Evaluating the Use of Combined Gamma Knife/Linac Stereotactic Radiosurgery (SRS) Treatments in Patients with 10 or More Brain Metastases

Abstract

Traditionally, patients with multiple brain metastases were treated with whole-brain-radiotherapy. With the introduction of stereotactic radiosurgery (SRS), it is possible to treat individual metastasis while sparing the healthy brain tissues using Gamma Knife (GK) or linear accelerator (Linac). However, treatment of multiple metastases with GK might take hours as each target is separately treated. The treatment time could increase precipitately for big targets and some patients do not tolerate it. Linac treatments can treat multiple targets simultaneously with a much less time but the normal tissue toxicity, e.g., brain volume receiving at least 12 Gy (or V12Gy), is a concern. The purpose of this study is to evaluate the use of combined GK/Linac SRS treatments in patients with 10 or more brain metastases. The criteria for this retrospective study are: 1) patients who had at least 10 brain metastases, 2) previously treated with GK, and 3) had CT scans of the head prior to the GK treatment (necessary for the Linac plan). Three treatment plans were performed for each case: a GK plan to treat all metastases, a Linac plan to treat all metastases, and a combined GK/Linac plan where lesions larger than 1 cc were planned for Linac while the rest planned for GK. Plans were performed with a prescription dose of 18 Gy and the goal of achieving a target volume coverage of V100%≥95% for each target. Additional constraint for organs at risk were included during planning. The plans were compared in terms of target coverage and normal tissue sparing. Data from one patient has been analyzed. This case had 23 brain metastases, where 9 metastases had a volume of at least 1 cc. The total volume of the targets was 42.3 cc (0.01 to 11.4 cc). Considering the whole brain minus targets, the Gradient Index (GI), calculated as V50%/V100%, had the highest value for the Linac plan, followed by the combined plan. The V12Gy had the lowest value in the combined plan, followed by the Linac plan. The Conformality Index (CI) calculated as V100% of whole brain/ Volume of total targets, had the lowest value for the Linac plan, followed by the GK plan and the combined plan respectively. The total beam on time (BOT) was 10.5 min, 552 min, and 245.8 min for the Linac, GK and combined plan, respectively. The V12Gy and BOT of the GK plan was dominated by the large targets. This could be improved by adding blocking at the expense of an increased treatment time. As it is, the BOT of GK plan was 552 min, which could be considered unreasonable for a single fraction treatment. This work shows preliminary results on using combined GK/Linac SRS treatments in patients with 10 or more brain metastases. In the case analyzed here the combined GK/Linac plan resulted in the most competitive plan in terms of V12Gy while keeping a feasible BOT. Future work includes performing and analyzing plans for at least nine more patients.