Journal of Medical Physics
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Year : 2005  |  Volume : 30  |  Issue : 1  |  Page : 20-28

Some Uncertainties In The Dose Delivery To Patients Using Telecobalt Machines

Correspondence Address:
S L Keshava

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Source of Support: None, Conflict of Interest: None

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When the telegamma source is new and of high activity, errors can occur if doses are delivered using radiation outputs whose measurements were based on longer periods of exposure time. This error is commonly referred to as the shutter error, and is due to the dose contribution during source movement, both before the start of treatment timer and after the end of treatment time. The commonly used double exposure method of quantifying shutter error (i.e., the dose contribution due to source in transit) does not correct the output at all exposure times. A method is suggested to overcome this problem in this paper, which will take the shutter time error into account for all treatment times. The radiation output factors get modified when shielding tray (solid/slotted) is used. The shielding tray transmission factors are field size-dependent. Error is introduced when a single tray transmission factor is used in patient dose calculations. Similarly, use of open field output factors when shielding tray is used also introduces the error, Since the introduction of shielding blocks also modifies the beam, a procedure is suggested to reduce the error. The errors due to the use of central axis depth doses (CADD) in certain types of shielded fields, where the equivalent squares are calculated with respect to the central axis of the beam, are also discussed and remedies suggested. The calculation aspects associated with the use of breast cone for treating tangential chest wall fields are also addressed. The contents of this paper are with intent to help newly qualified medical physicists working as lone physicists in cancer hospitals.

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