Tell her about your condition

"Anna, I will be blunt with you. Recently I have been diagnosed with a terminal illness."

Anna looked shocked at the sudden change of topic.

"I don't have long left but thankfully it has not given me many side effects, except for maybe one. The tumor is pressing on my pleasure center, making me tumescent almost all day long. Do you know what that means?"

Anna nodded. "It means you're...hard?" she gestured towards your crotch. A wave of understanding washed over her face as she blushed, realizing what you are going to ask her to do. She bit her lip and looked down embarassed.

"Yes it does. It also means I'm not afraid of consequences anymore. So what kind of extra credit are you going to do for me, Anna?"