Irregularly shaped fields are commonly used in radiation therapy in order to protect healthy tissue in a treatment field. The calculation methods of percentage depth dose or tissue-air ratio for irregularly shaped fields have been reported by many authors. However, there are few comments refered to the output used in depth dose calculations. We investigated the dose rate at reference depth (output) for the irregularly shaped fields on cobalt 60 gamma rays, Therateron 80. For obtaining the output of irregularly shaped fields, we measured the output both the equivalent square fields formed with lead blocks and the original collimators. With regard to the depth dose for some cases of irregularly shaped fields in clinical applications, we investigated to compare the measured values and calculated values obtained from our output study. In conclusion, it was shown that the values calculated by output of equivalent square fields formed with lead blocks would give the best accuracy for the depth dose of irregularly shaped fields.
