Single and multitarget stereotactic radiosurgery (SRS) with single isocenter in the treatment of multiple brain metastases (BM): institutional experience

SRS for the treatment of limited brain metastases (BM) is widely accepted, but there are still limitations in the management of numerous BM. Frameless single-isocenter multitarget SRS is a novel technique that allows for rapid treatment delivery to multiple BM. We report our preliminary clinical, dosimetric, and patient´s shifts outcomes with this technique.

We have reviewed clinical and dosimetric outcomes of patients with intact BM treated with SRS using one isocenter either for single (1BM) or multiple (≥ 2BM) targets). Immobilization was based on an SRS stereotactic mask. Elements Multiple Brain Mets SRS (Brainlab AG, Munich, Germany) software was used for registration, image fusion, target contouring, and treatment planning. Exactrac Dynamic (Brainlab AG, Munich, Germany) and a 6 degree of freedom couch were used for monitoring, correcting the position and assessing and applying residual errors also when couch rotations. During dose delivery, the patient position was monitored and registered using surface tracking and stereoscopic X-rays.

From May 2022 to December 2023, we treated 60 patients with a total of 255 BM. The 67% of patients had at least 2 BM treated and the average of treated BM per patient per course was 3.6 (range 1-13). The average total treated BM per patient (sum of all courses) was 4.4. Lung cancer was the most frequent (63%) primary tumor. 77% of cases were patients with a brain relapse and the remaining 23% had BM at diagnosis. Ninety-two percent of BM were treated with single fraction. The most used fractionations were 20 Gy (27.8%) and 21 Gy (43.5%), respectively, and the median PTV target volume (if single fraction) was 0,2 cc (range 0.016-4.32 cc). The median cumulative target volume per isocenter and the sum of all SRS courses were 1.37 and 1.46 cc, respectively. The 100% of patients completed the SRS treatment with no incidences. With an average follow-up of 8.3 months (0.1-19 months), we have not identified any local relapse, although 27% developed an intracranial relapse that was again treated with SRS in the 44% of cases. We did not find any relation between overall survival and the presence of any driver mutation (p = 0.97), presence of BM at diagnosis vs. recurrences (p = 0.113), number of courses of SRS (p = 0.688), number of isocenters (p = 0.679), or number of treated BM (1 vs. 2-3 vs. ≥ 4; p = 0.7). Healthy normal tissue constraints were adequately accomplished with a median V12 (if single dose) and V20 (if 5 fractions) of 0.2 and 5 cc, respectively. No acute toxicity > G2 was reported. Regarding patient positioning, monitoring, and registration based on X-ray imaging and surface guidance, patient shifts distributions were centered at 0.0 mm with standard deviations below 0.25 mm, except for the longitudinal shift based on X-rays, which was 0.35 mm. This implies an adequate fixation system, patient setup, and image guidance protocols. The mean total delivery time per fraction, from the first beam-on to the last beam-off, was 9.6 ± 4.8 min, with a range of 4.6-30.9 min. On average, repositioning occurred 1.2 times per fraction based on X-ray guidance and 0.6 times per fraction based on surface guidance.

Based on our preliminary experience, we find single isocenter for single and multitarget SRS technique is feasible, well tolerated and allows excellent local control. Regarding patient positioning, monitoring, and registration based on X-ray imaging and surface guidance, patients' shifts and repositioning rate are low enough to show an adequate fixation system, patient setup, and image guidance policies at our institution. Patient shifts during treatment are effectively managed by X-ray and SGRT verification. Low shift tolerances ensure patient stability, resulting in acceptable treatment times and patient repositioning rates. This dedicated workflow for SRS at our institution demonstrates excellent clinical outcomes. A longer follow-up period is necessary to evaluate the impact on long-term clinical outcomes.