Dosimetry disparities resulting from two accepted methods of calibration of cobalt-60 teletherapy machines

The primary concern of all involved in the management of radiotherapeutic patients is that the tumor dose delivered to each and every patient in one institution is identical to that delivered anywhere else. Despite the advocacy 1,2,3,4 for the calibration of high energy photon beams at 5 cm depth or beyond in a water phantom to reduce the effect of electron "contamination" from collimators, filters, applicators, etc., the "in-air" method of calibration is still in vogue in many institutions in the United States and elsewhere. The introduction of S.I. units in radiology is likely to create ambiguity in proper conversion factors from exposure to absorbed dose which can be avoided if field instruments are calibrated in terms of absorbed dose in water under specified conditions. The present study shows that the estimated error can be as much as 7% low when the "in-air" method of calibration is used instead of measurements in a large water phantom at 5 cm depth for cobalt-60 photon beams and hence recommends that the procedure for "in-air" calibration for high energy photon beam should be discontinued.