Interfractional change of tumor volume during fractionated stereotactic radiotherapy using gamma knife for brain metastases.

purposeFractionated stereotactic radiotherapy (FSRT) using gamma knife is useful for brain metastases. However, several uncertainties derived from fractionation pose issues for maintaining high-level accuracy. This study analyzed interfractional tumor change by performing radiological reassessment at the midterm of FSRT with ≥ 10 fractions, and the significance of replanning was evaluated.

methodsData of FSRT using gamma knife with ≥ 10 fractions were retrospectively collected. Interfractional volume changes in MRI at the midterm of the irradiation period were analyzed. Radiological changes after FSRT and final outcomes were also investigated.

resultsOverall, 114 lesions in 74 treatments from 66 patients were included, with previously irradiated lesions accounting for 46%. The median interval between planning and the interfractional MRI was 7 days. The interfractional change rates of tumor volume ranged from - 48 to + 72%. Significant interfractional enlargement was observed in 16 lesions (14%); evident regression was confirmed in 17 lesions (15%). Predictive factors for interfractional enlargement were small tumor and cystic lesion; high biologically effective dose was associated with regression. After FSRT, most lesions regressed within 6 months despite interfractional change type. The incidences of tumor control and radiation necrosis indicated no differences between interfractionally-regressed lesions and others.

conclusionThis is the first study to evaluate interfractional tumor change in FSRT using gamma knife with ≥ 10 fractions, indicating significant volume changes in 29% of the lesions. These preliminary results suggest that interfractional reassessment of a treatment plan in FSRT with irradiation periods exceeding a week is necessary for more adaptive treatment.