Background/Aim: In the era of immunotherapy, treatment-related lymphopenia (TRL) is gaining attention. In this study, TRL was investigated in patients with bone metastasis from hepatocellular carcinoma (HCC) treated with radiotherapy (RT). Methods: Clinical data of 302 patients receiving RT for 511 bone metastases from HCC between 2005 and 2018 were reviewed. Data on absolute lymphocyte count (ALC) from pre-RT to 12 months post-RT were collected. Severe TRL was defined as ALC <500 cells/mm3 and evaluated using ALC 2 months after initiating RT. Factors associated with TRL were analyzed, which include the amount of active bone marrow within the RT field. The amount of active bone marrow included in the RT field was calculated as the product of the percentage of the bone compartment included in the RT field and the active bone marrow percentage of the bone compartment. Results: Overall, 33.4% of patients developed TRL 2 months after initiating RT. The mean ALC decreased after initiating RT and remained persistently low during 12 months of observation. Overall survival (OS) was significantly worse in patients with TRL than in those without (median OS: 3.7 vs. 6.5 months, p < 0.001). In the prognostic factor analysis, TRL was an independent prognostic factor of OS (p = 0.036), along with known prognostic factors of HCC. The percentage of active bone marrow within the RT field was the only significant factor associated with TRL (p < 0.001). Conclusion: TRL was observed in patients receiving RT for bone metastasis from HCC, and it was associated with poor survival. The percentage of active bone marrow within the RT field significantly affected TRL development. The results suggest that a new strategy is required to prevent TRL.
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