Effective spatially fractionated GRID radiation treatment planning for a passive grid block.

Abstract

To commission a grid block for spatially fractionated grid radiation therapy (SFGRT) treatments and describe its clinical implementation and verification through the record and verify (R&V) system. SFGRT was developed as a treatment modality for bulky tumours that cannot be easily controlled with conventionally fractionated radiation. Treatment is delivered in the form of open-closed areas. Currently, SFGRT is performed by either using a commercially available grid block or a multileaf collimator (MLC) of a linear accelerator. In this work, 6-MV photon beam was used to study dosimetric characteristics of the grid block. We inserted the grid block into a commercially available treatment planning system (TPS), and the feasibility of delivering such treatment plans on a linear accelerator using a R&V system was verified. Dose measurements were performed using a miniature PinPoint(TM) ion chamber (PTW, Freiburg, Germany) in a water phantom and radiochromic film within solid water slabs. PinPoint ion chamber was used to measure the output factors, percentage depth dose (PDD) curves and beam profiles at two depths, depth of maximum dose (zmax) and 10 cm. Film sheets were used to measure dose profiles at zmax and 10-cm depth. The largest observed percentage difference between output factors for the grid block technique calculated by the TPS and measured with the PinPoint ion chamber was 3.6% for the 5 × 5-cm(2) field size. Relatively significant discrepancies between measured and calculated PDD values appear only in the build-up region, which was found to amount to <4%, while a good agreement (differences <2%) at depths beyond zmax was observed. Dose verification comparisons performed between calculated and measured dose distributions were in clinically acceptable agreements. When comparing the MLC-based with the grid block technique, the advantage of treating large tumours with a single field reduces treatment time by at least 3-5 times, having significant impact on patient throughput. The proposed method supports and helps to standardize the clinical implementation of the grid block in a safer and more accurate way. This work describes the method to implement treatment planning for the grid block technique in radiotherapy departments.