PTK7 helps detect T lymphoblastic leukemia/lymphoma by flow cytometry

T lymphoblastic leukemia/lymphoma (T-ALL) is a malignancy composed of proliferating T lymphoblasts. T lymphoblasts can be identified by flow cytometric analysis through the detection of aberrant antigen expression and/or immaturity marker expression. An ideal marker would be expressed brightly on T lymphoblasts but absent in mature T lymphocytes. One such marker is protein tyrosine kinase-7 (PTK7). However, PTK7 has not been widely adopted in clinical flow cytometry labs or incorporated into any T-ALL flow cytometry best practice recommendations. To this end, we demonstrate the utility of PTK7 in flow cytometry panels for T-ALL diagnosis, minimal/measurable residual disease (MRD) detection, and relapse. We retrospectively evaluated flow cytometry data on 175 patients. PTK7 was classified as positive, showing a near two-fold difference in brightness versus background mature T cells, in 87.76% of T-ALL cases at initial diagnosis, 75% of T-ALL at MRD, and 100% of T-ALL at relapse. PTK7 expression remained intact in cases of CD34 and/or TdT negative T-ALL (p = 0.992) and while expression was dimmer at MRD (72% decrease, p = 0.0313), PTK7 remained intact at relapse (33% increase, p = 0.8125). PTK7 should be included in flow cytometry panels when evaluating for T-ALL, both at initial diagnosis, relapse, and for the presence of MRD.