ABSTRACT
PURPOSE: The calculation model for the integral quality monitor (IQM) system does not take into account the characteristics of the HD120 multileaf collimator (MLC), which some Varian accelerators are equipped with. Some treatment plans prepared with this collimator are characterized by a high level of modulation. The aim of the work was to prepare a model for that collimator and to determine the influence of modulation on results of the verification carried out with the use of IQM system. METHODS: The short and long stabilities of the IQM detector response were verified by measuring the signal for a 6 MV flattening filter-free (FFF) beam with the static field of 10 × 10 cm2 size. The obtained results were compared with the measurements performed with the PTW Farmer chamber. Next, the signals for 35 static square fields 4 × 4 cm2 , covering the whole field 38 × 20 cm2 , were measured with the IQM. Based on the results of these measurements, the original calculation model has been changed in order to achieve the smallest differences between calculations and measurements. While tuning the model, the characteristics of the HD120 MLC were included. Measurements were performed for 30 clinical plans (86 arcs) prepared with 6 MV FFF beams. Among those 30 plans, there were were multitarget plans with single isocenter. For each plan, the modulation complexity score (MCS) was calculated. The measurement results were compared with the calculation results performed with the original and authors' calculation model. RESULTS: Very good stability of the short and long stabilities of the IQM detector response was obtained. Measurements performed for 35 static fields revealed that for the manufacturer's and for the authors' models, the deviation exceeded 3% for 12 and five of the 35 static fields, respectively. The differences for the manufacturer's and authors' algorithms were in the range of ±2% for the 15 and 26 of the fields, respectively. For original and the authors' models, the differences between measured and calculated signals (starting with the segment number 40) were within the range of ±3.5% for 87.6% and 96.7% of all arcs for the respective models. For both models, the dependence of the compliance of measurements and calculations on the MCS was observed. For most of the very modulated arcs, the measured signal was at least 3% lower than the calculated one. The largest differences between measurements and calculations were obtained for single-isocenter multitarget plans. CONCLUSIONS: The signal predicted by an algorithm taking into account the real geometry of the collimating system of the Edge accelerator (equipped with the HD120 MLC) made it possible to obtain greater consistency between the measurements and calculations. We characterized the dependence between the MCS of each arc and the compliance of the measurements and calculations. Much worse results were obtained for single-isocenter multitarget plans.
Subject(s)
Radiotherapy Planning, Computer-Assisted , Radiotherapy, Intensity-Modulated , Algorithms , Particle Accelerators , Phantoms, Imaging , Radiometry , Radiotherapy DosageABSTRACT
BACKGROUND: Effective diagnostic radiology system should be based on an efficient and suitable servicing of medical X-ray equipment. According to Polish requirements, all radiology departments are obligated to carry out quality control (QC) test of their X-ray set. In practice, testing is mostly performed by accredited external QC services, which have to participate in periodic interlaboratory comparison (ILC) to maintain or obtain the accreditation. Large-scale ILC for QC services were performed at the Nofer Institute of Occupational Medicine for three ranges of diagnostic radiology: dental, conventional and mammography. MATERIALS AND METHODS: During ILC, the metrological coherence of main physical parameters, which determine good quality of diagnostic image, were estimated and compared with appropriate reference values ensured by ILC organizer. The ILC participants comprised 29 QC services. The measurements were performed by ILC participants under laboratory conditions, using their own calibrated meters, according to routine procedures. All measurement results were assessed by calculating the E(n) value, normalized with respect to the uncertainties. RESULTS: Of the 328 evaluated results only 11 (3.4%) were classified as unsatisfactory. As much as 82% of them applied to mammography. Thus, the final evaluation revealed negative results in 2 of the 29 participants, which means that their satisfactory scores for the studied ranges were below 75%. CONCLUSIONS: In spite of predominant amount of satisfactory results, ILC indicated some regions of divergence e.g. large differences in evaluation of uncertainties and other inconsistencies.
