moreover, avoiding contrast medium related nephrotoxicity, this imaging modality can even be considered as a safe procedure in the setting of frail PTLD patients whose renal function is often impaired. In our analysis FDG-PET has higher PPV when compared to CT (90% vs 72%): the absence of metabolic activity at FDG-PET can rule out a supposed residual disease revealed at CT on the basis of morphological criteria. So while CT can still be considered the gold standard in staging assessment, due to its higher sensitivity, FDGPET may be better employed in the follow up, due to its capacity to detect the persistence of active disease in residual lesions described by conventional imaging.