Introduction/Background This study aimed to analyse the association between pelvic bone marrow (PBM) exposure and acute haematological toxicity (HT), in gynaecologic cancer patients treated with concurrent chemo-radiotherapy (CCRT) using VMAT technique.

Methodology Data of all gynaecologic cancer patients treated with VMAT CCRT were analysed. Haematological data were collected from CBC at baseline and week 5 of treatment. For each blood cell line, we calculated the difference between pre-treatment count and week 5 count(delta). Acute HT was graded according to WHO severity score. PBM delineation included iliac crests, lower pelvis and lumbosacral region. The following dosimetry metrics to PBM were assessed: Dmean, V5Gy, V10Gy, V20Gy, V30Gy and V40Gy. Statistical associations between dosimetry parameters, delta CBC and graded HT were analysed.

Results Thirty-one eligible patients were included in this study. The study cohort included 51.7%, 41.3% and 7% of endometrial, cervical and vulvar cancer patients, respectively. Radiotherapy dose ranged between 45 and 66Gy. Additional boost to positive nodes was prescribed in 9 cases. PBM exposure is described in table 1.

Significant correlation was found between delta white blood cells (WBC), platelets and neutrophils and all PBM dosimetry metrics (Pearson coefficients ranging between 0.39 and 0.76; p≤0.03). The strongest correlation was found between V30 Gy and delta platelets (Pearson coefficient=0.76; p<0.0001). The V30Gy had a strong predictive value of platelets, neutrophils and WBC decrease (r²≥0.5). The V40Gy and Dmean showed similar predictive values of radiation HT (r² ranging between 0.31 and 0.52). Grade1–2/grade3–4 HT were reported in 93%/48.3% of patients. Nodal boost and chemotherapy cycles’ number were statistically related to higher risk of HT (p<0.05).

Conclusion PBM exposure in pelvic VMAT CCRT yielded high rates of subsequent acute HT. Dmean, V30Gy and V40Gy strongly predicted WBC, platelets and neutrophils decrease. Accurate and tighter dose constraints for PBM sparing are needed to reduce the risk of HT.

Disclosures No disclosures to declare.