Maxwell Belanger, MS

Daily output variations of up to ±2% were observed for a protracted time on a Varian TrueBeam® STx; these output variations were hypothesized to be the result of atmospheric communication of the sealed monitor chamber. Daily changes in output relative to baseline, measured with an ionization chamber array (DQA3) and the amorphous silicon flat panel detector (IDU) on the TrueBeam®, were compared with daily temperature-pressure corrections (PTP ) determined from sensors within the DQA3. Output measurements were performed using a Farmer® ionization chamber over a 5-hour period, during which there was controlled variation in the monitor chamber temperature. The root mean square difference between percentage output change from baseline measured with the DQA3 and IDU was 0.50% over all measurements. Over a 7-month retrospective review of daily changes in output and PTP , weak correlation (R2 = 0.30) was observed between output and PTP for the first 5 months; for the final 2 months, daily output changes were linearly correlated with changes in PTP , with a slope of 0.84 (R2 = 0.89). Ionization measurements corrected for ambient temperature and pressure during controlled heating and cooling of the monitor chamber differed from expected values for a sealed monitor chamber by up to 4.6%, but were consistent with expectation for an air-communicating monitor chamber within uncertainty (1.3%, k = 2). Following replacement of the depressurized monitor chamber, there has been no correlation between daily percentage change in output and PTP (R2 = 0.09). The utility of control charts is demonstrated for earlier identification of changes in the sensitivity of a sealed monitor chamber.

PMID:32648368 | PMC:PMC7484838 | DOI:10.1002/acm2.12975

Calypso L-shaped Surface Beacon transponder has recently become available for clinical applications. We herein conduct studies to validate the Surface Beacon transponder in terms of stability, reproducibility, orientation sensitivity, cycle rate dependence, and respiratory waveform tracking accuracy. The Surface Beacon was placed on a Quasar respiratory phantom and positioned at the isocenter with its two arms aligned with the lasers. Breathing waveforms were simulated, and the motion of the transponder was tracked. Stability and drift analysis: sinusoidal waveforms (200 cycles) were produced, and the amplitudes of phases 0% (inhale) and 50% (exhale) were recorded at each breathing cycle. The mean and standard deviation (SD) of the amplitudes were calculated. Linear least-squares fitting was performed to access the possible amplitude drift over the breathing cycles. Reproducibility: similar setting to stability and drift analysis, and the phantom generated 100 cycles of the sinusoidal waveform per run. The Calypso system's was re-setup for each run. Recorded amplitude and SD of 0% and 50% phase were compared between runs to assess contribution of Calypso electromagnetic array setup variation. Beacon orientation sensitivity: the Calypso tracks sinusoidal phantom motion with a defined angular offset of the beacon to assess its effect on SD and peak-to-peak amplitude. Rate dependence: sinusoidal motion was generated at cycle rates of 1 Hz, .33 Hz, and .2 Hz. Peak-to-peak displacement and SDs were assessed. Respiratory waveform tracking accuracy: the phantom reproduced recorded breathing cycles (by volunteers and patients) were tracked by the Calypso system. Deviation in tracking position from produced waveform was used to calculate SD throughout entire breathing cycle. Stability and drift analysis: Mean amplitude ± SD of phase 0% or 50% were 20.01 ± 0.04 mm and -19.65 ± 0.08 mm, respectively. No clinically significant drift was detected with drift measured as 5.1 × 10-5 mm/s at phase 0% and -6.0 × 10-5 mm/s at phase 50%. Reproducibility: The SD of the setup was 0.06 mm and 0.02 mm for phases 0% and 50%, respectively. The combined SDs, including both setup and intrarun error of all runs at phases 0% and 50%, were 0.07mm and 0.11 mm, respectively. Beacon orientation: SD ranged from 0.032mm to 0.039 mm at phase 0% and from 0.084 mm to 0.096 mm at phase 50%. The SD was found not to vary linearly with Beacon angle in the range of 0° and 15°. A positive systematic error was observed with amplitude 0.07 mm/degree at phase 0% and 0.05 mm/degree at phase 50%. Rate dependence: SD and displacement amplitudes did not vary significantly between 0.2 Hz and 0.33 Hz. At 1 Hz, both 0% and 50% amplitude measurements shifted up appreciably, by 0.72 mm and 0.78mm, respectively. As compared with the 0.33 Hz data, SD at phase 0% was 1.6 times higher and 5.4 times higher at phase 50%. Respiratory waveform tracking accuracy: SD of 0.233 mm with approximately normal distribution in over 134 min of tracking (201468 data points). The Surface Beacon transponder appears to be stable, accurate, and reproducible. Submillimeter resolution is achieved throughout breathing and sinusoidal waveforms.

PMID:27455489 | PMC:PMC5627956 | DOI:10.1120/jacmp.v17i4.6152