3 Brain stereotactic radiotherapy: Four versus three table rotation position

Introduction The implementation of brain stereotactic treatments with the help of VMAT irradiation were configured in the way to be close to the initial practice based on cones. To optimize the dose distribution, four table rotation positions were fixed in the dose planning process. Even if the dose distributions were not comparable, VMAT offered the benefit of accessing the entire sphere around the cranium and new advantages appeared covering multiple targets with the MC. However, the four table position brought to an important session duration. The purpose was to consider removing one table position and validate that the dose distributions remained acceptable. Methods The brain targets are irradiated with the use of VMAT on four table rotations: 0°, 45°, 270° and 315°. The stereotactic brain plans of 15 patients were re-planned, removing the 270° table position and using the same inverse planning parameters. The prescribed total dose was between 24 Gy and 33 Gy. The number of targets were 1–3 with volumes between 2 and 41 cc. The collimator rotations were adapted to optimize the MLC effect on the modulation. The comparisons were established with the use of indexes: Homogeneity, Conformity, healthy tissue coverage and target coverage. Other parameters were collected: MU amount, CTV Dmax, PTV Dmax, Brainstem Dmax, Braim Dmean and V10 Gy. The session duration was evaluated for both configurations. Results The dose distributions were compared and found with similar clinical acceptability. This first analyze was confirmed with the different indexes and parameters (Table 1). HI, CI, HCO and TCO showed a low effect as the other parameters listed in Table 1. The session duration initially between 40 and 50 mn decreased until 30mn. Removal of the 270° rotation decreases the spread of low isodoses in the head-feet direction to create a redistributed stretch in the volume of the arc paths of the other 3 angulations. In the case of several target volumes, according to their positions in space, the dose distribution between them varies. This effect depends on the isocenter position, the arc path and the collimator rotations. Unfavorable arcs are those whose image of the BEV includes two target volumes. Table 1. Dosimetric parameters calculated for stereotactic brain plans of 15 patients. Conclusions An ambitious planning configuration for brain stereotaxy has been modified to reduce the session duration. Removing the 270° rotation had no significant dosimetric effect. The new configuration based on three table positions is acceptable. The indexes and parameters used show a low effect. The future will be to use new inverse planning software which take into account the arcs length and the table positions with variable combinations.