Treatment guidelines on care for survivors of the most common cancers often lack specifics or offer conflicting advice about when or how often to do certain tests, a review of recent recommendations finds.

Researchers examined cancer guidelines from North America and Europe that have been published since 2010 to advise doctors on the best ways to care for survivors of nine malignancies: breast, colorectal, lung, prostate, melanoma, uterine, bladder, thyroid and testicle.

They found ambiguous recommendations in 83 percent of the guidelines.

In particular, different guidelines for the same type of cancer often didn't address all the same surveillance tests, and few tests were universally recommended, noted lead study author Dr. Ryan Merkow of Memorial Sloan Kettering Cancer Center in New York. When guidelines did endorse tests, they rarely specified how long patients should continue to get tested.

"Often our instincts as providers are to order more tests - it makes us feel like we are caring for the patient, and it makes the patient feel like they are being cared for," Merkow said by email.

"In certain instances this is absolutely the correct approach, in others, it may lead to additional and unnecessary procedures and patient anxiety," Merkow added. "We have to approach surveillance in a deliberate and thoughtful manner as the risk of over and under-treatment is real."

Worldwide, an estimated 33 million people are cancer survivors, and their ranks are expected to grow due to rising cancer rates in an aging population as well as improved survival odds with advances in diagnosis and treatment, researchers note in JAMA Internal Medicine.

For the current study, Merkow's team focused on what's known as surveillance, or efforts to detect the return of cancer or the development of tumors in other parts of the body. This can include invasive tests like biopsies that come with their own risks and side effects as well as expensive imaging like positron emission tomographic (PET) scanning.

They reviewed 41 guidelines addressing post-treatment surveillance, with three to six recommendations focused on each of the nine cancer types examined.

Overall, 37 of these guidelines, or 90 percent, recommended physical exams and medical history. Most addressed some form of imaging (83 percent), while 63 percent covered what's known as endoscopic procedures that involve doctors using a flexible tube with a camera to examine the inside of the digestive tract.

In 23 of the guidelines, or 56 percent, recommendations touched on lab tests to look for biomarkers of certain types of tumors, which are typically found in blood, urine or body tissue.

Guidelines for using PET scans offer a snapshot of the mixed messages in surveillance recommendations. Only one of the guidelines in the study recommended patients get this scan, and it was for bladder cancer.

Other guidelines either didn't address PET scans or advised against them, which was the case for 67 percent of uterine cancer recommendations and 60 percent of lung cancer recommendations.

The most ambiguous recommendations for PET scan use were for tumors of the bladder, prostate and breast.

European guidelines were more likely than North American guidelines to contain ambiguous recommendations, the study also found.

Limitations of the study include its focus on national recommendations, which excluded widely followed provincial practice guidelines in Canada, the authors note. The study also didn't include recommendations published after February 2016.

Even so, the results highlight the work that still needs to be done to better standardize care for patients who have undergone cancer treatment, said Dr. Alexander Kutikov, a researcher at Fox Chase Cancer Center in Philadelphia who wasn't involved in the study.

"Clinical utility versus patient expectations versus costs must be thoughtfully balanced," Kutikov said by email. "Right now our approach to post-treatment cancer surveillance is somewhat arbitrary."