Noninvasive Stereotactic Radiosurgery Instrument-Based Spatially Modulated Radiotherapy (GRID) for Brain Metastasis: A Case Study.

Purpose/Objective(s) To compare (1) therapeutic ratio (TR) of spatially modulated radiotherapy (GRID) optimized with conventional collimator attached to the head of clinical linear accelerator (LINAC) and (2) planned with Gamma Knife System. Equivalent uniform dose to tumor (EUDT) and TR to normal tissue were evaluated. Materials/Methods GRID technique was used in our institution to manage bulky tumor in selected patients using a specially designed collimator installed in the head of LINAC. One of the patients in GRID study had a recurrent glioblastoma treated with 60 Gy and 40 Gy for the first and the second round followed by GRID boost. GRID dose was focused to GTV with small margin (PTVGRID), which is still much smaller than CTV and PTV. Considering inherently heterogeneous dose distribution in Gamma Knife (GK) plans where the prescription dose is specified to about 50% of the maximum dose, the clinical GRID plan using the conventional collimator was compared with the simulated GK plan. 14mm diameter of collimators with 21mm separation was used in the clinical GRID plan and two 16mm collimator and one 8 mm collimator were used in the simulated GK plan. Prescribed dose of GRID plan was 5 Gy and 12 Gy to the maximum in PTVGRID for the clinical and GK plan, respectively. Maximum dose to CTV and PTV outside of PTVGRID was evaluated. Linear quadratic cell surviving curve was used for TR and EUD calculation with 2 Gy surviving fraction of 0.4 and 0.5 for radio resistance tumor and normal tissue, respectively. Results Volume of GTV, PTVGRID, CTV and PTV are 57.4cc, 102.4cc, 290.8cc and 411.3cc, respectively. The minimum dose of PTVGRID was about 20% of maximum or 1 Gy in the conventional GRID and it was the same for GK GRID. TRs in GTV and PTVGRID are 1.6, 1.54 and 2.09, 1.54 in the clinical and GK plan respectively. TR in the middle of the GTV was 14% higher in the GK plan. EUDT in GTV and PTVGRID were 2.3 Gy, 2.2 Gy and 2.9 Gy, 2.3 Gy in the clinical and GK plan respectively. The maximum dose to CTV and PTV outside of PTVGRID were 5.4 Gy, 5.5 Gy and 4.7 Gy, 2.0 Gy. GK plan shows highly modulated dose distribution contained inside of the targets which leads similar or better TR and higher EUDT in the PTVGRID. Much reduced hotspot outside of PTVGRID assures safe delivery of high dose in the brain with radio-sensitive serial organs. Conclusion Feasibility of GRID plan using Gamma Knife was investigated in this case study. Larger TR and EUDT are achievable with GK compared to conventional collimator-based GRID. Safer dose delivery is also expected considering large number of radio-sensitive serial organs in brain. Further investigation is warranted to optimize GRID dose distribution using GK planning. Author Disclosure Y. Cho: None. J.S. Yu: None. J.H. Suh: Consultant; Philips, Novocure. Met to discuss current indications for TTF; Novocure. Met during 1 day retreat to discuss future of radiation oncology; Philips. Scientific advisory board; Neutron Therapeutics. B. Guo: None. S.T. Chao: Honoraria; Varian Medical Systems, Zeiss, AbbVie. Consultant; AbbVie.