## 3 Results
### Histological and Genetic Features of Necrotizing FLs  
In all cases, lymph node architecture was effaced by large-to-massive areas of necrosis with surrounding sclero tissue and variable numbers of atypical lymphoid cells. In two cases (case #1 and #2), necrosis was so abundant that it obscured the neoplastic population, which was only documented upon extensive sampling of the submitted specimens. The lymphoid proliferation had a nodular, interstitial and/or diffuse growth pattern with tick-walled, medium-sized vessels often infiltrated by neoplastic cells. Nodular areas were composed by small-to-medium lymphocytes with irregular nuclei, clumped chromatin and inconspicuous nucleoli and by numerous centroblasts. Diffuse areas consisted of large, mitotically active blasts with variably represented HRS-like cells (i.e., large, multi-nucleated blasts with prominent eosinophilic nucleoli and amphiphilic cytoplasm). The proportion between nodular and diffuse areas varied from case to case (2:1 to 1:2) (Figure 1).  
Immunohistochemical analysis revealed a strong positivity for CD20, CD79a, PAX5 and Bcl6 in both nodular and diffuse areas, with a heterogeneous expression of CD10. Bcl2 was positive in three out of four cases, while MUM1 and c-Myc were expressed in two and one cases, respectively. Immunostaining for CD23 showed residual dendritic cell meshworks in lymphoid nodules, while it was focally expressed in interstitial/diffuse areas with HRS-like cells. The latter were strongly positive for CD30 and pan-B cell antigens (CD20, CD79a and PAX5), with a sharp negativity for CD15 and EBER. In all cases, numerous CD3-positive reactive T cells were dispersed throughout the lesion and the Ki-67 proliferative index was moderate-to-high (range: 50% to 90%) (Figure 2). Of note, necrotic areas were diffusely positive for CD20 with only scattered positivity for CD3. In all cases, the overall histological picture was consistent with FL (G3A/G3B; follicular  
\begin{table}
\begin{tabular}{c c c c c} \hline  & **Case \#1** & **Case \#2** & **Case \#3** & **Case \#4** \\ \hline \multicolumn{5}{c}{**Clinical Features and Diagnosis**} \\ \hline Sex/age (y) & F/62 & M/74 & M/73 & M/66 \\ Clinical diagnosis & Inguinal hernia & Soft tissue & Soft tissue & Transformed \\ Local pain & Present & neoplasm & neoplasm & CLL/SLL \\ Disease Stage & IA & Present & Absent & Absent \\ Histological & FL (G3A) & IL & IA & IIA \\ diagnosis & w/ DLBCL & w/ DLBCL & FL (G3B) & FL (G3B) \\ Necrosis (\%) & 75 & 60 & 20 & 25 \\ Response to & Complete & Complete & Complete & \\ treatment & response & response & response & On treatment \\ \hline \multicolumn{5}{c}{**Immunophenotype/EBV status**} \\ \hline CD10 & + & + & + & + \\ Bcl6 & + & + & + & + \\ MUMI1 & - & + & - & + \\ Bcl2 & - & + & + & + \\ c-Myc & - & - & - & + \\ CD30 & + (HRS-like & + (HRS-like & + (HRS-like & + (blasts, cells) & HRS-like cells) \\ CD15 & - & - & - & - \\ Ki67 (\%) & 70 & 50 & 80 & 90 \\ EBER & - & - & - & - \\ \hline \multicolumn{5}{c}{**Gene Rearrangements (FISH Analysis)**} \\ \hline MYC & - & - & n.a. & - \\ translocations & - & - & n.a. & - \\ \multicolumn{5}{c}{_BCL2_} \\ translocations & - & - & n.a. & - \\ \multicolumn{5}{c}{_BCL6_} \\ translocations & - & - & n.a. & - \\ \hline \end{tabular} Notes: DLBCL = diffuse large B-cell lymphoma; FL = follicular lymphoma; G3A = grade 3A; G3B = grade 3B; HRS = Hodgkin/Reed–Sternberg; EBV = Epstein–Barr virus; w/ = with.  
\end{table}
Table 1: Clinical–pathological and genetic features of inguinal necrotizing follicular lymphoma (FL).  
growth pattern) with areas of diffuse large B-cell lymphoma (DLBCL) transformation. FISH analysis excluded _MYC_, _BCL2_ and _BCL6_ rearrangements in all tested cases (Table 1).