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Currently there is no consensus concerning dosimetric and clinical parameters to evaluate breast cancer radiotherapy treatment planning. Although fractionation schedules are harmonized, each institute uses its own plan evaluation criteria including its own OAR nomenclature. A quality improvement project was started to quantify the current variation and to formulate national consensus guidelines on plan evaluation.

A Benchmark Case including target and OAR delineations conforming to ESTRO guidelines was sent to all 20 radiotherapy departments. The institutes were requested to submit 4 photon treatment plans based on their own clinical practice (Table 1). A questionnaire was also distributed, in which for each case, an inventory was requested of which structures and dosimetry metrics were used during planning and evaluation. Additional information was requested on the possible compromise that could be made to PTV dose coverage. The benchmark results, in combination with a literature review, was then used to formulate a consensus proposal.

Table 1: Benchmark Cases

19 out of 20 departments participated in this project. 1 institute did not participate due to time limitations. There was a wide variation in dose evaluation parameters used clinically. For example, PTVs were evaluated using 10 different combinations of dose/volume criteria. Although target coverage did not differ much between institutions, substantial OAR dose differences were observed (Figure 1). These differences were more pronounced for the cases including the lymph nodes. Only 6% of the radiation oncologists said to always take into account clinical factors, social factors and gene mutation when delineating the CTV-breast and CTV-lymph nodes. 44% said to take this sometimes into account and 39% never. In order to spare OARs, 61% of the radiation oncologists said that they sometimes make a compromise to PTV dose coverage without there being an established protocol, 28% would make a concession to the PTV dose coverage with an established protocol for this, and 11% said to adjust the delineation of the PTV when they want to do a conscious concession. The project group formulated a proposal with regard to both dosimetric and clinical parameters to be evaluated. The parameters are planned to be an extension to the Dutch Nabon Breast Cancer Audit-Radiotherapy indicators. The harmonization proposal was discussed at an inventational conference on October 11^th 2019.

Figure 1: DVH parameter values relative to the average DVH parameter value over all institutions for case 1.

There is large variation in both volume and dosimetric parameters used for calculation and evaluation of a radiotherapy treatment plan for breast cancer. Radiation oncologists take into account clinical factors when performing the target volume delineations and assessing the treatment plans. Mostly this consideration is based on own experience, without established protocols. Approval of the consensus guidelines for plan evaluation by the Dutch Platform for Breast Radiotherapy is expected in November 2019.