Detecting anomalies in a deliberately biased tomotherapy plan: Comparison of two patient-specific quality assurance processes involving ArcCHECK® and Gafchromic® EBT3 films

Development and validation of a comprehensive patient-specific quality assurance program for a novel stereotactic radiation delivery system for breast lesions

PURPOSE

The GammaPod is a dedicated prone breast stereotactic radiosurgery (SRS) machine composed of 25 cobalt-60 sources which rotate around the breast to create highly conformal dose distributions for boosts, partial-breast irradiation, or neo-adjuvant SRS. We describe the development and validation of a patient-specific quality assurance (PSQA) system for the GammaPod.

METHODS

We present two PSQA methods: measurement based and calculation based PSQA. The measurements are performed with a combination of absolute and relative dose measurements. Absolute dosimetry is performed in a single point using a 0.053-cc pinpoint ionization chamber in the center of a polymethylmethacrylate (PMMA) breast phantom and a water-filled breast cup. Relative dose distributions are verified with EBT3 film in the PMMA phantom. The calculation-based method verifies point doses with a novel semi-empirical independent-calculation software.

RESULTS

The average (± standard deviation) breast and target sizes were 1263 ± 335.3 cc and 66.9 ± 29.9 cc, respectively. All ion chamber measurements performed in water and the PMMA phantom agreed with the treatment planning system (TPS) within 2.7%, with average (max) difference of -1.3% (-1.9%) and -1.3% (-2.7%), respectively. Relative dose distributions measured by film showed an average gamma pass rate of 97.0 ± 3.2 when using a 3%/1 mm criteria. The lowest gamma analysis pass rate was 90.0%. The independent calculation software had average agreements (max) with the patient and QA plan calculation of 0.2% (2.2%) and -0.1% (2.0%), respectively.

CONCLUSION

We successfully implemented the first GammaPod PSQA program. These results show that the GammaPod can be used to calculate and deliver the predicted dose precisely and accurately. For routine PSQA performed prior to treatments, the independent calculation is recommended as it verifies the accuracy of the planned dose without increasing the risk of losing vacuum due to prolonged waiting times.

Assessment of combined use of ArcCheck® detector and portal dosimetry for delivery quality assurance of head and neck and prostate volumetric-modulated arc therapy

Purpose

To assess the efficiency of combined use of ArcCheck^® detector (AC) and portal dosimetry (PDIP) for delivery quality assurance of head and neck and prostate volumetric‐modulated arc therapy.

Materials and methods

Measurement processes were studied with the Gamma index method according to three analysis protocols. The detection sensitivity to technical errors of each individual or combined measurement processes was studied by inserting collimator, dose and MLC opening error into five head and neck and five prostate initial treatment plans. A total of 220 plans were created and 660 analyses were conducted by comparing measurements to error free planned dose matrix.

Results

For head and neck localization, collimator errors could be detected from 2° for AC and 3° for PDIP. Dose and MLC errors could be detected from 2% and 0.5 mm for AC and PDIP. Depending on the analysis protocol, the detection sensitivity of total simulated errors ranged from 54% to 88% for AC vs 40% to 74% for PDIP and 58% to 92% for the combined process. For the prostate localization, collimator errors could be detected from 4° for AC while they could not be detected by PDIP. Dose and MLC errors could be detected from 3% and 0.5 mm for AC and PDIP. The detection sensitivity of total simulated errors ranged from 30% to 56% for AC vs 16% to 38% for PDIP and 30% to 58% for combined process.

Conclusion

The combined use of the two measurement processes did not statistically improve the detectability of technical errors compared to use of single process.