Stereotactic conformal radiotherapy based on a general-purpose treatment planning system

Abstract

Purpose: To report on commissioning and clinical implementation of a customized system for pediatric stereotactic conformal radiotherapy (SCRT). Methods and Results: The system is based on ADAC Pinnacle and its interfaces with other equipment: (a) Beam models were optimized for our compact blocking system and a new linear accelerator. Computed dose distributions agreed with measurements well within published acceptability criteria. A difference of up to 1.0 mm was measured between beam?s eye views of aperture blocks and calculated 50% isodose contours for a 2´2´2 mm dose calculation grid. (b) Three CT-to-density conversion tables were evaluated, one using tabulated data for a commercial phantom, the second including additional points from the manufacturer?s data for the inserts in an in-house phantom, and the third using measured densities for the in-house phantom materials combined with tabulated data for the commercial phantom. Improvement in the predicted isocenter dose with the third table was less than 0.5% in typical patient SCRT treatments and up to 1.0% in a phantom with 5 cm diameter inhomogeneity inserts. (c) Blocks were transferred to a computerized block cutter using in-house software that extracted the block shape from the export file and custom-fitted the additional necessary shapes. This process was shown to introduce no additional uncertainty. A 1 mm block production uncertainty was measured using film dosimetry on six blocks. (d) In the absence of a DICOM RT Image link, a method based on screen data capture was used to export digitally reconstructed radiographs to two portal imaging systems for treatment verification, without introducing uncertainty into the process. The use of multileaf collimator shielding in the anterior-posterior isocenter verification field reduced lens dose by 12-15 times. Conclusion: This customized system for planning and verification of pediatric SCRT provides a high level of precision as well as reasonable practical efficiency.