Eight patient treatments with 51 targets were planned with various margins using Elements Multiple Brain Mets SRS treatment planning software.
Forty-eight plans with 0 mm, 1 mm and 2 mm margins were created, including plans with variable margins, where targets more than 6 cm away from the isocenter were planned with larger margins. The dosimetric impact of the margins were analyzed with V5Gy, V8Gy, V10Gy, V12Gy values.
12 more patient motion data were analyzed to determine both the impact of the repositioning threshold and the distributions of the patient translational and rotational movements.
Results shown that the V5Gy, V8Gy, V10Gy, V12Gy volumes almost doubled when margins change from 0 to 1 mm and tripled when change from 0 to 2 mm.
With variable margins, the aggregated results are same as results from plans using the lower of two margins, since only 12.2% of the targets were more than 6 cm away from the isocenter. With 0.5 mm re-positioning threshold, 57.4% of the time the patients are repositioned.
Decreasing the threshold to 0.25 mm results in 91.7% repositioning rate, due to limitations of the fusion algorithm and actual patient motion.
The 90th percentile of translational movements in all directions is 0.7 mm, while the 90th percentile of rotational movements in all directions is 0.6 degrees. Median translations and rotations are 0.2 mm and 0.2 degrees, respectively.
Authors concluded that they have switched modus operandi from 2 to 1 mm PTV margins, with an eventual goal of using 0.5 and 1.0 mm variable margins when an automated margin assignment method becomes available. The 0.5 mm and 0.5 degrees repositioning thresholds are better with small residual patient movements.
Source: Medindia
