AbstractID: 6837 Title: Polymer gel dosimeter verification of junction doses in head and neck radiotherapy
The use of independent collimators in the abutment of two adjacent treatment volumes, as in head and
neck radiation therapy treatments, consists typically of positioning the collimator rotation axis (CRA)
at the junction of the volumes and asymmetrically offsetting each field. This has the effect of
positioning one of the collimator jaws at the CRA for each field. However, misalignment of the jaws
can lead to variations in dose uniformity in the junction region. Polymer gel dosimetry and MRI were
used to investigate junction dosimetry for a mono-isocentric treatment of two orthogonal pairs of
opposed (ant/post and lateral) 6 MV x-ray beams. The irradiations were made using a Philips SL 20
linear accelerator with independent jaws that were known to overlap at the isocentre for sequential
abutting offset (within manufacturer’s specifications for symmetric fields). An 11 cm diameter
cylindrical polymer gel phantom was imaged using a modified 32-echo multi-spin-echo pulse
sequence on a Siemens Vision 1.5 T MRI scanner . T2 images were calculated and converted to
relative dose distribution images. For comparison another gel phantom and Kodak X-Omat V films
were irradiated in a mono-directional geometry. Measurements of off-axis ratios, relative depth
profiles and full-width-half-maximum (FWHM) values using gel and perpendicular film were in
excellent agreement with each other. FWHM values and minimum doses agreed closely with the
results measured at the same depth in the mono-directional case. Gel dosimetry was shown to be a
valuable tool in complex radiotherapy situations including adjoining multi directional beams.