The 2008 WHO “Classification of Tumors of Hematopoietic and Lymphoid Tissues” has addressed the problem of “in situ” lesions among the early events in the evolution of lymphoid neoplasia . “In situ” lesions have been recognized for both follicular lymphoma (FL) and mantle cell lymphoma . The concept of in situ lymphoma/“intrafollicular neoplasia” has also been proposed for other lymphomas in which the putative normal counterpart of the tumor cell is located within the germinal center [3, 4]. However, with the exception of in situ FL, the precise histologic definition of these early lesions for other lymphomas is still lacking , in particular for Hodgkin lymphoma. As recently proposed [3, 6], the recognition of “intrafollicular neoplasia/in situ lymphoma for nodular lymphocyte predominant Hodgkin lymphoma (NLPHL)” relies on the identification of typical, strongly stained BCL6+ and CD20+ lymphocyte predominant (LP) tumor cells, which are surrounded by rosetting PD1+ T cells and develop within an environment reminiscent of a secondary follicle. Along with this presentation in the early phases of the disease, however, NLPHL may also consist of neoplastic nodules showing “follicle-like” structures, reminiscent of a primary follicle, with prominent follicular dendritic cell (FDC) meshworks. CD23 and CD21 immunostaining detects meshworks of FDC within the nodules, which entrap the LP cells and the surrounding T-cell rosettes. LP cells reside within these “follicle-like” structures, are not detected outside them, and do not invade the surrounding space (non-invasive growth pattern)  (Fig. 1).
NLPHL is an indolent neoplasm with a tendency for local recurrences as well as a low rate of progression to diffuse large B-cell lymphoma (DLBCL) [7, 8]. According to the current WHO classification, at least a partial nodular pattern is required for a diagnosis of NLPHL . Based on the established WHO criteria, the diagnostic features include a nodular or a nodular and diffuse proliferation of scattered LP tumor cells, set in a background of reactive lymphocytes reminiscent of a primary follicle. Typically, indeed, the LP cells reside in nodular meshworks of FDC processes . Noteworthy, a purely diffuse pattern would be classified as a DLBCL or T cell/histiocyte rich B-cell lymphoma (TCRBCL).
In the NLPHL pattern originally described by Fan et al. as “pattern A” , where LP cells reside within the classic neoplastic nodule, rare LP cells are also seen outside the nodule (early invasive pattern). During the progression of the disease, increasing number of LP cells extends outside the neoplastic nodules and infiltrate the perinodular space (invasive pattern). Importantly, the presence of numerous LP cells outside the nodules may characterize the frankly invasive growth pattern (originally described by Fan et al. as “nodular NLPHL with prominent extranodular LP cells”) (Fig. 1). It may represent early progression to a diffuse pattern and predicts for progression to a diffuse lymphoma, i.e. DLBCL .
To investigate the clinical relevance of recognizing pathologic features and the patterns of growth of NLPHL (see above), we reviewed a single institution series of 42 patients (total of 52 samples) affected by NLPHL. The patients entering the present study were diagnosed and treated at CRO of Aviano from 1986 to 2005 and selected among the total series of about 100 NLPHL patients observed during the same period of time. Patients with a follow-up >5 years from the initial diagnostic biopsy were included in this analysis. The majority of patients (28/42—67%) presented limited disease, (Stage IA–IIA). Twenty-seven patients (64%) received chemotherapy alone (10 patients) or in combination with radiotherapy (17 patients); 12 patients (29%) received radiotherapy alone and the remaining 3 patients (7%) did not receive any treatment after excisional diagnostic biopsy. All patients achieved a complete remission and 30/42 (71%) of them never relapsed. The rest of patients relapsed locally without upstaging (12/42—29%) and 2 patients received high dose chemotherapy and autologous bone marrow transplantation. Only two patients died due to the transformation of the lesion to a DLBCL. These findings are very important from a clinical point of view in that many patients who relapse do not need an aggressive approach, for example autologous bone marrow transplantation, as in classical HL.
From a pathologic point of view, in situ NLPHL and the non-invasive NLPHL pattern were recognized in 21 cases. In these cases, the immunohistologic study revealed “intrafollicular neoplasia” (in situ NLPHL) or detected “follicle-like” neoplastic structures (non invasive pattern), respectively. Interestingly, these findings were consistently associated with the pattern A of Fan (early invasive pattern), where rare LP tumor cells did expand outside the classic neoplastic nodules. A frankly invasive pattern was seen in 11 NLPHL cases in which LP cells were prominent outside the nodules. Non-invasive and/or early invasive pattern admixed with frankly invasive pattern were observed in the remaining seven cases. Three cases were unclassifiable for technical reasons. In conclusion, when applied to NLPHL, “intrafollicular neoplasia” could be observed in lymph nodes that otherwise showed the most common classic nodular involvement by NLPHL. Moreover, the association of non-invasive with early invasive pattern of growth was observed in 13 out of 21 patients presenting with limited disease (Stage IA–IIA) and having a high favorable outcome. Importantly, none of these patients had progression of disease.
Overall, the good clinical behavior displayed by most patients affected by NLPHL was related both to the clinical stage at presentation (early) and to the pathologic patterns (non invasive or early invasive). In fact, in these patients with favorable outcome, most NLPHL showed a prominent entrapping of LP tumor cells within the transformed follicles (in situ NLPHL) or the neoplastic nodules (non-invasive pattern). In these cases, classic neoplastic nodules were also observed but only rare LP tumor cells were present outside the nodules (early invasive pattern). Based on these results, we suggest that NLPHL displaying “intrafollicular neoplasia” associated with non-invasive and/or early invasive pattern of growth and limited clinical stage could be regarded as an early step of the disease, with highly favorable outcome.
The acknowledgement of pathologic features related to early phases of tumor progression is presently very important, since clinical studies aimed to establish whether lymph node resection alone in selected patients with NLPHL can be considered as a therapeutic option, are ongoing. The inclusion of immunoarchitectural patterns within prognostic factors (mostly clinical) might improve risk stratification of patients and drive therapy. Importantly, one clinical study from the European Network Group on Pediatric Hodgkin Lymphoma (EuroNet-PHL)  demonstrated that a substantial fraction of patients with surgically treated early-stage NLPHL experienced long-term remission and actually may have been cured. In these patients, who did not underwent any further treatment, the disease was clinically with limited stage, putatively early, and with a highly favorable outcome. A centralized morphological review of the histological specimens from these patients might contribute to highlight which immunoarchitectural patterns are related to clinical limited stage and to favorable outcome.