A comprehensive review of MR imaging changes following radiosurgery to 500 brain metastases

Abstract

Background and purpose: Stereotactic radiosurgery is known to control 85%-95% of intracranial metastatic lesions during a median survival of 6-8 months. However, with the advent of newer systemic cancer therapies, survival is improving; this change mandates a longitudinal quantitative analysis of the radiographic response of brain metastases to radiosurgery.

Materials and methods: MR imaging of 516 metastases in 120 patients treated with GK-SRS from June 2006 to December 2009 was retrospectively reviewed. Lesion volume at initial treatment and each follow-up was calculated by using the following formula: length × width × height / 2. Volume changes were correlated with patient demographics, histopathology, and radiation treatment variables.

Results: Thirty-two percent of lesions increased in volume following radiosurgery. Clinically, this translated into 54% of patients having ≥1 of their lesions increase in size. This increase begins at 6 weeks and can last beyond 15 months' post-SRS. Male sex (P = .002), mean voxel dose <37 Gy (P = .009), and initial treatment volume >500 mm(3) (P < .001) are associated with posttreatment increases in tumor size. Median survival following radiosurgery was 9.5 months for patients with all lesions exhibiting stable/decreased volumes, >18.4 months for patients with all lesions exhibiting increased volumes, and 16.4 months for patients with mixed lesional responses.

Conclusions: Most metastatic lesions are stable or smaller in size during the first 36 months post-SRS. However, a transient increase in volume is seen in approximately one-third of lesions. Sex, treatment dose, initial lesion size, and histopathology all correlate with variations in lesion volume post-SRS. The longer the patient survives, the more likely an increase in lesion size will be seen on follow-up imaging.

Figures

Fig 1.

Examples illustrating the study definitions of groups A, B, and C. A−D, Representative images from a group A lesion (increased in follow-up to a volume >120% of initial size). A, Gamma knife treatment plan for a 36-year-old woman nonsmoker with non-small cell lung carcinoma. The lesion was treated with 18 Gy to the 50% isodose line. B, Initial MR image. C, Six-month follow-up MR image. D, Twelve-month follow-up MR image. E−H, Representative images from a group B lesion (size fluctuated throughout follow-up but never increased beyond 120% of the initial size). E, Gamma Knife treatment plan for a 75-year-old man with colorectal adenocarcinoma. The lesion was treated with 20 Gy to the 50% isodose line. F, Initial MR image. G, Three-month follow-up MR image. H, Six-month follow-up MR image. I–L, Representative images from a group C lesion (remained stable or decreased in size throughout follow-up). I, Gamma Knife treatment plan for a 76-year-old woman with 2 metastatic breast cancer lesions. Both lesions were treated with 20 Gy to the 50% isodose line. J, Initial MR image. K, Six-month follow-up MR image. L, Twelve-month follow-up MR image.

Fig 2.

Average change in lesional size with time, relative to initial treatment volume, all lesions. Lesions decreased or remained stable in size for the first 9 months post-SRS. Subsequently, they increased in size until approximately 18 months post-SRS, at which point they began to decrease in size once again.

Fig 3.

Average change in lesional size with time, relative to the initial treatment volume, separated by histopathology (radiosensitive: lung, breast, colon, other; radioresistant: melanoma, renal). Radiosensitive tumors were more likely to increase in size during the first 12–18 months post-SRS than radioresistant tumors. However, this difference did not extend, consistently, beyond 18 months.

Fig 4.

Kaplan-Meier survival curve, by lesional profile, demonstrates that patients with lesions that all increased in size following SRS had significantly improved survival (P = .035).

Fig 5.

Example of radiologic and histopathologic changes in enlarging lesions. A, Gamma Knife treatment plan for a 56-year-old man with metastatic melanoma. The lesion was treated with 22 Gy to the 50% isodose line. B, Twelve-month follow-up T1 postcontrast MR imaging. C, Twelve-month FLAIR image. D and E, Twelve-month DWI/apparent diffusion coefficient images, respectively. F, Twelve-month FDG-PET image. G, Histopathology from stereotactic image-guided biopsies of the lesion. Specimens from the central, T1 hypointense portion of the lesion demonstrate coagulative necrosis. Specimens from the peripheral, T1 hypointense portion of the lesion demonstrate reactive gliosis and demyelination. Specimens from the T1 contrast-enhancing portion of the lesion demonstrate vascular hyalinization. This constellation of histopathologic findings suggests a diagnosis of radiation-induced changes.