Despite the machine's remarkable precision, radiation scattered each time, burning her skin.
In the annals of good-news/bad-news cancer stories, Hayek's stands out. Her two most significant acting roles so far have landed at exactly the same time as her two bouts of Hodgkin's lymphoma.
That bittersweet confluence has helped Hayek, now 30, handle the trauma of cancer.
The range of reactions to a Big C diagnosis is as varied as the permutations of the disease. For Hayek, it sparked a determination that has carried her through painful treatments and inspired followers on Instagram to lead healthier lives.
"I didn't think it would ever happen to me," Hayek said. "No one ever does."
The first sign
The disease crept up on her, disguised as a benign itch in May 2006.
She majored in theater arts at the University of Miami, but her starring role as shooting guard for the Division 1 basketball team kept her from stage productions. After graduation, she planned to move to Hollywood.
"It wasn't about being famous and rich," Hayek said. "I just always wanted to act."
At 5-foot-9 with long chestnut hair, honey eyes, luminous skin, and an athlete's sculpted muscles, Hayek often was told that she would light up the screen.
The itch, however, grew worse. It started on her elbow and spread to her shins and her back. She moved home to Lancaster and saw a dermatologist, who prescribed creams and lotions.
After four months, her doctor ordered a chest X-ray. It showed a three-inch mass in her mediastinum, the busy intersection in the chest cavity between the lungs.
Until then, Hayek had been ferociously healthy.
"I imagine I'm speaking for a lot of cancer patients when I say, 'I don't smoke. What did I do wrong?' "
Her mother, Joann, rests her hands on Katie's shoulders. "It can happen to anyone," she said.
They were at the family's vacation home in Cape May Court House for a week in July. Katie and her twin brother, Matthew, their older sisters, Elizabeth and Lori, and their families shared a lunch of watermelon with fresh figs, feta cheese, basil and balsamic vinegar, carrots and celery sticks with hummus, a bowl of sliced strawberries, and a small mountain of corn chips.
"Non-GMO," notes her 62-year-old father, Bill.
With encouragement from her sister, Katie started the Instagram blog, naming it "Fiber of Life," a metaphor for moving in a healthy direction, removing toxins from the body. She posts wholesome food-porn photos of dishes she's prepared and shares videos of her workouts. And then just because, every Friday, she sends a cathartic - and unprintable - message to cancer.
"It's a shame that it takes a disease like this to change," she said. "I wanted to help people lead a healthier lifestyle."
Her main thesis, "feed it or fight it," is that there are two kinds of food: those that encourage the growth of cancer cells and those that inhibit it.
"These are important messages for the public," said Ernest Hawk, vice president of cancer prevention at the M.D. Anderson Cancer Center in Houston. "There is a chance we can reduce the incidence of cancer in America by about 30 percent if everyone changed their habits."
It is impossible to measure how much specific foods or regular exercise will influence one person's odds of getting cancer, because studies cannot be that finely tuned.
"Cancer is a very individual disease," said Hawk, who also is a spokesman for the American Association for Cancer Research. "It is a combination of inherent genetics and environmental exposures that accumulate over time."
In general, however, he said you will reduce the odds of getting cancer by eating fruits and vegetables, avoiding saturated animal fats, maintaining a healthy weight, leading a more active lifestyle, and, of course, not smoking.
As a college athlete, Katie had played by all those rules. So it seemed inconceivable that she could have cancer.
She tried not to dwell on the karmic injustice and focused instead on her acting career.
At 22, she worried that she was already over the hill and needed to get to Los Angeles as soon as possible. She was afraid, too, that her scar might disqualify her for roles.
Pulling down the neck of her "Fight for It" muscle shirt, she reveals the glossy inch-and-a-half reminder of where surgeons dipped into her chest.
Before she had any inkling that she was sick, she auditioned for The Mighty Macs, a film about Immaculata College's 1971 women's basketball team, which rose from obscurity to win the national championship.
"They wanted real basketball players," Katie recalled. She was ushered into Immaculata's gym and was asked to do a righthanded layup, a lefthanded layup, dribble the length of the floor, and execute a jump shot.
Did she make the shots?
She flashes her dimpled smile. "Oh yeah."
She was offered the part of the main team player and once every two weeks for the next four months went for chemotherapy treatment. The director timed the filming of her scenes to accommodate her schedule. As her hair fell out, the film crew adjusted her wigs and makeup.
She lost weight, and her once-athletic arms and legs grew spindly, but the crew told her the look was in keeping with the 1970s.
One morning after a particularly bad reaction to the chemo, she went downstairs to be driven to the set.
"She had dark circles under her eyes and she was so pale," her father recalled. "I thought no amount of makeup was going to help."
Instead of telling her to go back to bed, he said: "You can do this."
That day, she worked for 12 hours.
"I wasn't one of those patients who was super positive," she said. "I was cranky and nauseous. But you adjust," she said. As soon as she showed up on set, she would suddenly forget about being sick.
Her father, a former director of sales training at Johnson & Johnson, was inspired by Katie's spirit. He had never been slothful, but over the years, had become out of shape, traveling for work and sitting behind a desk.
That spring, a friend asked Bill if he would join him in a fund-raising triathlon to raise money for childhood cancer in Katie's name.
"I figured that if she could make a movie and go through chemotherapy, I can do this."
Since then, he has lost 30 pounds and completed 29 triathlons - the last four with Katie.
When she recovered from chemo, she moved to Los Angeles, acting in commercials, appearing in the TV series One Tree Hill, and landing a few other small parts. In 2011, she returned to the East Coast. In December 2012, during a routine PET scan, her doctors found evidence that her tumor was growing back.
She opted for proton therapy. This time, the call from her manager came near the end of her treatment. She'd been cast in The Following. Her scene would be filmed the day after she received her last dose of radiation.
Her siblings joke that cancer has been the best thing for her career.
Maybe so, said Katie. And just as ironic, her illness has made her healthier. She is now a vegetarian and has adopted a rigorous workout routine, exercising six days a week.
"I started reading nutrition books," she said. "More knowledge is more power."
Her family were always active and ate well, but, influenced by Katie, they are even more deliberate about making healthy choices.
Throughout the afternoon, everyone made multiple visits to the kitchen counter to refill glasses from the daily vat of detox water, this one infused with lemon verbena and mint.
"Everyone has cancer cells," she said. "I know I'm more susceptible, so I know I have to do this."
While she continues to audition for acting roles, she is training to be a certified personal trainer. And to maintain perspective, she occasionally unearths photos from the darker days of her cancer treatment.
"Look at this," she said, searching through her phone for a picture of her blistered chest. It was taken the day she filmed her scene in The Following (Season 1, Episode 14).
Fans of the series would never notice - not because of any magic by the sympathetic makeup artist - but because Hayek's character, one of the cult members, was modestly dressed, hidden in deep shadows, and shot to death within a few seconds.
Her oncologist was horrified, she said. "She kept saying, 'Why did they have to kill your character! That's just wrong!' "
It was only make-believe, Katie said. In real life, she is very much alive, 448 days cancer-free and counting.