The former pastor-turned-photographer, who used to manage the careers of his singer/actress daughters Jessica and Ashlee Simpson, went public with his diagnosis in November (16), revealing he had already undergone surgery to combat the disease.

Now a source tells Us Weekly the 58-year-old has begun the next phase of his treatment.

"Joe just started radiation," the insider says. "He feels fine, he's getting through it and still working. It's all scheduled once a week.

"He's super optimistic. Doctors are hopeful that this will take care of it and be the last step, and he will hopefully then be in remission and cancer-free. Joe just says he feels like it's like going to get a check-up."

Meanwhile, Joe is pressing on with his work behind the camera to distract him from his health woes.

"He's just working and hanging with his kids," the source adds. "He's not slowing down at all. He's actually been doing some bigger campaigns and shoots. He wants to keep his mind off of it. His work helps it."

The treatment update comes weeks after Joe admitted he was praying for a miracle in his cancer fight.

"At this point, I am spending time with my family," he told In Touch in December (16). "They are my biggest support. The cancer continues to be monitored, and I am doing everything the doctors are telling me to do. I'm looking for a miracle."

Joe had his prostate removed upon diagnosis, adding, "As everyone knows, all it takes is one cell to multiply.

"Right now the outlook is great, but no matter what, I have to monitor this for the rest of my life. Once you have cancer, you have it - but I will get through this."