Narrow stereotactic beam profile measurements using N-vinylpyrrolidone based polymer gels and magnetic resonance imaging

In this work, polymer gel-MRI dosimetry (using VIPAR gels), radiographic fi lm and a PinPoint ion chamber were used for profile measurements of 6 MV x- ray stereotactic beams of 5 and 10 mm diameter, The VIPAR gel-MRI method ex hibited a linear dose response up to 32 Gy. VIPAR gels were found to resolv e the penumbra region quite accurately, provided that the in-plane image re solution of the related T-2-map is adequate (less than or equal to0.53 mm). T-2-map slice thickness had no significant effect on beam profile data, VI PAR measurements performed with a spatial resolution of 0.13 mm provided pe numbra widths (80%-20% distance) of 1.34 and 1.70 mm for the 5 and 10 mm co nes respectively. These widths were found to be significantly smaller than those obtained with the film (2.23 mm for the 5 mm cone, 2.45 mm for the 10 mm cone) and PinPoint (2.25 mm for the 5 mm cone, 2.52 mm for the 10 mm co ne) methods. Regarding relative depth dose measurements, good correlation b etween VIPAR gel and PinPoint data was observed. In conclusion, polymer gel -MRI dosimetry can provide relatively accurate profile data for very small beams used in stereotactic radiosurgery since it can overcome, to some exte nt, the problems related to the finite size of conventional detectors.