This is Steven Petrow at age 26 before chemotherapy treatment and before he lost his hair. (Courtesy of Steven Petrow)

“What are your values?” Paul Kalanithi is asked point-blank by his oncologist soon after he is diagnosed with Stage 4 lung cancer. By that, his doctor means: Does Kalanithi, a neurosurgical resident who was then 36, want to spend time with his wife, find another line of work or safeguard his hands so he can continue to operate?

Kalanithi recounts all this movingly in his posthumously published memoir “When Breath Becomes Air.”

“Many people, once diagnosed, quit work entirely. Others focus on it heavily. Either way is okay,” the oncologist explains. “You have to figure out what’s important to you.”

“I’ve been preparing my whole life” for surgery, Kalanithi replies. With that intention, the oncologist recommends a chemotherapy that should avoid any neurological damage to his hands, allowing him to continue to operate. “I don’t think it will change survival,” she pointedly adds, which is equally important to Kalanithi.

In the weeks to come, the young resident opened his veins to the carboplatin, started couples therapy with his wife and undertook an often-painful routine of physical therapy. “You’re doing great. . . . You’re finding your values, and that’s not easy,” the oncologist reminds him repeatedly.

“That’s not easy” is an understatement — and one I have come up against myself. I experienced the search for my values when I first faced cancer at age 26, and now I’m helping my mother — 84 and diagnosed with recurrent lung cancer — make her own decisions about what matters to her. At the same time, a lifelong friend, Susan, who is 60 and a widow, is facing a brain-cancer diagnosis with one son in college and the other finding his footing in the world. As she chooses amid the various treatment options before her, she is deciding what’s important to her.

If anyone had asked me to define my values at 26, I’d have drawn a blank. I was an ambitious grad student at Berkeley in Year 5 of a 10-year program. Then, suddenly, in my new life with testicular cancer, I had to ask the question: If I had only limited time left, would I want to spend five more years toiling at Berkeley? “NO WAY,” my inner voice shouted. There was simply too much else to do in the world than slave over my dissertation.

In hindsight, I can better understand my values by looking at my actions. Until my diagnosis, I’d taken risks as befitted a 20-something. Here’s just one example: No problem rocketing down a whitewater river, sans life jacket, until my kayak capsized and threw me into the rocky waters — and into the ER.

I loved the thrill of it all.

But there was no thrill in making decisions about cancer treatment. After surgery to remove the malignant testicle, the urologist declared me cancer-free and recommended what was then a new option, “watchful waiting,” over additional surgery or chemo. Such an approach at that time had very little science to support it, which didn’t stop me. I said yes in a nanosecond — not only because it was the shiny new thing but also because it dovetailed with another value of mine: vanity. Can you blame me, a young single guy, for not wanting a long, jagged scar?

Margot Petrow celebrates her 84th birthday with her children Steven Petrow, left, Jay Petrow and Julie Petrow-Cohen. (Caroline Petrow-Cohen)

My mom, however, was having none of my quick decision. First, because she has always believed in second opinions, and then because she laser-focused on the prize: long-term survival of her firstborn, not my vanity or my impulse to take a chance with an unproven option. I decided Mom knew best and flew cross-country to the Memorial Sloan Kettering Cancer Center in New York for a second opinion with urologic surgeon Harry Herr.

“You have metastatic disease,” he explained. “You’re not a candidate for monitoring. You need surgery right away.” (He recommended a lymph node dissection, an eight-hour procedure that, among other side effects, can leave a guy unable to ejaculate. Oh, and yes, there was the question of that scar.)

Struggling between two paradigms — risk-taker Steven and someone I was on the verge of becoming — I phoned Herr, asking for help. “If you follow my advice and I’m wrong, you’ll have had an unnecessary surgery. If you follow [your San Francisco doctor’s] recommendation and he’s wrong, you’ll be dead,” he explained very matter-of-factly. But certainly persuasively.

With that declaration, I found my values: Above all, I wanted to live. I wouldn’t let my vanity or fear of disfigurement, or possible sexual impacts, drive my decisions. I would do everything necessary to live as long as I could. That, I finally realized, is what really mattered to me.

Hello, New Steven.

Paul Kalanithi found his values when confronted with brain cancer. He returned to the OR, started a family with his wife and lived to witness the birth of his daughter. A year after his initial diagnosis, Kalanithi suffered a recurrence and decided he could no longer perform surgery. He started writing a memoir, which he largely completed before his death in March 2015. His life was cut short, but not before he accomplished his mission.

Several years back, a study of breast cancer patients in the journal Health Psychology reported that “positive strategies for coping” tended to lead to “feelings of inner peace, satisfaction with one’s current life and the future . . . [while] the absence of such strategies predicted reports of loss of meaning and confusion.” This resonated with my experience.

Faced with a cancer recurrence, my mother has found what she values, choosing palliative care for her cancer recurrence. She wants to enjoy this last chapter among her loved ones, displaying her deep-rooted dignity daily. I applaud her, though it’s hard for me to accept that her time is now limited.

As for my friend Susan, she has chosen to fight her malignancy with every tool in the kit. She doesn’t want her sons orphaned. She wants to see her younger son graduate from college next year and then go on to start a family. If her strategy is successful, she’ll have won a double prize: She’ll see her sons set off on their own paths and she’ll have reaffirmed what matters most to her. Her boys.

Petrow writes The Post’s Civilities column. Follow him on Twitter: @stevenpetrow.