Stereotactic Radiosurgery (SRS) vs. Hypofractionated Stereotactic Radiotherapy (SRT) in Brain Metastases: Kinetic Responses

Abstract

To evaluate the treatment responses of patients with brain metastases treated with SRS or SRT. Specifically, we intend to assess the differences in the kinetic response of SRS versus SRT. Brain metastasis patients who underwent SRS/SRT were collected consecutively between January 2014 and March 2021. The prescribed dose varied according to the tumor size. For SRS, the dose prescription followed the RTOG 90-05 protocol with 15 Gy, 18 Gy, and 24 Gy to the PTV for tumor diameters of 3.1-4 cm, 2.1-3 cm, and ≤ 2 cm, respectively. SRT was commonly used to treat larger lesions (3 cm in diameter) or those close to critical organs. The most common doses prescribed for SRT were 27 Gy in 3 fractions and 30 Gy in 5 fractions. All patients must have at least one imaging after treatment. If the tumor volume increased after the first imaging, which could be due to radiation necrosis or tumor progression, the following images would be excluded from the analysis. Multilevel linear regression was used to compare the kinetic response of SRS and SRT. The initial tumor volumes were adjusted in the model. Fifty-five patients with 128 brain metastases were treated with SRS (n = 85 lesions) or SRT (n = 42 lesions) between January 2014 and March 2021. At the time of analysis, 285 follow-up MRI scans were evaluated. Most patients had non-small cell lung cancer (75 lesions with SRS vs 20 lesions with SRT), followed by breast cancer (6 lesions with SRS vs 13 lesions with SRT). The median of the total dose (IQR) was 24 Gy (21-26 Gy) for SRS and 34 Gy (26-37 Gy) for SRT. For plan quality metrics in SRS/SRT, the median of Paddick's conformity index was 0.76 (IQR = 0.57-0.89) in SRS vs 0.84 (IQR = 0.69-9) in SRT with no statistically significant difference (p-value = 0.095). Likewise, the median homogeneity index in SRS was 1.34 (IQR = 1.22-1.43), compared to 1.36 (IQR = 1.21-1.57) in SRT, with no statistically significant difference (p-value = 0.21). The mean (range) of the initial tumor volume of SRS and SRT was 1 cc (0.01-13.7 cc) and 4 cc (0.02-17.70 cc), respectively. All tumors decreased in volume after treatment. The rate of tumor shrinkage in SRS was 0.13 cc per month (0.004 to 0.25, p-value 0.043). Compared to SRS, SRT made the tumors smaller at an additional rate of 0.45 cc per month (0.24 to 0.66, p-value < 0.001), even after adjusting for initial tumor volume. Most tumor progression or radiation necrosis appeared between 9 and 12 months after treatment. Following treatment, SRT exhibits better kinetic responses than SRS.