• Primary hyperoxaluria is a rare disease. It often results in end-stage renal failure. It is inherited in an autosomal recessive manner. It is classified into types 1, 2, and 3, which are due to mutations in AGXT, GRHPR, and HOGA, respectively.

• The deposition of oxalic acid crystals in the bone marrow can be associated with varying degrees of cytopenias and/or leukoerythroblastic reaction and/or hepatosplenomegaly. Involvement of the bone marrow rarely precedes diagnosis (for reference, see PMID  39464227, 37551519, 28943803).

We thank the Nephrology Clinic of the General Hospital of Athens 'Evangelismos' for the opportunity to study this interesting case. 

• The 'pattern 1' of angioimmunoblastic lymphoma mimics lymphoid follicular hyperplasia. Therefore, it presents a diagnostic challenge as it may go unnoticed.

• The diagnosis of 'pattern 1' requires a high degree of suspicion, the use of a combination of specific and sensitive TFH markers, and molecular confirmation through the detection of T-cell clonality or mutations in RHOA p.G17V or IDH2 p.R172 (for reference, see PMID 19329936).

We thank our colleague, hematologist Ms. K. Skarmoutsou-Tsimirika, for the opportunity to study this interesting case.

• This is a lymphoma of T or NK-cell origin, characterized by: (a) extranodal localization, (b) cytotoxic immunophenotype, (c) necrosis, (d) angiocentrism, and (e) association with EBV infection. It is also referred to as "nasal-type" due to its frequent primary localization in the midline structures of the viscerocranium.

• The combination of all the above features is highly characteristic and allows for reliable distinction from other lymphoma types. The biological behavior is aggressive (for reference, see PMID 28410601).

We thank the Hematology Clinic of the General Hospital of Athens 'Evangelismos' for the opportunity to study this interesting case. 

• Fluid overload–associated large B-cell lymphoma (FO-LBCL) presents as serous effusions without detectable tumour masses.

• The differential diagnosis primarily includes primary effusion lymphoma (PEL). In contrast to PEL, FO-LBCL is not associated with KSHV/HHV8, expresses B-cell markers without a terminal B-cell differentiation phenotype (CD20+, PAX5+, CD138-), and is associated with a more favorable prognosis (for reference, see PMID 32936906).

We thank the Hematology Clinic of the General Hospital of Athens 'Alexandra' for the opportunity to study this interesting case. 

• NPM1-mutated AML typically exhibits a myeloid, myelomonocytic, or monocytic phenotype. Erythroblastic and/or megakaryoblastic differentiation is rare (for reference, see PMID 40370262, 19490647).

• Cytoplasmic positivity for NPM1 indicates a mutation in the corresponding gene (either exon 12 or non-exon 12) (for reference, see PMID 34343258). In the present case, the NPM1 mutation was confirmed molecularly.

We thank the 2nd Department of Internal Medicine, National and Kapodistrian University of Athens, at “Hippokration” General Hospital for the opportunity to study this interesting case.

• The neoplastic nature of the T-cell population is supported by the combination of morphological atypia, abnormal immunophenotype (loss of BCL2, see PMID 35452696, 23702649), molecular detection of clonality, and a double TET2 mutation (non-specific but characteristic, see PMID 22760778, 38128263).

• Cases of TFH lymphoma with paradoxically indolent clinical course without therapy have been reported in the literature (see PMID 38128263).

We thank the 3rd Department of Internal Medicine, 417 NIMTS Hospital, for the opportunity to study this interesting case.