Dikembe Mutombo reflects on his career in the NBA and how the game is expanding around the world. (2:24)

BOSTON -- Even before he entered the room, Marcus Smart felt the familiar pang of nerves. Every visit to Boston Children's Hospital leaves Smart feeling inspired but also comes with the unavoidable flood of emotion because of all the reminders of the older brother he lost to cancer more than a decade ago.

Navigating the sixth floor at Children's, where most of the hospital's inpatient pediatric oncology services are rendered, Smart breathed deep on a September afternoon in 2016 then turned the corner into Sammy Dubois' room.

And then came the unexpected hail storm of Nerf darts.

Dubois, then 16, had been diagnosed with high-risk acute myeloid leukemia a month earlier. A softball junkie, the resident of Rindge, New Hampshire, didn't know much about Smart before his visit but had excitedly asked the nurses at Children's what she should do in the presence of a famous visitor.

The nurses all told Dubois, who was undergoing an initial round of induction chemotherapy in advance of a bone marrow transplant, to relax and just be herself.

So the teenager eased any awkwardness by pelting one of the NBA's most rugged defenders with rubber darts and foam balls. Smart quickly sought out his own toy weapon to return fire. In that moment, a rare bond was forged between player and patient, a bond that would only grow stronger as Smart routinely checked in on Dubois during her treatments.

This week, a now-cancer-free Dubois is in London with Smart and the Celtics, the trip a surprise gift from a Celtics minority owner who was moved when Dubois accompanied Smart on stage at the Celtics' annual Shamrock Foundation Tip-Off Gala before the start of the 2017-18 season.

Smart was honored that night with New England Baptist Hospital's Community Champions Award and gave an impassioned speech about why it's important for him to give back. It floored everyone in attendance, including his teammates. Later in the program, Dubois read a letter she penned to Smart detailing what his visit had meant to her during her battle with cancer.

Sammy Dubois joined Marcus Smart on stage at the 2017 Champion for Children's event, where Smart was honored by Boston Children's Hospital. Darren McCollester/Getty Images for Boston Children's Hospital

On a night the Shamrock Foundation raised more than $1 million for its charities, the big-ticket prize was a chance to join the Celtics on their weeklong trip to London, including Thursday's game against the Philadelphia 76ers.

James Pallotta, a member of the Celtics' executive board and co-owner of Italian soccer club A.S. Roma, won the live auction with a bid of $45,000. Pallotta huddled with Celtics TV play-by-play legend Mike Gorman, the emcee for the night, and it was announced that the trip would go to the Dubois family.

Celtics newcomer Kyrie Irving, on stage at the time, offered to match Pallotta's $45,000 donation and said he'd also provide game gear from the team's overseas voyage.

"I couldn't believe what had happened," Smart said recently while reflecting on the generosity of Pallotta and Irving. "[Dubois was] ecstatic about it, and her family, too. It just showed that, in today's society, as an athlete or somebody with a high profile, we're always in the media but you always see the bad things. You never really see the good things.

"This allows us to show that we have more to offer than just basketball."

SMART IS NO STRANGER to negative press. Back in February 2014, during his sophomore season at Oklahoma State, he drew national headlines when he shoved a Texas Tech fan after spilling into the crowd in the final seconds of a loss in Lubbock.

Smart's character was questioned and many wondered whether the incident might hurt his draft stock. He found a supporter in Celtics president of basketball operations Danny Ainge -- no stranger to on-court flare-ups during his own playing days -- who raved about Smart during ESPN's broadcast of the draft combine that year and then backed up his talk by selecting Smart with the No. 6 overall pick.

Now in his fourth season with the Celtics, Smart remains a polarizing figure. Renowned for his defensive tenacity and his ability to make game-changing plays in key moments, Smart is still widely criticized for his shooting woes (career 35.6 percent shooter, including 29.1 percent beyond the 3-point arc).

Still, Celtics coach Brad Stevens tells anyone who will listen that the way Smart impacts winning is impossible to truly quantify.

"[The Celtics] took a chance on me, and decided to give me a different path to take to help and better my life. If it wasn't for that, man, I wouldn't be here today."

What's no longer in question is Smart's character. Inspired both by his brother Todd Westbrook's battle with cancer and the dark path that a young Smart spiraled down in the aftermath of his brother's passing, Smart started the YounGameChanger Foundation, which aims to help families of chronically ill children while also providing a guiding voice to inner-city athletes.

Smart laid himself bare in his acceptance speech at the Shamrock Gala. He spoke of Westbrook and his father-like impact on a young Smart. He detailed how, at 9 years old, Smart told his mother that all he wanted for the holidays was to spend one final Christmas with Westbrook, whose health was failing.

That year, Smart's brothers surprised Marcus with his first basketball hoop. And Smart still smiles now, cherishing that his wish came true in receiving one final Christmas as a family. Westbrook lost a 15-year battle with leukemia at age 33 on Jan. 9, 2004.

Smart admits he acted out in the aftermath. He became a bully and a petty thief. He watched one of his best friends get shot and killed and nearly suffered a similar fate in a Dallas suburb after throwing rocks at a gang member who chased and fired at Smart in the aftermath.

Smart detailed how one of his other brothers, Michael, nearly overdosed on cocaine after Westbrook's death.

"I went down a dark path in my life," Smart said. "My life has flashed before my eyes plenty of times. ... [The Celtics] took a chance on me, and decided to give me a different path to take to help and better my life. If it wasn't for that, man, I wouldn't be here today."

Now Smart wants to help others. And this isn't an NBA player going through the motions at community events. Smart routinely invests himself in the patients he visits during local hospital tours, and sometimes people like Dubois end up having a greater impact on him than the other way around.

"Marcus, he had a certain upbringing that he has an outer shell, of course, but on the inside, Marcus is a great guy," Celtics teammate Jaylen Brown said. "He's as considerate and as passionate about helping people as anybody. That's dope that [Dubois is in London]."

Echoed Celtics big man Al Horford: "Marcus, you see this tough guy, but he's a very caring type of guy. He's a guy that you want around because he's just a good guy. Yes, he's tough and pesky on the court. Off the court, it's different. He has a good heart."

Sammy Dubois and her family got to spend some time with celebrity chef Gordon Ramsay while in London. Dubois Family

AFTER A NERF TRUCE was negotiated, Smart peppered Dubois with questions about her hobbies during that initial meeting. A ukulele lesson ("I showed him a few chords on the instrument and he tried it as well," Dubois said) and a hair makeover ("I found my pink hair extensions on the table and thought it would be funny if he would wear one," she said) followed soon after.

At some point, Smart was gifted an orange rubber bracelet with the hashtag "#SammyStrong" in white lettering.

Maybe it helped that it was the color of his college team, or that it paired nicely with Smart's rust-colored mohawk. But the orange wristband meant enough to Smart that he immediately slipped it on his wrist. It was still there two weeks later as the Celtics gathered for media day. One of the snapshots taken then ended up being used for a promotional pillar inside Boston's North Station, home of TD Garden, and the #SammyStrong bracelets were impossible to miss in the oversized picture.

Smart ended up wearing the wristbands for nearly all of the 97 games that he appeared in during the regular season and playoffs last year. It was such a common presence that versions of the popular NBA 2K video game shipped this year showed Smart wearing the wristband.

About the only time the bracelet came off on the court was when an opponent ripped it off Smart during a scramble for a loose ball.

"That bracelet means so much to me," said Smart, who also kept the colored hair extensions that Dubois gave him that day.

During his gala speech, Smart detailed why: "I played every game with [the #SammyStrong bracelets] because I wanted [Dubois] to know that I was still thinking about her, even when I was playing those games. When I was having hard times, I would look at those bracelets and it would remind me that you're blessed and someone is always in a different situation than you are. You never know what's going on in somebody else's life.

"So when you're feeling down, look at those bracelets and think of Sammy and it will always bring joy to me."

Smart's visit made Dubois an instant Celtics fan. And it meant the world to her and her family to see Smart wearing the #SammyStrong bands. "Seeing him wearing the bracelets sent me a message saying that he was thinking about me and supporting me," Dubois said.

Marcus Smart wore the #SammyStrong bracelet during the 2016-17 season. Winslow Townson-USA TODAY Sports

Smart showed it with his actions as well. When a Celtics contingent visited Children's for a holiday event in December 2016, Smart asked the nurses if Dubois was still admitted at the hospital.

Dubois had undergone her bone marrow transplant in late October (cells from a 33-year-old man in Poland helped to eradicate her cancer) and was initially discharged in November. But infections forced her to return to Children's soon after. During the Celtics' December visit, Smart asked Stevens and teammates Terry Rozier and Jonas Jerebko to join him on a surprise visit to Dubois' room.

Dubois has made incredible progress since that visit. Most of her restrictions were lifted this past summer and she was able to travel with her New Hampshire-based softball team. She even got back in the batter's box once -- a strikeout in the box score but, as her mother, Tammy, noted, "a grand slam in our book."

After missing all of the 2016-17 school year, Dubois is now completing her junior year at St. Bernard's High School in Fitchburg, Massachusetts. She had always hoped to be an orthopedic surgeon, but the nurses at Children's have inspired her to consider a future as a pediatric oncology nurse. Dubois is starting to tour area colleges and Simmons, only a couple of blocks away from Children's, is currently a top option.

This week, however, she's enjoying a brief respite in London with her father, Errick. She has bombarded family back home with Snapchats of her London cuisine and the sights she has seen.

The big moment comes Thursday afternoon, when she attends her first Celtics game. And after all those nights rooting for Smart from her hospital bed, she'll finally get to cheer for him in person.

What's clear from his actions is that Smart has been rooting for Dubois all along as well.