IS IT NECESSARY TO REPEAT QUALITY-CONTROL PROCEDURES FOR HEAD AND NECK PATIENTS

Using serial verification films for detection of localization errors and in vivo measurements of the delivered dose, a comparison was made of the information obtained from a single check on the first treatment session or from repeated checks in subsequent irradiations, leading to an assessment of the predictive value of a single check. A total number of 215 films and 261 entrance dose measurements have been performed on 34 fields for 10 head and neck patients. The patients are immobilized with individual plastic masks fixed on the couch and treated on a 6 MV linac, supplied with an automatic verification system excluding the couch parameters. The global results show Gaussian frequency distributions with standard deviations of 4 mm for port film measurements and 3.4% for the dose measurements. Large errors (> 5 mm displacement and > 4% deviation from the expected dose) have been detected in 16% in the cranio-caudal direction and 24% in the antero-posterior direction with port films and in 15% of the in vivo measurements. In order to identify the nature of the errors, which can be random or systematic, the first measurement is taken as the reference value and shows that consecutive measurements on the same field were reproducible with standard deviations of respectively 2.5 mm and 1.8%. This means that a large part of the spread of the global results can be explained by systematic errors in the treatment preparation chain. With the first check, 6 out of 10 systematic localization errors and 7 out of 7 systematic errors leading to erroneous dose delivery have been detected. Therefore, most of the systematic errors, which affect the overall quality of the treatment, can be identified with the first check. The four systematic localization errors, missed with the first film, were of rather limited size: only one of them showed a mean displacement larger than 7 mm. Because the first measurement is an acceptable indication of the overall quality of the treatment delivery, the authors propose a code of practice for checking the treatment quality at the patient level.