118 Chapter 5 as the threshold dose for the risk of developing ORN as described in the Dutch National Protocol is empirical [12, 13]. Several other studies suggest using 50Gy or 60Gy for the mandible or even 70Gy for the maxilla as a reference value for the development of ORN [28-32], with only one Delphi study discussing a critical radiation threshold for prophylactic removal of teeth [33]. While the Canadian Dental Oncology Network seems to accept a certain risk of developing ORN, the Dutch guidelines prefer minimizing this risk as much as possible. There are previous publications describing radiation doses to portions of the mandible and maxilla [34-37]. One study retrospectively delineated each tooth within the radiation fields in 18 HNC patients and used a Dmean cut-off point of >50Gy to assess the need for pre-RT extractions or similarly invasive procedures [34]. Two studies did not report the doses in ipsi- and contralateral which made it difficult to compare the results [35, 36]. Another study looked at the mandibular volume percentages receiving >55Gy for 28 patients with base of tongue malignancies [37]. The strength of our study includes the large sample size (358 patients with 1759 precisely delineated extraction sites) and the detailed information on radiation doses (mean and maximum dose). Another strength of this comprehensive study is the fact that all extraction sites were delineated by a single experienced radiation technician (MG) in close collaboration with an experienced prosthodontist (DB). This contributes significantly to the consistency of the results. A limitation of the present study is that the exact diagnosis for tooth extraction is missing. For some teeth, the prognosis might have been so poor that extraction would be the treatment of choice regardless of planned radiotherapy. Tooth extractions in these patients is also partly triggered by the insurance system in the Netherlands. The treatment of possible oral infection sites and the resulting prosthetic rehabilitation are covered by the national insurance system in the Netherlands. This opportunity leads to acceptation of more frequent tooth extractions in order to favorably access standardized prosthetic denture rehabilitation. This means that the actual percentage of redundantly removed teeth for reasons of planned radiotherapy is probably lower. Another limitation is the indication of the location of the radiation field at the time of the comprehensive dental assessment. This can lead to a bias in judgement, especially since the guidelines for the expected dose still date from the early days of IMRT and have not yet been adapted to the much more appropriate VMAT-RT.
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