Implementation of in vivo dosimetry using diodes as part of quality assurance in radiotherapy
L. I. Villegas 1*, W. P. Espinoza 2,3, M. V. Bayas 1
1 Physics Department, Escuela Politecnica Nacional
2 Radiotherapy Department, Hospital Oncológico SOLCA-QUITO
3 Oncology Department, Hospital Carlos Andrade Marín
An in vivo dosimetry system for photon beams was implemented in the Oncology Hospital Solón Espinosa A. SOLCA-NÚCLEO DE QUITO. The implementation included: calibration of two diodes and a preliminary study with patients. The diodes were calibrated to provide doses in grays and were adapted to the clinical conditions of patients. The calibration and correction factors were determined for 6MV and 18MV photon beams. The correction factors were obtained for field size, source-skin distance, off-axis distance, wedge and tray. The preliminary study determined the dose measurements with diodes and compared them with those obtained from the Treatment Planning System in 33 patients treated. A total of 263 dose measurements were performed during sessions for patients having treatment sites: pelvis, abdomen, chest, breast and head-neck, in specific treatment fields. The variations between measure and reference dose were statistically less than 5% and were dependent of photon beam energy with a confidence level of 99.9%. The minimum acceptable tolerance values for this deviations were between 3.7%-5.3% to 6MV and 1.5%-3% for beams of 18MV, showing some values over reference tolerance. In addition, 134 variation measures for 18MV beams were less than 3% and 80 measures for 6MV beams exceed 5%, some even exceed 7%. For this reasons, we defined a tolerance of 5% for 6MV beams and 3% for 18MV beams with two action levels: 5% and 7% for 6MV, 3% and 5% for 18MV beams. This tolerances are used for in vivo dosimetry system by personnel in the Hospital. This system provides additional information and complements the quality control in the radiotherapy department.
Variations between in vivo dose and reference dose in treatment fields. Patients treated with 6MV and 18MV photon beams