Gamma Knife Radiosurgery

All patients with melanoma BrMets treated with Gamma Knife stereotactic radiosurgery (SRS) between 2007 and 2015 who also received either anti-CTLA-4 or anti-PD-1 immunotherapy were identified from an IRB-approved institutional database. Patients were excluded if they had leptomeningeal disease or no follow-up imaging after SRS. Individual lesions were also excluded within each patient’s data if they were post-operative resection cavities or if the lesions were associated with extensive extralesional hemorrhage. In patients who underwent SRS treatment more than once, each new lesion was studied independently and included in this study.
All patients were treated using the Leksell Perfexion Gamma Knife (Elekta Medical Systems, Inc.). Lesions were treated to a median of 20 Gy (range, 12-24 Gy) to the tumor margin, with doses individualized using institutional standardized modifications of RTOG 90-05,^{11 (link)} which take into account both tumor volume and number of lesions. Lower doses were prescribed for both increasing tumor volume and increasing number of lesions to be treated. Most patients treated with anti-CTLA-4 therapy received up to four doses of ipilimumab at either 3 mg/kg or 10 mg/kg; several of these patients later received a re-induction course. Patients treated with anti-PD-1 therapy received pembrolizumab, at doses of either of 2 mg/kg or 10 mg/kg every 2 or 3 weeks, or nivolumab, at doses of 3 mg/kg every 2 or 3 weeks. Examination of the number of days elapsed between SRS and either the first or last dose of immunotherapy for each lesion (with lesions treated during immunotherapy assigned a value of 0) demonstrated a cluster of lesions around +/− 4 weeks (Supplemental Figure 1). On this basis, immunotherapy and radiosurgery treatment to any single lesion was considered concurrent if SRS was administered within 4 weeks of the start or end of immunotherapy; all other lesions were defined as having had non-concurrent treatment.
3D MPRAGE, T1-weighted gadolinium enhanced MR images with 1 mm slice thickness of the whole brain were obtained on the day of SRS treatment and at each follow-up, as described in a previous publication from this institution.^{12 (link)} To determine lesional response, the maximal diameter of the T1 contrast-enhancing portion of each SRS-treated lesion was measured in three orthogonal planes at the time of treatment and at each follow-up by a single individual, to reduce inter-reader measuring errors. Lesion volumes were calculated using the formula (length × width × height)/2, as previously published.^{12 (link)} Data collection was censored for any single lesion if it required local intervention, such as surgery, laser thermocoagulation, or salvage radiation or if the patient received bevacizumab therapy. In addition, data collection was also terminated if the patient was switched from anti-CTLA-4 therapy to anti-PD-1 therapy during follow-up or vice versa. Volume changes at each follow-up were normalized to the baseline treatment volume. For a descriptive graphical analysis of temporal changes in volume, scans were grouped into intervals clustered at 1.5 months, 3 months, 6 months, 9 months, 12 months, 18 months, 24 months, and 36 months.

Rats of 250-300 g were exposed to local heart irradiation with the Small Animal Conformal Radiation Therapy Device (SACRTD) developed at our institution. The SACRTD has a 225kVp X-ray source (GE Isovolt Titan 225) mounted on a custom made “gantry”, a stage on a robotic-arm positioning system (Viper™ s650 Adept Technology, Pleasanton, CA), and a flat panel digital X-ray detector of 200 μm resolution (XRD 0820 CM3 Perkin Elmer, Fremont CA). For the purpose of local heart irradiation, a brass and aluminum collimating assembly produced a field of 19 mm diameter at the isocenter.
The dose rate at the isocenter was measured using a pin-point ion chamber (PTW N301013, ADCL calibrated for 225 kV) following the TG-61 protocol of the American Association of Physicists in Medicine. In addition, dosimetry was performed with Gafchromic® EBT-2 film (Ashland Specialty Ingredients, Wayne, NJ). A set of films was calibrated by exposing it to known doses on a Gamma Knife (Co-60) system, and the films were analyzed according to Devic et al. (24 (link)). A calibration curve was also drawn by exposing films with the SACRTD 225 kV X-ray beam. The films were energy independent and could be used for measurements of dose in the range used in our experiments. To measure relative depth dose, 11 pieces of film were placed in between 11 slabs of solid water phantom, each of 5 mm thickness. The top of the phantom was placed at the isocenter, perpendicular to the beam, and the films were exposed to 5 Gy at the isocenter (225 kV, 13 mA).
For local heart irradiation, rats were anesthetized with 3% isoflurane and placed horizontally in a Styrofoam holder. The X-ray source was tilted horizontally and a digital X-ray image was acquired with the detector (65 kV, 5 mA). The heart was localized and the gantry was tilted vertically for irradiation. The heart was irradiated at 225 kV, 13 mA, (0.5 mm Cu-filtration) resulting in 1.92 Gy/min at 1 cm tissue depth. The hearts were exposed in three 19 mm-diameter beams of 6 Gy or 8 Gy each. An angle of 30° between the beams (one vertical beam, one beam -30° from vertical, and one beam +30° from vertical) was established by tilting the platform.
Rats were observed for 3 months or 6 months after irradiation to determine cardiac function, structure, and molecular changes as described below.