Home » Pet Health » Cancer 101 » What Is Relapse or Recurrence?


Relapse or recurrence refers to cancers that come back after initial treatment (such as chemotherapy or radiation therapy). Generally, malignant cancers diagnosed at a later stage are more likely to relapse. Such cancers are more advanced, widespread, and fast-growing.

The reason that cancers recur stems from the fact that not all tumor cells are the same but are substantially different from each other due to their genetic instability. Cancer cells are cells that have gone awry; they both multiply unchecked and function incorrectly. They are not as genetically stable as the normal cells in the developed tissues and organs and evolve to become more aggressive.

As cancer progresses from an early nodule to a late-stage metastatic (spreading) tumor, the cells will acquire many resistance features–for example, the muti-drug resistance (MDR) mechanism. When a cancer cell develops an MDR mechanism, it will pump the drug outside the cell and resist treatment with a new drug. This resistance leads to the ineffectiveness of the applied chemotherapy where, after a period of improvement, the cancer cells will resume growth, allowing tumors to flourish and spread. The resistant cells can also learn to evade attacks from the body’s immune system.

Relapse can also happen if some cancer cells remain unchecked after surgically removing the tumor. As a precaution, vets also remove tissues surrounding the initial tumor to minimize recurrence. It is challenging to remove all the cancerous tissue from the body, and a few microscopically undetectable cancer cells that remain can resume growth and may spread in the body.

There are three different types of recurrences:

    1. Local recurrence is when cancer comes back to grow at the same place or very close to where it originated.
    1. Regional recurrence is when cancer comes back to grow in the lymph nodes or tissues near the original tumor. Regional recurrence may occur because cancer cells can spread via the lymphatic system and lodge in the nearby lymph nodes.
    1. Distant recurrence occurs when cancer spreads to organs or tissues far from the original site. The most common areas of distant recurrences are the lungs, brain, liver, and bones. When cancer spreads, the cells are fundamentally different from their original type as they become very mobile, penetrate blood vessels, break down surrounding tissues, and settle at distant sites.

Recurrent cancer displays properties resembling the original disease. Still, your vet must perform tests to confirm if the recurrent cancer is the same type as the original tumor or if it is a new type. It might require a different therapy approach than the initial disease if diagnosed as a new type of cancer.

The Pet Cancer Foundation’s Website Editorial team is comprised of veterinarians, veterinary oncologists, and veterinary technicians, as well as scientific writers and editors who have attained their PhD’s in the life sciences, along with general editors and research assistants. All content found in this section goes through an extensive process with multiple review stages, to ensure this extended resource provides pet families with the most up-to-date information publicly available.

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Last Updated: October 14, 2022

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The following sources were referenced to write the content on this page: 

Neuman, HB, Schumacher, JR, Francescatti, AB, Adesoye, T, Edge, SB, Vanness, DJ, Yu, M, McKellar, D, Winchester, DP & Greenberg, CC 2018, ‘Risk of synchronous distant recurrence at time of locoregional recurrence in patients with Stage II and III breast cancer (AFT-01)’, J Clin Oncol, vol. 36, no. 10, pp. 975-980.

Riggio, AI, Varley, KE & Welm, AL 2021, ‘The lingering mysteries of metastatic recurrence in breast cancer’, Br J Cancer, vol. 124, no. 1, pp. 13-26.

Siegel, RL, Miller, KD & Jemal, A 2020, ‘Cancer statistics’, CA Cancer J Clin, vol. 70, no. 1, pp. 7-30.

Wu, Q, Yang, Z, Nie, Y, Shi, Y & Fan, D 2014, ‘Multi-drug resistance in cancer chemotherapeutics: mechanisms and lab approaches’, Cancer Lett, vol. 347, no. 2, pp. 159-166.

The Pet Cancer Foundation’s medical resource for pet owners is protected by copyright.

For reprint requests, please see our Content Usage Policy.