Does an Indeterminate Result Mean Cancer? What to Expect

An indeterminate result does not mean you have cancer. It means the test could not sort you neatly into “definitely benign” or “definitely malignant,” so the answer landed somewhere in between. Depending on the organ and the type of test, the chance that an indeterminate finding turns out to be cancer ranges from under five percent to roughly one in three. That is a wide range, and the next steps your doctor recommends will depend heavily on where in that range your particular situation falls.

What “Indeterminate” Means in Practice

In everyday language, indeterminate sounds like a non-answer, and that is essentially what it is. A pathologist looked at cells under a microscope, or a radiologist studied an image, and saw features that did not fit cleanly into either category. The cells might look slightly unusual but not unusual enough to call suspicious. The imaging finding might have some worrying characteristics mixed with reassuring ones. Rather than force a call and risk being wrong in either direction, the reporting system flags the result as indeterminate, and the question gets handed to additional tests, closer monitoring, or a different kind of biopsy.

Most cancer screening and diagnostic pathways have a formal category for this gray zone. Thyroid biopsies use the Bethesda system, breast imaging uses BI-RADS scores, prostate MRI uses PI-RADS scores, and lung nodules follow Fleischner Society guidelines. Each system was designed partly to handle indeterminate findings in a standardized way, so clinicians everywhere follow a similar decision tree.

Thyroid Nodules and the Bethesda Gray Zone

Thyroid nodules are where most people first encounter the word “indeterminate.” About one in five thyroid biopsies returns a result that falls into Bethesda Category III (atypia of undetermined significance) or Category IV (follicular neoplasm or suspicious for one).¹ Those labels do not mean cancer was found. They mean the cells looked atypical enough that the pathologist could not confidently call them benign, but not atypical enough to call them suspicious or malignant.

The actual malignancy rates within these categories are not trivial but are far from guaranteed. One study found cancer in about 30 percent of Bethesda III nodules and 47 percent of Bethesda IV nodules.² A separate analysis of indeterminate nodules across all subcategories found malignancy rates of roughly 14 to 25 percent depending on the specific cytology pattern, with a notably higher rate of about 69 percent in the “suspicious for malignancy” category, which sits just above the indeterminate zone.³ The variation is large, and the numbers shift depending on the institution and the population being studied.

One detail that surprises many people: nodule size alone does not reliably predict whether an indeterminate thyroid nodule is cancerous. In a study of over 400 indeterminate nodules, size was not significantly associated with malignancy across most subcategories.³ So a larger nodule is not necessarily more dangerous than a smaller one when both have indeterminate cytology, which is a common misconception.

What does seem to matter is what kind of atypical features the pathologist saw. Research on subcategorizing Bethesda III nodules found that nuclear atypia was the strongest predictor of malignancy among the different patterns that can produce an indeterminate reading.⁴ In other words, not all indeterminate results carry equal weight, even within the same Bethesda category.

Breast Imaging and BI-RADS 3

If you have had a mammogram or breast MRI and received a BI-RADS 3 result, that means “probably benign.” It is the breast imaging equivalent of indeterminate. The category was designed to identify findings that have a very low probability of being cancer but that cannot be dismissed entirely based on how they look. When used correctly, the BI-RADS 3 designation reduces the number of unnecessary biopsies while still catching early cancers that might otherwise be missed.⁵

The standard recommendation for a BI-RADS 3 finding is short-interval follow-up, typically another imaging study in six months, rather than an immediate biopsy. The idea is that if the finding stays stable over time, it is almost certainly benign. If it changes, biopsy follows. For findings categorized higher, at BI-RADS 4 or 5, biopsy is recommended right away because the suspicion of malignancy is substantially higher.⁶ The waiting period for a BI-RADS 3 result can feel anxiety-inducing, but it exists because the data support it: immediate biopsy on every probably-benign finding would subject far more people to invasive procedures than would ever turn out to have cancer.

Prostate MRI and PI-RADS 3

Prostate MRI uses a parallel scoring system called PI-RADS, where a score of 3 means indeterminate, a score of 4 means likely significant cancer, and 5 means very likely. PI-RADS 3 lesions are common, showing up in roughly one out of every three to five men who undergo diagnostic prostate MRI, depending on whether they are having their first biopsy or have had a prior negative one.⁷

The chance that a PI-RADS 3 lesion harbors clinically significant prostate cancer ranges from roughly one in five to one in six men, depending on the patient group.⁷ That is lower than for PI-RADS 4 and 5 lesions, but it is not negligible. One study of 163 men with PI-RADS 3 lesions found that about 56 percent were entirely benign, about 32 percent had insignificant cancer that might never cause harm, and about 12 percent had clinically significant cancer.⁸ Another study concluded that PI-RADS 3 lesions carry a low risk of significant cancer overall, but that does not mean zero risk.⁹

Whether to biopsy a PI-RADS 3 lesion is an active debate. Skipping biopsy entirely in that French study would have left about 12 percent of clinically significant cancers undiagnosed.⁸ Factors like age, PSA density, lesion size, and prior biopsy history all feed into the decision. For many men, the answer ends up being a targeted biopsy of the suspicious area combined with continued monitoring.

Lung Nodules Found by Accident

Indeterminate pulmonary nodules are incredibly common. CT scans of the chest, often done for unrelated reasons, frequently turn up small lung nodules that cannot immediately be classified as harmless or dangerous. The Fleischner Society guidelines help radiologists sort these by size, number, and the patient’s smoking history to determine follow-up intervals.¹⁰

The vast majority of incidentally discovered lung nodules are benign. A study of Medicare patients found that only about 3.6 percent of people with indeterminate pulmonary nodules went on to be diagnosed with lung cancer during follow-up, compared to 0.8 percent in a control group without nodules. The follow-up process involved considerably more imaging and procedures than the control group experienced, with an excess of 63 chest CTs per 100 people with nodules over two years. But those extra scans and procedures did prevent some cancers from reaching advanced stages, estimating about 1.3 late-stage cases avoided per 100 people in the nodule group.¹¹

In other words, finding a lung nodule on a scan is almost always the beginning of a monitoring period, not the beginning of a cancer diagnosis. Small nodules are usually watched with repeat imaging at intervals, and many never change.

Cervical Screening and ASCUS

Cervical cytology has its own version of indeterminate: ASCUS, which stands for atypical squamous cells of undetermined significance. It is the most common abnormal Pap smear result, and the name itself signals the uncertainty. The cells look a bit off but the pathologist cannot say why. Most ASCUS results do not lead to a cancer diagnosis.

The key next step is usually HPV testing. If the HPV test is positive, colposcopy (a closer look at the cervix with magnification) is recommended. If HPV-negative, the risk is very low and routine screening can continue. In a study of menopausal women with ASCUS, about 7.8 percent overall had positive findings on colposcopy biopsy, including a range from mild precancerous changes to, in rare cases, actual carcinoma. HPV testing identified about 70 percent of those abnormal cases, making it a useful filter for deciding who needs further workup.¹² The finding that some HPV-negative women still had significant disease is a reminder that no triage system is perfect, but the overall risk for an ASCUS result remains low.

How Molecular Testing Is Changing the Equation

One of the biggest shifts in managing indeterminate results, especially in thyroid nodules, has been the arrival of molecular testing. Rather than going straight to surgery when a biopsy is unclear, doctors can now send the sample for genomic analysis to help sort benign from malignant.

A randomized trial comparing two molecular tests for indeterminate thyroid nodules found that both achieved high specificity and allowed roughly half of patients to avoid diagnostic surgery entirely.¹ That is a meaningful reduction in unnecessary operations. One widely used genomic sequencing classifier showed a negative predictive value of 99 percent for both Bethesda III and IV nodules in a recent study, meaning that when the test says benign, it is almost always right.¹³ The practical takeaway is that a genomic-classifier-benign thyroid nodule can be monitored over time rather than removed surgically.¹⁴

These tests are not free. One cost analysis found that molecular testing added about $104 per patient to the overall cost of thyroid nodule evaluation but decreased the number of diagnostic surgeries and reduced total costs when accounting for the expense of two-stage operations that would otherwise be needed.¹⁵ A separate analysis found that one type of multi-mutation testing decreased unnecessary surgeries by about two-thirds and actually saved money per patient compared to standard care.¹⁶ So while the upfront cost exists, the downstream savings from avoided surgeries generally offset it.

When the Biopsy Itself Can Miss

One concern that rarely gets discussed with patients is false-negative biopsies, where the sample comes back benign but cancer is actually present. This is different from an indeterminate result, where the answer is openly uncertain. A false negative feels definitive and reassuring when it should not be.

In a study of thyroid fine-needle aspirations, the false-negative rate (excluding tiny incidental cancers) was about four percent. Interestingly, only about one in five of those false negatives were caused by the needle simply missing the tumor. In the majority of cases, the needle had sampled the malignant nodule but the cells still looked benign under the microscope. More than half of the false-negative cases involved solitary nodules, and a third had underlying thyroiditis, which can make interpretation harder.¹⁷ The takeaway is that a benign biopsy result, while overwhelmingly likely to be correct, is not a lifetime guarantee, which is why continued follow-up imaging is standard even after a reassuring result.

The Role of Second Opinions

Pathology is more subjective than most people realize. When a tissue sample sits on the borderline between benign and malignant, two equally qualified pathologists can look at the same slide and reach different conclusions. A study of second-opinion consultations in prostate pathology found that the subspecialist reviewer disagreed with the original diagnosis more than half the time. Among significantly altered diagnoses, about five percent of cases were changed from benign to malignant, and about six percent were changed from malignant to benign.¹⁸

Those reversal rates are not small. If you have received an indeterminate result and are facing a decision about surgery or aggressive treatment, asking for a second pathology review is reasonable and increasingly routine at major cancer centers. Specialized immunohistochemical stains can help, too. In prostate pathology, stain cocktails that highlight specific proteins have been found to improve the distinction between true cancer and benign conditions that mimic it, especially in small tissue samples.¹⁹

Watching Versus Acting

The hardest part of an indeterminate result is often the wait. When you are told your cells or imaging look ambiguous, the instinct is to want something done immediately: take it out, scan it again, test everything. But in many cases, the evidence supports careful monitoring over rushing to treatment.

For thyroid nodules specifically, no clear consensus exists on whether every indeterminate nodule should be surgically removed or simply watched. Most indeterminate thyroid nodules turn out to be benign after surgery, which means that many people undergo operations they did not actually need.²⁰ The goal of all the molecular testing and refined risk-stratification discussed earlier is to avoid exactly this scenario: operating on someone whose nodule was never going to cause them harm.

Active surveillance, where the nodule or lesion is monitored with regular imaging and possibly repeat biopsies, is becoming the preferred approach for many indeterminate findings across different cancer types. For PI-RADS 3 prostate lesions, as noted earlier, factors like age and PSA density can help decide who needs a biopsy now and who can safely wait. For BI-RADS 3 breast findings, the established protocol already is surveillance. For lung nodules, follow-up intervals are tailored to size and risk factors. The common thread is that indeterminate findings require ongoing attention, not necessarily immediate intervention.

Artificial Intelligence and Where Things Are Headed

One of the more interesting developments is the application of AI to indeterminate imaging findings. Researchers have begun benchmarking AI systems against radiologists for estimating the malignancy risk of indeterminate-size lung nodules found on low-dose CT screening.²¹ The potential value is clear: if an algorithm can reliably sort the “almost certainly benign” nodules from the “worth biopsying” ones, it could spare thousands of people from the anxiety and expense of repeated scans or unnecessary procedures.

AI-assisted pathology is moving in a similar direction. Digital analysis of cell morphology and tissue architecture is being explored as a way to help pathologists resolve ambiguous slides. None of these tools have replaced human judgment yet, and the regulatory landscape for clinical deployment is still evolving. But for people sitting with an indeterminate result right now, the relevant takeaway is that the gray zone these results occupy is one of the most active areas of medical research. The tools available to refine your risk are substantially better than they were even a decade ago, and they continue to improve.

1
PubMed Central. Effectiveness of Molecular Testing Techniques for Diagnosis of Indeterminate Thyroid Nodules: A Randomized Clinical Trial
2
PubMed Central. Malignancy rates in thyroid nodules classified as Bethesda categories III and IV; a subcontinent perspective
3
PubMed. Bethesda Category III, IV, and V Thyroid Nodules: Can Nodule Size Help Predict Malignancy?
4
PubMed. Indeterminate Bethesda System Category (Bethesda Category III) Thyroid Nodules: Cytomorphologic Subclassification and Its Impact on Malignancy Risk
5
PubMed Central. BI-RADS 3: Current and Future Use of Probably Benign
6
PubMed Central. Magnetic resonance imaging in diagnosis of indeterminate breast (BIRADS 3 & 4A) in a general population
7
PubMed Central. MRI in early prostate cancer detection: how to manage indeterminate or equivocal PI-RADS 3 lesions?
8
The French Journal of Urology. PI-RADS 3 MRI lesions: Are biopsies still necessary?
9
PubMed Central. mp-MRI Prostate Characterised PIRADS 3 Lesions are Associated with a Low Risk of Clinically Significant Prostate Cancer
10
PubMed. Fleischner Society Guideline Recommendations for Incidentally Detected Pulmonary Nodules and the Probability of Lung Cancer
11
PubMed Central / Wiley Online Library. Diagnostic follow-up of indeterminate pulmonary nodules in the Medicare population
12
PubMed Central. HPV Reflex Testing in Menopausal Women
13
PubMed Central. Utility of genomic sequencing classifier in managing cytologically indeterminate oncocytic thyroid nodules
14
PubMed. Outcomes of Cytologically Indeterminate Thyroid Nodules Managed With Genomic Sequencing Classifier
15
PubMed Central. Cost Impact of Molecular Testing for Indeterminate Thyroid Nodule Fine-Needle Aspiration Biopsies
16
PubMed. Utility and cost-effectiveness of molecular testing in thyroid nodules with indeterminate cytology
17
PubMed Central. False-negative fine-needle aspiration of thyroid nodules cannot be attributed to sampling error alone
18
PubMed. Diagnostic issues in second opinion consultations in prostate pathology
19
PubMed. Immunohistochemistry in diagnostic surgical pathology of the prostate
20
PubMed Central. Current controversies in the management of patients with indeterminate thyroid nodules
21
PubMed. Benchmarking of AI and Radiologists for Indeterminate Lung Nodule Malignancy Risk Estimation at Screening CT: The LUNA25 Challenge