METHODS
Using different SSD's for supraclavicular and lateral fields
asymmetric collimators were set to 0 mm, 1 mm, 2 mm position
and treatment planning was done for 45 different situations.
Films were irradiated, dose distribution along the junction
were determined.
RESULTS
2 mm and 1 mm overlap and gap produced 22% and 13%
overdose, 21% and 15% underdose respectively. 0 mm the
dose nonuniformity is about 2%. The difference in dose profiles
along the junction obtained from films and TPS wasn't
statistically significant. TPS and linear accelerator were in accordance
with each other. Besides the penumbra and scattered
radiation also contributed to the uniformity of dose distribution
along the junction point.
CONCLUSION
Single-isocentre technique produces a dose similar to the prescribed
tumor dose along the field junction. The dose distribution
is homogeneous at 0 mm and treatments were delivered
with accuracy.
Keywords: Asymmetric collimation; head and neck cancers; abutted fields; radiotherapy; single isocentre
