Did I Miss It? Discovering Hidden Coexisting Hematological Neoplasms: A Single Institutional Review of 100 Collision Tumors.

A collision tumor is defined as two histologically distinct tumor types identified at the same anatomic site. Hematolymphoid proliferative disorders (HLPDs), which coincide with non-hematological neoplasms, can mimic an immune response and can easily be overlooked as an immune reaction to a solid organ neoplasm, especially when low grade. In order to avoid a delay in the diagnosis of a HLPD during the workup for a non-hematological neoplasm, we identified a cohort of 100 cases with a HLPD diagnosis during the initial workup and treatment of a non-hematological neoplasm, or vice versa. Among the 100 collision tumors, the most common non-hematological neoplasms associated with a HLPD were from the colon (17%), breast (15%), and prostate (12%). The most commonly identified HLPDs were chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL; 18%), diffuse large B-cell lymphoma (17%), follicular lymphoma (14%), marginal zone lymphoma (10%), acute myeloid leukemia (8%), and classical Hodgkin lymphoma (5%). Interestingly, in this cohort 5% of the low-grade HLPDs, all of them CLL/SLL, were missed at initial sign-out and subsequently required an addendum report. The other 95% of cases were reviewed or signed out by a hematopathologist before the report was finalized for the non-hematological neoplasm. In summary, high-grade hematological malignancies are less likely to be missed; however, low-grade coexisting HLPDs can be overlooked as a reactive immune response to a solid organ neoplasm. Therefore, it is important to keep in mind the existence of collision low-grade HLPDs before assuming the lymphoid infiltrates as an immunological response.