AbstractID: 7404 Title: Total body irradiation with a re-conditioned Cobalt unit
The trend in radiotherapy is to replace cobalt-60 teletherapy units with modern linear
accelerators (linacs), however there are still some applications for which a cobalt unit is
simpler and more suitable than a linac.
Following a departmental expansion, we
decommissioned our existing cobalt-60 treatment room and affixed its source head and
neck assembly to a wall bracket in the corner of a bunker with an existing 10 MV linac.
The modified cobalt unit provides a stationary fixed photon field for total body
irradiation (TBI). The unit is at a nominal source-to-floor distance of 251.2 cm and a
custom made collimator provides a field size of about 1.3 x 2.7 m2.
A lead sheet
compensator flattens the beam to + 3% over 190 cm at a depth of 10 cm. Surface dose,
PDD and TPR measurements have been performed, and the beam is calibrated according
to the TG-51 protocol. The patient lies in the AP/PA orientation thus facilitating the
treatment of children and patients requiring partial organ shielding.
Partial lung
attenuators are constructed out of lead, and the current treatment time for 75 cGy is about
10 minutes. This installation provides a dedicated large field TBI technique using an
existing cobalt unit slated for decommissioning.
We believe that there remain some
useful applications for cobalt units and that these units may conveniently share bunker
space with an existing linac installation without requiring excessive departmental
expansion.