Introduction. The limited selectivity of antitumor agents is the reason behind toxicity developing as a result of chemotherapy, radiation therapy and other treatment modalities for malignant neoplasms. Hematological toxicity in patients with various types of malignant neoplasms manifests as a change in the bone marrow (BM) structure with an increase in fat in the setting of inflammatory cascade activation with the elevation of pro-inflammatory cytokines such as interleukins 2 and 6, tumor necrosis factor and others. The non-invasive quantitative BM assessment by magnetic resonance imaging (MRI) performed in the patient of interest described below allows us to move to a new level of verification of the fundamental principles of hematological toxicity. Clinical case. Here, we discuss the results of BM fat assessment in the clinical case of a 12-year-old patient who underwent rehabilitation therapy after completing treatment for pleuropulmonary blastoma of the right lung. Quantitative MRI revealed a heterogeneous fat compartment in the bones, with maximum values registered at the tumor site and in the irradiated area: in the Th4 vertebral body – 57%, in the Th5 body – 76%, Th6 – 75%, Th7 – 70%. Conclusion. The use of quantitative BM assessment in children will make it possible to develop personalized approaches to radiation therapy as well as to monitor BM reconstitution processes following the end of cancer treatment. Future research also needs to more thoroughly examine the involved pathogenetic mechanisms and the BM lipodystrophy phenomenon in cancer patients. © 2025 by «D. Rogachev NMRCPHOI».