Quality assurance in daily treatment procedure: patient movement during tangential fields treatment

Fifteen women undergoing breast radiotherapy following wide local excision of an early stage breast cancer were submitted to repeated measurements of surface landmarks to check the reproducibility of patient positioning, and to portal imaging using a megavoltage imaging device. When the patient is being set-up the mean rise and fall of a lateral skin mark (tattoo) was within 4 mm in 95 observations of 15 patients. At the end of the lateral field exposure, the mean displacement of the lateral tattoo was close to zero, with only 15/95 (16%) observations falling outside the range +/- 2 mm. The daily measurements of lung thickness fell above and below the simulated lung thickness, consistent with random fluctuations. Eighty-eight percent of lung thickness measurements were within +/- 5 mm of the simulator position. A tentative conclusion is made that more sophisticated immobilisation and imaging devices may be unnecessary for breast irradiation with a high degree of reproducibility.