「1992年にIFN単独療法が保険適用になって以来」という一節を英訳するなら、

since independent treatment with IFN has been approved for coverage by insurance in 1992

ではなく、

since the time in 1992 that treatment administered exclusively with interferon (IFN) was approved for coverage under the medical scheme in Japan

とするか、もう少し簡潔に

since the time in 1992 that treatment administered exclusively with interferon (IFN) was approved for coverage by medical insurance in Japan

とすれば良いでしょう。この例文における「単独療法」を「independent treatment」と訳すべきではなく、「exclusive treatment」または「treatment administered exclusively with」が適切です。