ICD-11’s Approach to Classifying Neoplasms

When a doctor writes “neoplasm” on a medical report, that single word can mean almost anything – from a harmless skin tag to an aggressive brain tumour. To make sense of this huge spectrum, the World Health Organization developed a global coding system called the International Classification of Diseases (ICD). Its newest version, ICD-11, has completely reorganised how the world records cancers and other tumours. For students of population and family health, this matters because every cancer statistic, every treatment guideline, and every research paper eventually traces back to how these conditions are classified.

What exactly is a neoplasm? 🔗

A neoplasm is simply an abnormal growth of tissue that forms when cells divide more than they should, or fail to die when they normally would. The word literally means “new growth.” Not every neoplasm is cancer, and this distinction is the foundation of the entire ICD-11 chapter on the topic.

Neoplasms are sorted by their behaviour – that is, how the abnormal cells act inside the body. A pathologist looks at tissue under a microscope and assigns one of several categories based on whether the cells stay put, invade nearby tissue, or spread to distant organs.

Malignant neoplasms 🔗

These are what most people call cancer. Malignant cells grow uncontrollably, invade surrounding tissue, and can travel through blood or lymph to form secondary tumours elsewhere in the body – a process called metastasis. Lung cancer, breast cancer, and oral cancer all fall in this group. In the ICD-11 coding system, malignant behaviour is marked with a behaviour code of “3” for primary site malignancy and “6” for metastatic site malignancy.

Benign neoplasms 🔗

Benign tumours grow slowly, stay in one place, and do not invade surrounding tissue. A common example is a uterine fibroid or a fibroadenoma of the breast. They are not cancer-free in the sense of being risk-free – a benign brain tumour can still be dangerous because of where it sits – but they do not spread. In the ICD-11 morphology framework, benign behaviour carries the code “0.”

In situ neoplasms 🔗

“In situ” is Latin for “in place.” These are abnormal cells that look like cancer under the microscope but have not yet broken through the basement membrane of the tissue they started in. Carcinoma in situ of the cervix is the classic example, often picked up through Pap smears. If left untreated, many in situ lesions progress to invasive cancer, which is why screening programmes target them so aggressively.

Uncertain and unknown behaviour 🔗

Sometimes a pathologist cannot decide whether a tumour is benign or malignant. These borderline cases include conditions like borderline ovarian mucinous tumours or follicular thyroid tumours of uncertain malignant potential. ICD-11 separates “uncertain behaviour” (where the pathologist genuinely cannot tell) from “unknown behaviour” (where information is simply missing). This split was new in ICD-11 and lets researchers track the two situations separately.

How ICD-11 reorganised the neoplasm chapter 🔗

ICD-10, the previous version, sorted neoplasms into just four broad groups. ICD-11 expanded this into eight subchapters, giving each major category of tumour its own dedicated space. Chapter 2 of ICD-11 covers all neoplasms, and every code in it begins with the number “2.”

Five of these subchapters still represent the behaviour-based groups – malignant, in situ, benign, uncertain, and unknown. The remaining three are entirely new groupings that pull specific tumour types out of the old behaviour-only sorting system because they need their own treatment pathways. According to a 2025 European study on ICD-11 implementation, these three new groups cover central nervous system neoplasms, haematopoietic and lymphatic tissue neoplasms, and inherited cancer-predisposing syndromes.

Key subcategories worth knowing 🔗

Brain and central nervous system neoplasms 🔗

In ICD-10, brain tumours were scattered across the malignant and benign sections depending on their behaviour. ICD-11 pulls them all into one subchapter, regardless of whether they are benign or malignant. The reason is practical – a “benign” meningioma pressing on the brainstem can be just as life-threatening as a malignant tumour, and grouping these conditions together helps clinicians and registries study brain tumours as a single problem. This block includes gliomas, meningiomas, medulloblastomas, and tumours of the spinal cord and cranial nerves.

Haematopoietic and lymphoid neoplasms 🔗

These are cancers of the blood and immune system – the leukaemias, lymphomas, and multiple myeloma. Like the CNS group, they now sit in a dedicated subchapter because they behave so differently from solid tumours. They do not form a single lump you can biopsy; they spread through the bloodstream and bone marrow from the start. Pulling them into their own block reflects how oncologists actually think about and treat these diseases.

Inherited cancer predisposing syndromes 🔗

This is perhaps the most forward-looking addition to ICD-11. Hereditary cancer syndromes are responsible for an estimated 5 to 10 per cent of all cancer cases and include conditions like Lynch syndrome, Li-Fraumeni syndrome, hereditary breast and ovarian cancer syndrome caused by BRCA1 and BRCA2 mutations, and familial adenomatous polyposis. In ICD-10, these were buried in various places. ICD-11 gives them their own subchapter so that patients carrying inherited risk can be tracked, even before they develop a tumour.

This matters enormously for prevention. A woman with a BRCA1 mutation has a sharply higher lifetime risk of breast and ovarian cancer, and recording her status in a standardised way means health systems can offer earlier screening and counselling.

The coding revolution: stem codes and extension codes 🔗

The way ICD-11 actually records information about a tumour is fundamentally different from before. ICD-11 introduces new concepts called stem codes, extension codes, precoordination, and postcoordination. A stem code is a six-character code that captures the essential identity of the disease – for example, the site and the histological type combined into a single code. Extension codes then attach additional information like the stage of the cancer, the grade of the tumour, laterality (left or right side), and the method of diagnosis.

This is a major improvement over ICD-10, which split topography (where the tumour is) and morphology (what the tumour cells look like) into separate codes that had to be used together. ICD-11 effectively folds the morphology section of the older International Classification of Diseases for Oncology directly into Chapter 2. The result is a single, more powerful coding system that can describe a cancer in much greater detail with fewer separate codes.

Why detailed classification matters for treatment and research 🔗

Better treatment planning 🔗

Modern cancer treatment is intensely specific. A breast cancer that is HER2-positive needs different drugs from one that is hormone receptor-positive. A lung adenocarcinoma with an EGFR mutation responds to targeted therapy that does not work on other lung cancers. ICD-11 captures these biological details directly in the code, which means when a clinician looks up a patient’s diagnosis, they immediately see the information that drives treatment choice.

Stronger cancer registries 🔗

India runs the National Cancer Registry Programme through the Indian Council of Medical Research, with population-based registries and hospital-based registries across the country. The most recent national estimates project that the number of cancer cases will keep rising sharply, driven by an ageing population and changing lifestyles. Detailed coding lets these registries answer increasingly specific questions – not just how many people got cancer last year, but which subtypes, at what stages, with what genetic backgrounds.

Comparable global data 🔗

Because ICD-11 was officially adopted by WHO member states for use starting January 2022, researchers can now compare cancer statistics across countries with much greater confidence. A rare childhood brain tumour coded in Mumbai can be matched with cases recorded in São Paulo and Stockholm, building the larger datasets that rare cancer research desperately needs.

Genomic medicine integration 🔗

The inherited cancer predisposition subchapter, in particular, is built for an era where genetic testing is becoming routine. Families with Lynch syndrome or BRCA mutations can be flagged in health records, surveillance programmes can be triggered automatically, and population-level data on hereditary cancer risk can be tracked over time.

The road ahead 🔗

Adopting ICD-11 is a major project for any country. Coders need retraining, hospital software has to be upgraded, and existing records have to be mapped from the old system to the new one. WHO has been supporting countries through this transition, and several have already moved their mortality and morbidity reporting to ICD-11. For students of population health, the takeaway is that classification is not just paperwork. It is the scaffolding on which every cancer statistic, every screening guideline, and every research grant ultimately rests. A clearer scaffold means a clearer view of the disease.

What do you think? If a more detailed coding system can capture inherited cancer risk before any tumour appears, should screening and counselling for high-risk families become a routine part of primary healthcare? And how should the cost of implementing ICD-11 be balanced against its long-term benefits for cancer research in resource-limited settings?

References

1. https://www.who.int/news-room/fact-sheets/detail/cancer
2. https://pmc.ncbi.nlm.nih.gov/articles/PMC9758819/
3. https://icd10monitor.medlearn.com/coding-clinic-raises-questions-about-uncertain-behaviour/
4. https://pmc.ncbi.nlm.nih.gov/articles/PMC10960217/
5. https://www.sciencedirect.com/science/article/abs/pii/S1386505625000383
6. https://pmc.ncbi.nlm.nih.gov/articles/PMC9993141/
7. https://www.ncdirindia.org/
8. https://ijmr.org.in/cancer-incidence-estimates-for-2022-projection-for-2025-result-from-national-cancer-registry-programme-india/
9. https://www.who.int/standards/classifications/classification-of-diseases