Cobalt unit for teletherapy
Almost for 40 years the most important external beam radiotherapy device of tumour therapy was the cobalt unit. It’s true, that the Cs-137 source seemd hopeful, but that’s proved to be a dead end.
A suitable artificial radioactive source should be relatively small having a γ-energy above 1 MeV (skin sparing effect), and a sufficiently long half-time. The average energy of the two γ-lines of a Co-60 source is 1,25 MeV, it has a 5,28 year half-life, a source can be made with 1-2 cm diameter and 3-400 TBq activity, which can result in about 3 Gy/min dose rate at 80 cm source-surface distance. It’s enough to replace the source in 5-8 year intervals, depending on patient’s number.
The unit consists of a stand, a moving C-arm with the head, and a (at least rotating and moving in 3 axis) treatment couch. Because of the head’s heavy weight, it’s advisable to apply a counterweight on the opposite end of the C-arm. The head contains the source, the field-limiting system and the field projection system. The machine is operated from an external operator room. The modern heads can be moving also around the long axis of the head.
The source is stored usually in the head, when it is in beam off position. The source movement from beam on to beam off (storage) position can be done with mechanic devices (pushing rod, sliding clutch, etc.), pneumatic way, or the source isn’t moving, only the beam is broken by a metallic block. The machine is supplemented with many latches to improve the safety of the staff and the patient.
Cobalt unit
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