Output trends, characteristics, and measurements of three megavoltage radiotherapy linear accelerators

Abstract

The purpose of this study is to characterize and understand the long‐term behavior of the output from megavoltage radiotherapy linear accelerators. Output trends of nine beams from three linear accelerators over a period of more than three years are reported and analyzed. Output, taken during daily warm‐up, forms the basis of this study. The output is measured using devices having ion chambers. These are not calibrated by accredited dosimetry laboratory, but are baseline‐compared against monthly output which is measured using calibrated ion chambers. We consider the output from the daily check devices as it is, and sometimes normalized it by the actual output measured during the monthly calibration of the linacs. The data show noisy quasi‐periodic behavior. The output variation, if normalized by monthly measured “real’ output, is bounded between ± 3%. Beams of different energies from the same 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 normalized by the monthly measurements. These results suggest that the origin of these correlations is both the linacs and the daily output check devices. Beams from different linacs, independent 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 normalized by the monthly measured output. Some scatter plots of pairs of beam output from the same linac show band‐like structures. These structures are blurred when the output is normalized by the monthly calibrated output. Fourier decomposition of the quasi‐periodic output is consistent with a 1/f power law. The output variation appears to come from a distorted normal distribution with a mean of slightly greater than unity. The quasi‐periodic behavior is manifested in the seasonally averaged output, showing annual variability with negative variations in the winter and positive in the summer. This trend is weakened when the daily output is normalized by the monthly calibrated output, indicating that the variation of the periodic component may be intrinsic to both the linacs and the daily measurement devices. Actual linac output was measured monthly. It needs to be adjusted once every three to six months for our tolerance and action levels. If these adjustments are artificially removed, then there is an increase in output of about 2%–4% per year.PACS numbers: 87.56bd, 87.56Fc, 87.55Qr

Radiation Therapy Deficiencies Identified During On-Site Dosimetry Visits by the Imaging and Radiation Oncology Core Houston Quality Assurance Center

“…Rather, as discussed in the previous paragraph, the TPS calculation had never agreed with the actual dose distribution generated by the linac, and the error had typically never been identified because the TPS calculation was never incorporated into the QA program. Naturally, this doesn’t mean machine QA isn’t important; many parameters (such as output) do drift and need to be properly maintained [20,21], and many tests in TG-142 were not relevant to the specific tests conducted during the on-site audit so this evaluation does not capture the full impact of a linac QA program. Nevertheless, clear and substantial improvements could be achieved in clinical practice by focusing relatively more medical physics effort on the TPS beam model commissioning and QA (e.g., MPPG5a [16] and TG-53 [22]) and directly linking these calculations to machine QA tests.…”

Reference dosimetry in magnetic fields: formalism and ionization chamber correction factors

“…Systematic uncertainties also exist including the fact that the room temperature was lower by an average of 4.3 • C on the days when measurements were taken with the magnetic field off (16.7 • C) compared to the days when the magnetic field was on (21.0 • C), which may have affected the output of the beam on those days. Although the sealed monitor chamber design would likely have prevented this, the output of linacs with sealed monitor chambers has been shown 37 to have seasonal variations so this may have been a factor. The strength of the magnetic field at the monitor chamber is on the order of 25 G and thus the effect of the field on the chamber calibration is expected to be negligible.…”

Seasonal variations in measurements of linear accelerator output

“…These events are referred to as service events . In a manner similar to that described by Hossain et al ,. we attempt to represent the machine output as a function of time with the impact of these servicing events removed from the data.…”

“…Output measurements that exceed tolerance levels require investigation, create delay in patient treatment, and may require adjustments to restore functionality of the equipment to within tolerance. Seasonal variations in output measurements have been observed and reported in the literature by at least one center . That center's report suggests that the variations observed may be related to both the measurement device and the accelerator itself.…”