In last week’s episode, Casey headed to Portland to visit the teenage sons of his best friend Andy Darden, who died in the show’s pilot. They weren’t doing well — especially with their mom in prison and their aunt, who was supposed to be taking care of them, not around — and were at risk of being placed in separate foster homes. When Casey inquired about bringing them back to Chicago, that was a dead end.

Once Casey returned to Chicago and told Christopher Herrmann (David Eigenberg) that “any firefighter’s child is every firefighter’s child,” the writing was on the wall, and this week became about his goodbyes.

“This was a difficult decision because I have loved the show from the start but there are other things that I would like to do in the future, and there’s some family that I need to take care of and 18 years is a long time,” Spencer explained in a roundtable interview. (The Australian actor, 42, starred on House before jumping straight into Chicago Fire.) “I was marking off these milestones and looking to the future … and it was time.

He added, “It felt so organic for me and a perfect full circle and a really reasonable way for Casey to organically leave.”

For Spencer, one of the most difficult elements of his decision was the timing in regards to Casey’s relationship with Sylvie Brett (Kara Kilmer), which was years in the making and only finally came to fruition at the end of season 9. Now, they’re facing three years of a long-distance relationship but are determined to stay together.

“It was difficult because I really like Kara,” he said. “But I think it was written really well into the episode because they’re trying to make the relationship work but they know that he’s leaving. So they’re sort of saying nothing’s going to change.”

Added showrunner Derek Haas: “We’re going to keep it alive. … This is a three-year commitment that Casey’s making but hopefully we’ll see him before those three years are done.”

One of those moments where Spencer could return may just be the impending wedding of Kelly Severide (Taylor Kinney) and Stella Kidd (Miranda Rae Mayo), for which Casey is supposed to be the best man.

“I’m hoping,” said Haas. “I never try to predict what’s going to happen production-wise, but we’re in the camp of whenever Jesse wants to come do an episode with us, we will move mountains and earth to make that happen. We’ve been talking. We’ve got some ideas on what that would be and I would say you hit the nail on the head on what some of those moments might be.”

Spencer was also quick to point out that he still lives in Chicago with his wife, research scientist Kali Woodruff Carr, whom he married last year.

“I’m not running off to Los Angeles or anything, although I might escape for a little bit of the winter,” he teased. “There is the potential for me to come back.”

As he looked back at the show which launched a massive franchise that now includes Chicago P.D. and Chicago Med, Spencer’s pride was palpable.

“It just felt like we really built something from the ground up that no one thought would survive and now we’ve hit 200 episodes,” he said. “There are so many points to be proud of, and I was always proud of the show. I always liked it. … There are so many things that I’ve loved about it and memories and points that we hit. That makes [leaving] bittersweet.”

Spencer’s final day on set was filming on the “burn stage” for the scene where he and the rest of Firehouse 51 are cleaning out the burned-out church.

“Everyone just started hitting the pipes on the ground and chanting,” he recalled. “It sort of became this semi-pseudo tribal sendoff for me. … It was something else. It was really nice and then I finished and they wheeled out the cake and we had speeches. Eamonn [Walker] gave a speech and Taylor gave a speech.”

“I couldn’t have asked for anything more,” he added. “It was a kind of fitting way to finish out on the burn stage. We’re a fire show. We started with fire and we ended with it.”