BONE MARROW INFILTRATION IN METASTATIC NEUROBLASTOMA: DISTINCT CYTOHISTOLOGICAL PATTERNS AND THEIR PROGNOSTIC IMPLICATIONS

Background. Neuroblastoma (NB) is the most common extracranial solid paediatric tumor, and it is responsible for 15% of all childhood cancer-related deaths. Bone marrow (BM) is a frequent site of metastasis at diagnosis in stage M NB. The BM microenvironment plays a crucial role in tumour progression, and BM infiltration at diagnosis is a key factor in staging and prognosis. Despite aggressive multimodal therapies, treatment resistance in BM-infiltrating tumour cells remains a major obstacle to improved outcomes. Materials and methods. We retrospectively reviewed BM blood smears and trephine biopsies from patients with stage M NB at diagnosis treated at the Paediatric Haematology-Oncology Unit, Department of Woman’s and Children’s Health of Padua University from January 2004 to April 2023. The presence/ absence of BM infiltration and, in the positive cases, the pattern of cytohistological infiltration, the percentage of tumour-occupied area, the grade of fibrosis, osteosclerosis and collagen deposition were assessed. Clinical data were retrieved, and correlations between morphological features and prognosis were evaluated. Results. Fifty-two patients with diagnosis of stage M NB according to the INRGSS criteria were identified. Bilateral blood smears from 52 patients and bilateral trephine BM biopsies from 23 patients were reviewed. The median age was 2.95 years (0.25-6.83 years), and the mean age was 2.75 years (range 0.25-6.83). Thirty-three of 52 (63.5%) patients were males and 19/52 (36.5%) were females. Eight of 52 (15%) patients tested negative for BM infiltration, while blood smears from 44/52 (85%) patients and trephine BM biopsies from 19/23 (83%) patients were tested positive. Trephine BM biopsies from 4/23 patients were inadequate. Cytologically and histologically, two different patterns of BM infiltration emerged: i) “A” pattern characterized by the presence of clumps of NB cells in the BM blood smears (25/44, 57%) and by paratrabecular and diffuse aggregates of neoplastic cells with fibrosis, without intermingled haematopoietic cells, in the trephine biopsies (9/19, 47%), and ii) “B” pattern characterized by diffuse scattered and disperse neoplastic cells in the BM blood smears (19/44, 43%) and by intrasinusoidal/interstitial and diffuse infiltration of neoplastic cells intermixed with hematopoietic cells, without fibrosis, in the trephine biopsies (10/19, 53%). One discordant case showed diffuse scattered and disperse neoplastic cells on the blood smear and paratrabecular and diffuse infiltration with massive fibrosis on the trephine biopsies. Five-years OS was 49% for “A” pattern, and 29% for “B” pattern. Five-years EFS was 50% for “A” pattern, and 7% for “B” pattern. Patients with “B” pattern had significantly higher rates of progression or relapse (85%). The patient with discordant pattern between blood smear and trephine biopsy is still alive in complete remission. The presence of fibrosis noted in “A” pattern was associated with systemic symptoms (fever and pain) and better outcomes. Patients with “B” pattern infiltration had higher median SIOPEN scores (41 vs. 21 for “A” pattern and 9 for BM-negative patients), correlating with a worse prognosis despite initial good treatment responses. Conclusion. BM infiltration patterns in high-risk NB have distinct prognostic implications. “B” infiltration pattern is associated with poorer OS and EFS, while “A” infiltration pattern, in the presence of fibrosis, correlates with better outcomes, potentially due to a stronger inflammatory response. Understanding these patterns can guide more personalized therapeutic strategies for high-risk NB patients.