Therefore, its known narrative elements are not a part of the Doctor Who universe. It may have been the basis for a similar story in another medium, however — and that story may indeed be a part of the DWU.
Horror of Det-Sen was the planned second novel in the Lethbridge-Stewart novel series. Penned by Lance Parkin, the book would have featured Alistair Gordon Lethbridge-Stewart and Edward Travers returning to Tibet. It was cancelled due to scheduling issues, and is currently not in planned production.  The first several pages were printed in the back of The Forgotten Son.