The purpose of this study is to characterize and understand the long term behavior of the output from megavoltage radiotherapy linear accelerators. We consider the output from your daily check products as it is definitely and sometimes normalized them from Laquinimod (ABR-215062) the actual output measured during the regular monthly Laquinimod (ABR-215062) calibration of the Linacs. The data shows noisy quasi-periodic behavior. The output variance if normalized by Laquinimod (ABR-215062) regular monthly measured “actual’ output is definitely bounded between ±3%. Beams of different energies from your same Laquinimod (ABR-215062) Linac are correlated with a correlation coefficient as high as 0.97 for one particular Linac and as low as 0.44 for another. These maximum and minimum correlations drop to 0.78 and 0.25 when daily output is Laquinimod (ABR-215062) normalized from the monthly measurements. These results suggest that the source of these correlations are both the Linacs and the daily output check products. Beams from different Linacs self-employed of their energies have lower correlation coefficient with a maximum of about 0.50 and a minimum of almost zero. The maximum correlation drops to almost zero if the output is definitely normalized from the regular monthly measured output. Some scatter plots of pairs of beam-output from your same Linac display band-like constructions. These constructions are blurred when the output is definitely normalized from the regular monthly calibrated output. Fourier decomposition of the quasi periodic output is definitely consistent with a 1/f power regulation. The output variation appears to come from a distorted normal distribution having a mean of slightly greater than unity. The quasi-periodic behavior is definitely manifested in the seasonally averaged output showing annual variability with bad variations in the winter and positive in the summer. This trend is definitely weakened Mouse monoclonal to AMACR when the daily output is definitely normalized from the regular monthly calibrated output indicating that the variance of the periodic component may be intrinsic to both the Linacs and the daily measurement devices. Actual Linac output was measured regular monthly. It is definitely needed to be modified once every 3-6 weeks for our tolerance and action levels. If these modifications are artificially eliminated then there is an increase in output of about 2-4% per year. Keywords: Linac output Daily output constancy check 1 noise seasonal output variation I. Intro Radiotherapy linear accelerators are calibrated to deliver a specific dose under standard conditions following an accepted protocol (for example AAPM’s TG-51 protocol by Almond et al. 1). The Linac output is definitely calibrated to deliver 1.0 cGy per Monitor Unit (MU) in the depth of maximum output. Beams of photons or electrons pass through a monitor chamber located in the Linac head which turns off the beam once the prescribed MU is definitely delivered. The medical end result of radiotherapy demands the Linac output do not deviate from your calibrated level by more than a few percent. The output is definitely measured Laquinimod (ABR-215062) regular monthly by physicists and re-calibrated if necessary to keep the output within ±1%. The output is also measured daily from the therapists like a constancy examine and individuals are treated if it is within particular tolerance levels. If the output is definitely beyond the tolerance the Linac is definitely re-calibrated. In our institution patient treatment may continue if output is definitely measured to be within ±3% of baseline in accordance with AAPM TG-40 recommendations2. If output is definitely measured to be between ±3-5% individual treatments may continue but output is definitely verified by a physicist at the earliest convenience. Recalibration may then be performed if needed and output is definitely brought back to an acceptable level. If the output is definitely measured to be greater than ±5% patient treatment is definitely halted and re-calibration is performed before the treatment continues. Three types of action levels were suggested in AAPM TG-142 statement3. Short and long term styles can help us understand the intrinsic and controlled switch of Linac beam output4-6. The rate of recurrence and action level can be modified based on the results of this type of studies along with comprehensive statistical analysis of the type carried out by Sanghangthum et al7. II. Methods and materials Four oldest Linacs in our Institution are manufactured by Varian (Varian Oncology Palo Alto CA). Each Linac gets replaced in about 7 to 10 years time. The output for the three of the four oldest Linacs are analyzed with this work. The fourth the first is identical to the third one both in age and behavior therefore not included in this study. The three Linacs included are a Varian Trilogy Linac (internally designated as T3) and two Varian iX Linacs (internally designated as V2 and iX5). All three Linacs have.