Mandatory dental treatment claim before cancer radiation therapy for eye tumor was rejected

My wife was diagnosed left nasolacrimal duct - poorly differentiated squamous cell carcinoma ( Malignant Eye tumour)

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She took up the treament plan as radiation therapy on her eye areas, mouth and neck areas at SGH. To start with the radiation therapy, she needs to go thro dental treament care e.g tooth extraction, tooth filling etc. However, the claim for the cost of dental care treament at national dental centre was rejected even with doctor's memo stated that dental care is part of the radiation therepy treament. why the dental claim is not considered thought it was related to cancer treament plan? is hopostalization plan for other insurnace e.g AIA, NTUC etc same as Prudential? Pls advise.