Evaluation of Treatment Plan Quality of Single and Multiple Isocenter Stereotactic IMRT for Multiple Intracranial Tumors

Abstract

To investigate whether single isocenter stereotactic IMRT (SRT) plan for multiple intracranial tumors can achieve similar dose conformity and normal brain tissue doses to that of multiple isocenter SRT plan. Fifteen patients (ten with two and five with three brain metastatic tumors) were selected for this retrospective study. Patients treated with multiple isocenter treatment plans using 9 to 12 non-coplanar beams per lesion underwent repeat planning using single isocenter and 10 to 12 non-coplanar beams. Identical contour sets and dose-volume constraints were applied to all plans. The prescribed dose to each target was 25 Gy to be delivered over 5 fractions with a minimum of 95% dose required to cover ≥ 95% of the target volume. Treatment plans were evaluated by a single radiation oncologist. All single isocenter plans were judged clinically acceptable while satisfying the dose-volume constraints. The Paddick conformity index for two target single and multiple isocenter plans was 0.77±0.11 and 0.77±0.13 (p = 0.93), respectively, and for three target single and multiple isocenter plans was 0.78±0.03 and 0.75±0.06 (p = 0.31), respectively. The average normal brain volumes receiving 10 and 15 Gy for two target plans (single vs multiple isocenter) were (6.25% vs 5.73%) and (3.23% vs 3.00%) while for three target plans these volumes were (9.72% vs 6.51%) and (4.32% vs 3.28%), respectively. At 10 and 15 Gy dose levels, no significant differences were observed in normal brain volumes between two and three target single vs multiple isocenter plans for distances ≤ 5cm or > 5cm in between isocenter and the furthest point of a target. The estimated total treatment times were approximately 1.75 and 2.5 times lower with two and three target isocenter delivery, respectively. Single isocenter plans were capable of achieving clinically acceptable and comparable quality treatment plans similar to multiple isocenter plans. Treatment delivery times were much higher and normal brain tissue doses slightly lower at 10 and 15 Gy dose levels for multiple isocenter plans compared to single isocenter plans. Up to 9 cm separation between the target volumes and isocenter results in a slight deviation at 10 and 15 Gy dose levels. The clinical consequences of these normal tissue integral doses are still unknown.