Dosimetric Comparison of Multiple vs Single Isocenter Technique for Linear Accelerator-Based Stereotactic Radiosurgery: The Importance of the Six Degree Couch

Abstract

Previous studies have evaluated the feasibility of a single isocenter technique (SIT) for linear accelerator-based stereotactic radiosurgery (SRS). However, recent data has demonstrated that not controlling for rotational error reduces target coverage. With the advent of the robotic six degree of freedom (6DoF) couch, we sought to evaluate its effect on patients with multiple lesions. Patients treated with SRS at our institution to ≥ 3 metastases were identified. Original treatment plans involved multiple isocenters (MP). The lesions were re-planned at same prescription dose and normalization using SIT with 2 coplanar arcs and 3 non-coplanar half arcs at approximately 45, 90, and 135 degrees (SP). SPs were evaluated to identify planning treatment volumes (PTVs) >5cm from isocenter. Lesions 5-10cm from isocenter had an additional 1mm of margin, and lesions > 10cm from isocenter had an additional 2mm of margin added. Plans were re-run with these margins to account for inability to correct rotational error (SP+1). Volume of brain receiving 12 Gy (V12) and mean brain dose (MBD) were evaluated. Dosimetry planning time (DPT) was estimated at 0.5 hours for normal structures, 2 hours per isocenter for MPs, and 5 hours for SPs. Patient time on table (PTT) was estimated at 20 minutes per isocenter for set-up and beam on time of 2 minutes per arc. Statistics were calculated using Related-Sample Wilcoxon Signed Rank test. A total of 10 patients with 73 brain metastases receiving SRS to a median of 6 lesions (range 3-16) were analyzed. All PTVs had max heterogeneity between 105% and 135%. MP treated 73 lesions with 63 isocenters. On average, MP had a 19.2% higher V12 than SP (range -7.9%-45.6%, p=0.017). However, MBD was higher with SP compared to MP, with MBD increased by 34.9% on average (range 0.25%-80.1%, p=0.005); the highest mean brain dose recorded was 4.6 Gy. For creation of SP+1, 30 lesions required 1mm of margin, while none required 2 mm of margin. V12 increased by 47.8% on average per patient (range 6.9%-97.6%, p=0.008) from SP to SP+1. MBD also increased from SP to SP+1 by an average of 19.2% (range 3.6-73.3, p=0.008), with max MBD of 4.7 Gy. DPT was 5.5 hours for SP, while median for MP was 12.5 hours (range 6.5-30.5, p=0.005). PTT was 30 minutes for SP, while median for MP was 144 minutes (range 72-352, p=0.005). SIT results in dosimetry comparable to multi-isocenter plans for patients being treated to multiple brain metastases as long as rotational error can be corrected for with the use of a 6DoF couch. While mean brain dose is slightly higher, this is unlikely to have clinical significance. The time savings to both DPT and PTT are significant using SIT. Increasing margin to account for rotational error (rather than correcting for it with 6DoF couch) leads to a nearly 50% increase in V12, which could result in higher rates of radiation necrosis.