Pastor Tim Keller is undergoing more immunotherapy treatment after finding out that he has new tumours. 

The popular writer and theologian has been fighting stage 4 pancreatic cancer for nearly three years. 

He shared an update on Facebook about his health where he said that he will be going back to the National Cancer Institute in Bethesda Maryland for a "variation" of the immunotherapy he had done there last June. 

Last year's immunotherapy successfully eradicated "99%" of the tumours, he said, but now there are new ones and they need targeted treatment.

"They are unfortunately in some fairly inconvenient places, so the doctors encouraged us to go through the treatment again, this time targeting a different genetic marker of the cancer," he said. 

Keller is asking Christians to pray for him because the last round of immunotherapy was "fairly brutal". 

"Please pray for our trust and dependence on God, for his providential oversight of the medical preparations now in process, and for our desire to glorify God in whatever comes our way. Thank You," he said. 

Keller recently said that his cancer had transformed his prayer life. 

"This is going to sound like an exaggeration. My wife and I would never want to go back to the kind of prayer life and spiritual life we had before the cancer, never," he said.