Authors' reply—Perfomance of a commercially available field flatness device

Hermes, a commercially available field flatness device (manufactured by Precitron AB, UK agents Graytech Ltd), has been used to measure the value of field flatness of radiotherpay treatment fields including γ-rays (cobalt 60), X rays (55 kV to 8 MV) and electrons (5 MeV to 14 MeV). A secular variation has been observed over a period of 12 months use. It is postulated that this may be partly attributable to radiation damage.

The device contains 10 P type semiconductor diode detectors, nine of which are strategically placed for the measurement of field flatness and one for measurement of depth dose.