# Newly diagnosed B-cell acute lymphoblastic leukemia demonstrating localized bone marrow infiltration exclusively in the lower extremities

## TL;DR

A five-year-old boy with B-cell ALL showed localized bone marrow infiltration only in his lower extremities, challenging the usual pattern of leukemia spread.

## Contribution

This case report presents a rare instance of localized B-cell ALL in newly diagnosed pediatric patients.

## Key findings

- The patient had B-cell ALL confined to the lower extremities with normal bone marrow at conventional biopsy sites.
- Standard ALL treatment led to complete remission with no relapse after two and a half years.
- Localized ALL may be missed if only standard bone marrow sites are sampled.

## Abstract

Acute lymphoblastic leukemia (ALL) is generally thought to be widely distributed in the bone marrow (BM). Localized involvement of BM from particular bones is extremely rare, especially in newly diagnosed cases. Here, we described a newly diagnosed case of B-cell ALL localized to the lower extremities in a five-year-old boy.

The patient presented with pain in both knees for over one month. Imaging findings indicated pathological fracture of the right distal femur and bilateral proximal tibiae. An open biopsy of the right distal femur lesion was performed, and BM cytology and histopathological analysis confirmed the diagnosis of B-cell ALL. However, his whole blood and BM findings on usual biopsy sites (the sternum and iliac bone) were normal. He received standard treatments for ALL, and achieved complete remission. He has been on maintenance therapy till now without evidence of relapse for two and a half years.

The present case highlights the fact that, in exceptionally rare circumstances, ALL may initially manifest as localized infiltration within BM of specific bones, rather than exhibit diffuse BM involvement, which renders BM aspiration and biopsy only from the usual sites insufficient for the diagnosis of leukemia. Therefore, clinicians should maintain a high index of suspicion for acute leukemia when facing any child with unexplained persistent skeletal pain and radiographic abnormalities even in the absence of abnormal hematologic findings, and strongly consider performing targeted BM biopsies of radiologically abnormal bone lesions alongside conventional BM sampling sites.