Acceptance, commissioning and clinical use of the WOmed T-200 kilovoltage X-ray therapy unit

Abstract

Objective: The objective of this work was to characterize the performance of the WOmed T-200-kilovoltage (kV) therapy machine. Methods: Mechanical functionality, radiation leakage, alignment and interlocks were investigated. Half-value layers (HVLs) (first and second HVLs) from X-ray beams generated from tube potentials between 30 and 200 kV were measured. Reference dose was determined in water. Beam start-up characteristics, dose linearity and reproducibility, beam flatness, and uniformity as well as deviations from inverse square law were assessed. Relative depth doses (RDDs) were determined in water and water-equivalent plastic. The quality assurance program included a dosimetry audit with thermoluminescent dosemeters. Results: All checks on machine performance were satisfactory. HVLs ranged between 0.45-4.52 mmAl and 0.69-1.78 mmCu. Dose rates varied between 0.2 and 3 Gy min 21 with negligible time-end errors. There were differences in measured RDDs from published data. Beam outputs were confirmed with the dosimetry audit. The use of published backscatter factors was implemented to account for changes in phantom scatter for treatments with irregularly shaped fields. Conclusion: Guidance on the determination of HVL and RDD in kV beams can be contradictory. RDDs were determined through measurement and curve fitting. These differed from published RDD data, and the differences observed were larger in the low-kV energy range. Advances in knowledge: This article reports on the comprehensive and novel approach to the acceptance, commissioning and clinical use of a modern kV therapy machine. The challenges in the dosimetry of kV beams faced by the medical physicist in the clinic are highlighted.

Excessive applicator radiation leakage for a common therapeutic kilovoltage system

“…This result is lower than the 1.9-3.6% maximum doses reported by Aspradakis and Zucchetti. 28 In saying this, measurement conditions were not identical as Aspradakis and Zucchetti's measurements were conducted at 1 cm depth in water and did not block the CAX of the field with lead. The 10 × 15/40 applicator has a larger applicator wall, so the space between the wall and the primary collimator is reduced.…”

“…the kilovoltage energy range for therapeutic purposes. In a publication on kilovoltage therapy machine commissioning, Aspradakis and Zucchetti 28 reported applicator leakage for the 200 kV (1.78 mm Cu Half Value Layer (HVL)) beam of a WOmed T-200 unit (Wolf Medical Systems, St Gangloff, Germany) with a Varian HPX-226 tube (Varian Medical Systems, Palo Alto, CA) for both 6 × 8 cm and 5 cm diameter applicators. Maximum reported leakage was 3.6 and 1.9% of central axis dose measured at 1 cm in water, 6 cm and 4 cm from central axis for each applicator respectively.…”

Current status of kilovoltage (kV) radiotherapy in the UK: installed equipment, clinical workload, physics quality control and radiation dosimetry

“…Farmer-type chambers are appropriate for relative dose measurements at depth but are not suitable for measurements at the water surface; here, parallel-plate chambers are more appropriate. 14 Discrepancies have been reported between measured depth doses and BJR supplement 25 data at energies below 75 kV (2.6-mm Al HVL), 16 with this being attributed to differences in the detectors, irradiated medium and measurement methodology for the BJR data. The 2001 Report 75 from the American Association of Physicists in Medicine 17 recommends that if it is not possible to identify a suitable detector for relative dosimetry, then the BJR supplement 25 data should be used.…”

Off‐axis dose distribution with stand‐in and stand‐off configurations for superficial radiotherapy treatments

“…Previous studies have almost exclusively focused on dose variation with stand‐off at the central axis. While this shows that dose varies with stand‐off, it neglects the additional dose variation off‐axis which is compounded by the loss of dose due to increased source to surface distance (SSD).…”

“…The applicators used in this study featured open ends for the investigated energies of 100 and 140 kVp enabling protrusion into the applicator. Li et al and Aspradakis and Zucchetti investigated both open‐ended and closed‐ended applicators but did not investigate stand‐in. Li et al used two open‐ended applicators, a 4 cm circular cone and an 8 × 8 cm 2 square cone, at 100 and 300 kVp.…”