Safety Impact of Exceeding Linac Vault Design Workload Limits

Abstract of the technical presentation presented at:
Canadian Organization of Medical Physicists 64th Annual Scientific Meeting, 2018
September 12-15, 2018

Prepared by:
Yani Picard and Jeff Sandeman
Canadian Nuclear Safety Commission

Abstract:

Purpose

To determine whether exceeding the design workload limits potentially results in safety significant increases to occupational and public exposure to radiation around linac vaults in radiotherapy centers.

Method

Licensees must provide data on accelerator workloads to the CNSC as part of their annual compliance reports (ACR). Over recent years, CNSC staff reviewing these reports have observed a trend towards increased annual workloads, to the extent that in some cases, it has exceeded the design workload limit for the vault. In such cases, it is necessary to evaluate whether or not this increase could result in a significant increase in exposure to facility staff or the public.

Annual workloads exceeding design workload limits assumptions were reviewed. When the ACR contained workloads for the different photon beam energies of the linac, the annual workload was calculated by simply summing them. The resulting occupational and public exposure to radiation around the vaults were evaluated using the vault design submitted by the licensee at the commissioning phase, following NCRP Report No. 151 dose estimation formalism and using conservative assumptions.

Results

Historically, design workload limits were typically established using the linac’s highest photon beam energy. Where intensity-modulated radiotherapy (IMRT) was considered, it was generally assumed to be performed at the highest beam energy, which may include consideration of the impact of using the linac flattening filter free (FFF) mode if available. Furthermore, design workload limits were normally established on the basis of ALARA limiting occupational doses to less than 1 mSv per year and public doses to less than 50 µSv per year.

The review found that observed increases in workload are generally at lower beam energies, usually in conjunction with IMRT techniques, FFF mode, or a combination of both. Therefore, marginal increase of the workload over the design workload limit usually had a negligible impact on occupational and public exposure to radiation around linac vaults in radiotherapy centers. There are circumstances where increases in dose could occur, and this must be carefully evaluated.

Conclusion

When building a new vault or installing a new linac in an existing vault, it has been common practice in the past to establish the design basis workload using extremely conservative assumptions, to ensure the long term viability of the facility as treatment techniques evolve. However, the recent trend to more complex treatment modalities at lower energies makes validating the adequacy of the shielding design more complicated. Licensees must carefully review the impact of the radiotherapy techniques (e.g., IMRT) and the operating modes (FFF or FF) used, and establish or review their design workload limits and demonstrate that they are still ALARA before these limits are exceeded.

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