Multicentric Castleman's disease: Closest mimic of lymphoma on FDG PET/CT

Sir,

Intense lymph nodal concentration of tracer on 18 F— Fluoro-deoxyglucose Positron Emission Tomography/Computed Tomography (FDG PET/CT) is a characteristic of high grade lymphomas. However, a similar imaging pattern is seen in Castleman's disease (CD). We report one such case of a 43-year-old lady, presenting with fever and significant weight loss since 3 months. Patient was seronegative for HIV (Human Immuno-deficiency Virus and HHV-6 (human herpes virus-6). Ultrasonography (USG) of abdomen showed enlarged retroperitoneal nodes. With a high suspicion of neoplastic etiology, whole body FDG PET/CT was carried out. Maximum intensity projection (MIP) image revealed intense tracer concentration in the retroperitoneal, pelvic, and right inguinal nodal chain [Figure 1a — arrow heads], with discrete foci in mediastinum [Figure 1a — arrow] and pelvic bones. Axial fused PET/CT images confirmed showed hypermetabolic conglomerate enlarged retroperitoneal [Figure 1b — arrow], right pelvic [Figure 1c — arrow] and inguinal nodes [Figure 1e — arrow], extending from renal hilum upto aortic bifurcation. Also seen was tracer uptake in bone marrow at D6 [Figure 1d — arrow], L2 and L5 vertebra, and in right pelvic bones [Figure 1d — arrowhead]. Post-contrast CT images show mild enhancement in retroperitoneal [Figure 2a — arrow], pelvic [Figure 2b and 2c — arrow] and inguinal [Figure 2d — arrow] nodes, which show hypermetabolism on PET images. Discrete hypermetabolic focus was seen in right pelvic subcutaneous region. Scan features and uptake pattern was strongly suggestive of stage IV lymphoma. Bone marrow biopsy showed plasma cell aggregates with no malignant cells. However, histopathological examination of retroperitoneal nodal specimen revealed plasma cell variant of CD, with numerous atrophic germinal centers. Patient was treated with steroids and follow-up study carried out 3 months later showed significant metabolic and morphological regression [Figure 3— arrow and arrow heads].

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Figure 1 MIP image (a) showing intense tracer uptake in retroperitoneal and pelvic nodal chain (arrow-heads) and focal uptake in mediastinum (arrow). Axial fused PET/CT showing hypermetabolic retroperitoneal nodes (b – arrow) with (FDG) avid right pelvic (c – arrow) nodes. FDG uptake seen in the right iliac (d – arrow head), D6 vertebra (d – arrow). Also seen is hypermetabolic right inguinal node (e – arrow)

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Figure 2 Axial post contrast CT images show mild enhancement in nodes in retroperitoneal (a – arrow), pelvic (b and c – arrow) and right inguinal (d – arrow) regions, which on axial PET images show FDG uptake

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Figure 3 MIP image of FDG PET/CT carried out after 3 months of steroid therapy shows significant metabolic and morphological regression of disease (compared to Figure 1a)

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CD was first described by Benjamin Castleman in 1956.[1] It is of unknown etiology; most popular hypothesis labeling it as chronic low grade inflammation with Interleukin-6 overproduction.[2] It commonly involves the lymphatics, with extra-lymphatic involvement seen in lungs, bones, meninges, and muscles.[3] Its association with inflammatory cells is the most likely explanation of FDG uptake in CD. FDG is known to concentrate in many non-specific infections and inflammations, seen as hypermetabolic adenopathy.[4] Hypermetabolic nodes are also seen in granulomatous diseases.[5] All these are commonest false positives for FDG uptake in lymph nodes in cancer patients. In these cases, distinction can be made from lymphoma, by the intensity of nodal FDG concentration and morphology of nodes. However, CD, though a benign lymphoproliferative disorder; the pattern of lymphatic involvement, extranodal sites; and FDG avidity of these lesions, makes it to appear similar to lymphoma. Thus, FDG PET/CT offers little help in distinguishing between the two; making CD the closest imaging mimic of lymphoma on FDG PET/CT. However, whole body metabolic — morphological fusion imaging helps in picking up sites of involvement, separating unicentric from multicentric disease. This can have an impact on disease management, since localized disease is amenable to surgery.[6] Thus, on a positive note, PET/CT can be used for mapping the extent of disease involvement by uncovering sites of disease and can also be used for assessing treatment response, which are otherwise difficult to document.