Shannen Doherty is exposing an apparent misconception about cancer: It's not contagious.
The 45-year-old actress made the point while detailing a stranger's "horrified" reaction to hearing that she had just finished radiation treatment during an elevator ride on Thursday.
"Today, we were in an elevator and I was asked how I was feeling. I responded by saying that I had just gotten a radiation treatment so therefore very tired," Doherty wrote next to a car selfie with her husband, Kurtis Warienko. "The random girl on the elevator with us physically lunged away as far as she could. She then proceeded to look horrified for the next [three] floors."
Shannen Doherty through the years
Shannen Doherty through the years
WEST HOLLYWOOD, CA - APRIL 19: Actress Shannen Doherty attends Jennie Garth's 40th birthday celebration and premiere party for 'Jennie Garth: A Little Bit Country' at The London Hotel on April 19, 2012 in West Hollywood, California. (Photo by Angela Weiss/Getty Images)
LOS ANGELES, CA - FEBRUARY 13: Actress Shannen Doherty attends Shepherd Conservation Society's 2nd Annual 'World Love For Dolphins Day' at Japanese Consulate on February 13, 2015 in Los Angeles, California. (Photo by Joe Kohen/Getty Images)
PARIS, FRANCE - MARCH 13: (EXCLUSIVE COVERAGE) Actress Shannen Doherty arrives at Charles-de-Gaulle airport on March 13, 2014 in Paris, France. (Photo by Marc Piasecki/GC Images)
LOS ANGELES, CA - APRIL 09: Shannen Doherty is sighted at The Grove on April 9, 2012 in Los Angeles, California. (Photo by Noel Vasquez/FilmMagic)
WATCH WHAT HAPPENS LIVE -- Pictured: Shannen Doherty -- (Photo by: Peter Kramer/Bravo/NBCU Photo Bank via Getty Images)
LOS ANGELES, CA - MAY 10: Shannen Doherty attends STK Los Angeles' 3 Year Anniversary Party at STK on May 10, 2011 in Los Angeles, California. (Photo by Todd Williamson/WireImage)
LOS ANGELES, CA - DECEMBER 14: Shannen Doherty signs copies of her new book 'Badass' at Barnes & Noble bookstore at The Grove on December 14, 2010 in Los Angeles, California. (Photo by Jason LaVeris/FilmMagic)
Shannen Doherty arrives at the Los Angeles premiere of 'Burning Palms' held at ArcLight Cinemas on January 12, 2011 in Hollywood, California.
NEW YORK - NOVEMBER 02: Actress Shannen Doherty promotes 'Badass' at Borders Books & Music, Columbus Circle on November 2, 2010 in New York City. (Photo by Jason Kempin/Getty Images)
Shannen Doherty celebrates the grand opening of Bravada Women's Athletica at Bravada - Women's Athletica on June 17, 2010 in Los Angeles, California. (Photo by Jean Baptiste Lacroix/WireImage)
NEW YORK - APRIL 01: Dancer Mark Ballas (L) and television personality Shannen Doherty pose for photos in Midtown Manhattan on April 01, 2010 in New York City. (Photo by Ray Tamarra/Getty Images)
LOS ANGELES, CA - MARCH 27: Shannen Doherty attends Perez Hilton's 'Carn-Evil' Theatrical Freak and Funk 32nd birthday party at Paramount Studios on March 27, 2010 in Los Angeles, California. (Photo by Jean Baptiste Lacroix/WireImage)
LOS ANGELES, CA - MARCH 26: Shannen Doherty is seen on March 26, 2010 in Los Angeles, California. (Photo by Jean Baptiste Lacroix/WireImage)
LOS ANGELES, CA - JANUARY 16: Actress Shannen Doherty arrives at The Art of Elysium's 3rd Annual Black Tie Charity Gala 'Heaven' on January 16, 2010 in Los Angeles, California. (Photo by Jeffrey Mayer/WireImage)
LOS ANGELES, CA - DECEMBER 18: Actress Shannen Doherty arrives at Flaunt Magazine's 10th Anniversary Party and Annual Holiday Toy Drive at the Wayne Kao Mansion on December 18th, 2008 in Holmby Hills, CA. (Photo by Michael Bezjian/WireImage)
PASADENA, CA - JULY 11: Actress Shannen Doherty speaks during the 2006 Summer Television Critics Press Tour for the Oxygen Network at the Ritz Carlton Hotel July 11, 2006 in Pasadena, California. (Photo Frederick M. Brown/Getty Images)
NEW YORK - NOVEMBER 3: (ITALY OUT, NY DAILY NEWS OUT, NY NEWSDAY OUT) Actress Shannen Doherty attends the Fendi Flagship store opening November 3, 2005 in New York City. (Photo by Arnaldo Magnani/Getty Images)
Shannen Doherty during DKNY Jeans Presents Vanity Fair in Concert With a Live Performance by Alanis Morissette Benefiting Step Up Women's Network at Hiro Ballroom at The Maritime Hotel in New York City, New York, United States. (Photo by Jamie McCarthy/WireImage)
Shannen Doherty during Olympus Fashion Week Fall 2005 - Sweetface by Jennifer Lopez and Andy Hilfiger - Backstage Arrivals at The Tent, Bryant Park in New York City, New York, United States. (Photo by KMazur/WireImage)
Shannen Doherty at the HP Portrait Studio in Park City, Utah (Photo by Jeff Vespa/WireImage)
Shannen Doherty (Photo by Jeffrey Mayer/WireImage)
Shannen Doherty during The 56th Annual Primetime Emmy Awards - Fox After Party - Arrivals at Spago in Beverly Hills, California, United States. (Photo by J.Sciulli/WireImage for The jLINE Group, Inc.)
Shannen Doherty during World Premiere of 'S.W.A.T.' at Mann Village Theater in Westwood, California, United States. (Photo by Jon Kopaloff/FilmMagic)
Shannen Doherty (Photo by Ron Galella/WireImage)
Jason Priestley, Shannen Doherty and Luke Perry (Photo by Ron Galella/WireImage)
394962 09: ***EXCLUSIVE*** (Publicist Approval Required for ALL Usage) Actress Shannen Doherty poses for a portrait January 2001 in Los Angeles, CA. (Photo by Jeff Katz/Getty Images) (Clients must call or email Getty Images with EXACT frame number, client name, name/type of publication, date of intended publication and type of usage (cover, inside full page, etc) and WAIT for Getty Images to confirm publicist approval before allowing client to commit these images to use.)
Shannen Doherty, circa 1988. (Photo by Getty Images)
Actresses Shannen Doherty and Shalane McCall attend Starlight Foundation Benefit on September 22, 1988 at Ed Debevic's Restaurant in Beverly Hills, California.
LITTLE HOUSE ON THE PRAIRIE -- Season 9 -- Pictured: Shannen Doherty as Jenny Wilder (Photo by Frank Carroll/NBC/NBCU Photo Bank via Getty Images)
Doherty has been in the fight of her life since being diagnosed with breast cancer in March 2015. The Beverly Hills 90210 alum recently opened up to ET about the scary health battle while revealing that the cancer had spread to her lymph nodes.
Celebrities who have publicly battled breast cancer
Celebrities who have publicly battled breast cancer
Toronto, Canada - September, 17 2015 - Actress Cynthia Nixon (best known for Sex and the City) was in Toronto promoting her indie film JAMES WHITE. TIFF15, Toronto International Film Festival September 17, 2015 (Richard Lautens/Toronto Star via Getty Images)
Sheryl Crowe arrives at the 2014 Vanity Fair Oscar Party on March 2, 2014 in West Hollywood, California. (Photo credit ADRIAN SANCHEZ-GONZALEZ/AFP/Getty Images)
LOS ANGELES, CA - AUGUST 01: Christina Applegate arrives at Dizzy Feet Foundation's 5th Annual Celebration Of Dance Gala held at Club Nokia on August 1, 2015 in Los Angeles, California. (Photo by Michael Tran/FilmMagic)
Actress Maura Tierney of Showtime's 'The Affair' poses in the Getty Images Portrait Studio powered by Samsung Galaxy at the 2015 Summer TCA's at The Beverly Hilton Hotel on August 10, 2015 in Beverly Hills, California. (Photo by Maarten de Boer/Getty Images)
US actress and UNHCR ambassador Angelina Jolie stands during a visit to a camp for displaced Iraqis in Khanke, a few kilometres (miles) from the Turkish border in Iraq's Dohuk province, on January 25, 2015. Run by authorities from the three-province autonomous Kurdish region of north Iraq with the help of the United Nations refugee agency, the UNHCR, Khanke aims to house 18,000 people, said the agency's Liena Veide. AFP PHOTO/SAFIN HAMED (Photo credit should read SAFIN HAMED/AFP/Getty Images)
LOS ANGELES, CA - AUGUST 13 NBCUNIVERSAL EVENTS -- NBCUniversal Portrait Studio, August 2015 -- Pictured: TV personality Giuliana Rancic from 'Fashion Police' poses for a portrait at the NBCUniversal Summer Press Day during the 2015 Summer TCA Tour at The Beverly Hilton on August 12, 2015 in Beverly Hills, California. (Photo by: Christopher Polk/NBC/NBCU Photo Bank via Getty Images)
SIMI VALLEY, CA - JULY 05: Nancy Reagan attends The Grand Opening of D23 Presents Treasures of The Walt Disney Archives on July 5, 2012 in Simi Valley, California. (Photo by Michael Kovac/FilmMagic)
BEVERLY HILLS, CA - MAY 24: Reporter Linda Ellerbee speaks during the 36th Annual Gracie Awards Gala at the Beverly Hilton Hotel on May 24, 2011 in Beverly Hills, California. (Photo by Frederick M. Brown/Getty Images)
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"The unknown is always the scariest part," the actress told ET's Jennifer Peros in August. "Is the chemo going to work? Is the radiation going to work? You know, am I going to have to go through this again, or am I going to get secondary cancer? Everything else is manageable. Pain is manageable, you know, living without a breast is manageable. It's the worry of your future and how your future is going to affect the people that you love."