An article authored by Ash Bullement has been published in the Journal of Medical Economics on the topic of survival extrapolation for patients treated with cancer immunotherapies.
The article presents a review of extrapolation methods presented in company submissions to NICE, and assesses the accuracy of these extrapolations compared with later data.
You can read the article for free here
https://www.tandfonline.com/doi/full/10.1080/13696998.2018.1547303