Castleman disease (CD), designated after Benjamin Castleman [1,2] who initially described the morphologic features of CD, is currently recognized to be a group of at least four lymphoproliferative disorders [3,4]. Castleman disease can be subdivided into unicentric and multicentric types and unicentric CD is most common. Unicentric CD can exhibit a range of morphologic features which also can be subdivided into hyaline-vascular and mixed/plasmacytic variants. In large part, the pathogenesis of unicentric CD is unknown. Rare cases assessed have shown cytogenetic abnormalities, often involving chromosome 7, as well as complex karyotypes [5]. Chang and colleagues used the human androgen receptor assay and suggested that HV-CD is a benign neoplasm of lymph node-based stromal cells [6]. Gene mutations also have been reported in unicentric CD with mutations of PDGFRB being most common, in up to 10 % of cases (5.7).

Indolent T-lymphoblastic proliferation (iT-LBP) is a rare, clinically benign lymphoproliferative disorder characterized by a polyclonal proliferation of precursor T-lymphoblasts. In 2022, Saglam and colleagues reviewed the literature and summarized 45 cases of iT-LBP. Subsequent cases have been reported, but we estimate that no more than 60 cases of iT-LBP have been reported to date. As defined by others, iT-LBP is an extrathymic expansion of TdT+, CD3+ small to medium-size T-cells that form dense cell clusters or sheets, has a nondestructive growth pattern, shows no evidence of an aberrant immunophenotype or a monoclonal T-cell population, is not associated with thymic epithelium and is clinically indolent [8]. iT-LBP does not involve the bone marrow or mediastinum [8]. To date, iT-LBPs are regarded as reactive, polyclonal processes without recurrent genetic aberrations.

Cases of iT-LBP have been reported in association with many conditions. Two of the more frequent associations include unicentric CD [[9], [10], [11]] and follicular dendritic cell sarcoma [11,12]. iT-LBP also has been associated with nodal T-follicular helper cell lymphomas [13] and carcinomas including hepatocellular carcinoma [14] and acinic cell carcinoma [15].

Here we describe a case of stroma-rich variant CD presenting as a pelvic mass which evolved to an iT-LBP over the course of two years. At the time of diagnosis of iT-LBP a PDGFB mutation was identified. We review the literature on the association between CD and iT-LBP. We also discuss the presence of PDGFRB mutation and the differential diagnosis of iT-LBP.