Room: AAPM ePoster Library
Purpose: Treatment of brain tumors for a pregnant patient requires thorough understanding of peripheral dosimetry to reduce fetal dose. We performed comprehensive peripheral dosimetry measurements as a function of linac parameters which is a valuable tool in optimizing plan quality and minimizing fetal dose.
Methods: 490 ion chamber measurements were acquired on a Varian Truebeam linac with HDMLCs using an anthropomorphic phantom and solid water. Isocenter was at the center of the head. Measurement distances were 50 - 95 cm mimicking the fundus position during pregnancy. Measurement depths were 1 - 10 cm. The following parameters were assessed: Energy (6MV, 6MVFFF), jaw (15x15 - 5x5 cm2) and MLC (retracted, 15x15 - 4x4 cm2), gantry (0 - 315°), collimator (0, 45, 90°), couch (345 - 90°), and a custom 7 mm lead shield.
Results: Dosimetry was reported relative to these reference conditions: Dose was 0.0499 cGy for 6MVFFF, 100 MU, distance 50 cm, depth 10 cm, jaw and MLC 10 x10 cm2, gantry 0°, collimator 0°. For the same conditions, the 6MV reading was 36% higher. Smaller jaw and MLC drastically reduced the dose. Collimator 45° and 90° generally resulted in lower doses than collimator 0°. Collimator 90° was preferred at closer distances (50 cm), while collimator 45° was preferred at distances (65 - 95 cm). Non-coplanar beams produced similar dosimetry as coplanar beams for certain beam combinations 0.022 cGy (Gantry 90° Couch 0°) vs 0.019 cGy (Gantry 90°, Couch 345°). For couch 90° and 45°, gantry 90° should be avoided. Shielding was most effective for posterior gantry 135° and 180° reducing the dose up to 15%.
Conclusion: Optimal linac parameters to reduce peripheral dosimetry include: 6MVFFF, small collimation, large distances from isocenter, and non-zero collimator angles. Optimal selection of gantry angles can allow for the use of small couch kicks.