AbstractID: 8243 Title: Micro-IMRT treatment delivery: Coplanar versus non-coplanar beam angles
The micro-IMRT stereotactic system utilises a PRIMUS® linear accelerator, a Radionics MMLC and XPlan® RTP software. With
four to six intensity levels and 5-7 beams, a plan of 30-50 field segments is produced. 17 patients have been treated using this
technique, with on average 12 minutes required for beam delivery. The PTVs have included benign and malignant lesions, in cranial
and head/neck sites, for children and adults. In general, the brainstem has been the main OAR. Fixed field and IMRT treatments using
coplanar and non-coplanar beam angles are compared, considering the number of beams, total segments, total MUs, average MU per
beam, and average treatment time per fraction. Cranial sites generally use a non-coplanar arrangement because of the shallow depths
to the PTV. This superior beam direction increases brain stem sparing. Coplanar beams are more common in the lower cranial and
neck regions. Six beams have usually been used for coplanar treatments and five beams for non-coplanar, indicating more effective
target coverage than with non-coplanar beams. The number of delivery segments is proportionate to beam number (6.2 for both
cases). More MUs are required for the coplanar case, although the average time per fraction is similar. Treatment times for IMRT
have been approximately double that of fixed fields. This difference is reduced when considering the total treatment time, which
includes patient setup and localisation, a feature common to both fixed field and IMRT treatments. The average MUs delivered per
treatment increases by 2.5 between fixed field and IMRT treatments.