The reproducibility of polyacrylamide gel dosimetry applied to stereotactic conformal radiotherapy

The reproducibility of polyacrylamide gel (PAG) dosimetry has been evaluate d when used to verify two radiotherapy treatment plans of increasing comple xity. The plans investigated were a three-field coplanar arrangement, using the linac jaws for field shaping, and a four-held, conformal, non-coplanar plan using precision-cast lead alloy shielding blacks. Each treatment was performed three times using phantoms and calibration gels manufactured in-h ouse. Two phantoms were specially designed for this work to aid accurate po sitioning of the gels for irradiation and imaging. All gels were imaged post-irradiation using a Siemens Vision 1.5T MR scanne r. T-2 relaxation images were calibrated to absorbed dose distributions usi ng a number of smaller calibration vessels to produce distribution maps of relative dose. The relative dose distributions were found to be reproducibl e, with the standard deviation on the mean areas enclosed by the greater th an or equal to 50% isodose lines measured in three orthogonal planes being 6.4% and 4.1% for the coplanar and non-coplanar plans respectively. The mea sured distributions were also consistent with those planned, with isodose l ines generally agreeing to within a few millimetres. However, the measured absolute doses were on average 23.5% higher than those planned. Although the polyacrylamide gel dosimetry technique has some limitations, p articularly when calibrating distributions to absolute dose, the ability to resolve sharp dose gradients in three dimensions with millimetre precision is invaluable when verifying complex conformal treatment plans, where avoi dance of proximal, critical structures is a treatment criterion.