Purpose: To develop and clinically implement a trigeminal neuralgia (TN) program using an Elekta VersaHD, with Agility MLCs for collimation, XVI CBCT registration system, and the HexaPod 6DOF robotic couch for patient positioning.
Methods: A dedicated 6MV Flattening Filter Free small-field beam model was developed in the RaySearch RayStation treatment planning system. Small-field output factors (OFs) were measured down to 5mm x 5mm, and benchmarked against published data. Multiple detectors were used to redundantly measure OFs; small-field ion chamber and diode, parallel plate chamber, GafChromic Film and Sun Nuclear SRS MapCheck. A 28-field modified Winston-Lutz test was developed to fully characterize collimator, gantry and table isocentricity. A TN-specific GafChromic film patient-specific QA process was developed using the Sun Nuclear Stereophan with film holder-insert and custom analysis software. Additional monthly QA tests were added to better monitor table isocentricity, HexaPod calibration, MLC positional accuracy and small-field OFs. Inverse and forward planning techniques generated non-spherical distributions maximizing target-to-brainstem dose gradients. Elekta Fraxion immobilization system (head frame, custom pillows, thermoplastic masks and suction bite blocks) were commissioned. Custom XVI presets were developed to generate higher resolution and lower noise cranial images.
Results: 1% agreement in OF between literature and measurements for fields greater than 1cm x 1cm, with increasing disagreement down to 5mm x 5mm. Repeat Winston-Lutz tests indicate overall kV-MV isocentricity better than 0.5mm, with maximum ranges from couch isorotations. GafChromic film tests show geometric accuracy of the dose cloud and maximum dose placement to within 0.5mm. Treatment plans with non-spherical dose distributions can produce a target-to-brainstem dose fall-off of over 17 Gy/mm. The Fraxion immobilization system has demonstrated average intra-fraction motion less than 0.4mm.
Conclusion: An Elekta MLC-linac based TN program was developed and has, thus far, successfully treated nine patients.