Quality assurance by systematic in vivo dosimetry: results on a large cohort of patients

Background: In vivo dosimetry is widely considered to be an important tool for quality assurance in external radiotherapy. Introduction: In this study we report on our experience over more than 4 ye ars in systematic in vivo dosimetry with diodes. Materials and methods: From November '94 an in vivo entrance dosimetry chec k was performed for every new patient irradiated at one of our treatment un its (Linac 6/100, 6 MV X-rays). Diodes were calibrated in terms of entrance dose; appropriate correction factors had been previously assessed (taking SSDs, field width, wedge, oblique incidence and blocking tray into account) and were individually applied to in vivo diode readings. The in vivo measu red entrance dose was compared with the expected one, with a 5% action leve l; if a larger deviation was found, all treatment parameters were verified, and the in vivo dosimetry check was repeated. During the period November ' 94-May '99, 2824 measurements on 1433 patients were collected. Results: Nine out of 1433 (0.63%) serious systematic errors (leading to a 5 % or more on the delivered dose to the PTV) were detected by in vivo dosime try; four out of nine would produce a 10% or more error if not detected. Th e rate of serious systematic errors detected by an independent check of tre atment chart and MU calculation was found to be 1.5%, showing that less tha n 1/3 of the errors escapes this check. One hundred and twelve out of 1433 (7.8%) patients had more than one check: the rate of second checks was sign ificantly higher for breast patients (31/250, 12.4%) against non-breast pat ients (81/1183, 6.8%, P = 0.003). A number of patients demonstrated a persi stent relatively large error even after two or more checks. For almost all patients the cause of the deviation was assessed; the most frequent cause w as the difficulty in correctly positioning the patient and/or the diode. Wh en analyzing the distribution of the deviations between measured and expect ed entrance doses (excluding first checks in the case of repetition of the in vivo dosimetry control) the mean deviation was 0.4% with a standard devi ation equal to 3.0%. The rates of deviations larger than 5 and 7% were 9.9 and 2.6%, respectively. When considering the same data taking the average d eviation in the case of opposed beams, the SD became 2.6% and the rates of deviations larger than 5 and 7%, respectively, 5.2 and 0.8%. When dividing the beams according to their orientation, significantly higher rates of lar ge deviations (>5 and 7%) were found for oblique and posterior-anterior (PA ) fields against lateral and anterior-posterior (AP) fields (P < 0.05). Sim ilarly, higher rates of large deviations were found for wedged fields again st unwedged fields (P < 0.03) and for blocked fields against unblocked fiel ds (P < 0.01). When dividing the data according to the anatomical district, accuracy was worse for breast (mean deviation 0.1%, 1 SD: 3.5%) and neck A P-PA fields (mean deviation 1%, 1 SD: 3,4%). Better accuracy was found for vertebrae (0.1%, 1 SD 2.1%) and brain patients (-0.7%, 1 SD: 2.6%). During the considered period, in vivo dosimetry was also able to promptly detect a systematic error caused by a wrong resetting of the simulator height couch indicator, with a consequent error in the estimate of patient thickness of about 4 cm. Conclusions: In our experience, systematic in vivo dosimetry demonstrated t o be a valid tool for quality assurance, both in detecting systematic error s which may escape the data transfer/MU calculation check and in giving an effective way of estimating the accuracy of treatment delivery. (C) 2000 El sevier Science Ireland Ltd. All rights reserved.