Minimally Invasive Adenocarcinoma of the Lung: Understanding Your Pathology Report

by Jason Wasserman MD PhD FRCPC
June 20, 2025

Minimally invasive adenocarcinoma (MIA) is an early form of lung cancer that sits on the border between non-invasive and invasive disease. It is called “minimally invasive” because although a small number of cancer cells have begun to grow into the supporting tissue of the lung, the area of invasion is very limited — no more than 5 millimeters. The tumor must be 3 centimeters or smaller overall. Beyond that tiny, invasive focus, the cancer cells grow along the inner surfaces of the lung’s air sacs (alveoli) in a pattern called lepidic growth, without destroying the underlying lung structure. When completely removed by surgery, minimally invasive adenocarcinoma is associated with an excellent outcome. This article will help you understand the findings in your pathology report — what each term means and why it matters for your care.

What causes minimally invasive adenocarcinoma?

The primary cause of minimally invasive adenocarcinoma is tobacco smoking. Smoking exposes the lungs to harmful chemicals that damage the DNA inside lung cells, which can eventually cause cancer to develop. However, MIA also occurs in people who have never smoked — in fact, like other early lung adenocarcinomas, it is proportionally more common in never-smokers than in more advanced lung cancers.

Other risk factors include:

• Radon gas exposure — Radon is a naturally occurring radioactive gas that can accumulate in homes and buildings. Long-term exposure is a recognized cause of lung cancer.
• Occupational exposures — Prolonged contact with substances such as asbestos, silica dust, or diesel exhaust in certain workplaces increases the risk of lung cancer development.
• Air pollution — Long-term exposure to outdoor air pollution is associated with a modestly increased risk of lung cancer.
• Genetic factors — Some people carry inherited changes in genes that increase the risk of developing lung adenocarcinoma, though this is uncommon.

What are the symptoms of minimally invasive adenocarcinoma?

Most people with minimally invasive adenocarcinoma have no symptoms at the time of diagnosis. Because the tumor is small and causes minimal disruption to the surrounding lung tissue, it does not typically obstruct airways or cause noticeable changes in breathing. Most cases are found incidentally — discovered during imaging performed for another reason, such as a chest CT for an unrelated condition, or identified through lung cancer screening.

Rarely, symptoms such as a persistent cough, mild shortness of breath, or chest discomfort may occur, particularly if the tumor is located near an airway.

What conditions are associated with minimally invasive adenocarcinoma?

Minimally invasive adenocarcinoma falls within a well-recognized spectrum of early lung adenocarcinoma that progresses through distinct stages. Understanding this spectrum helps explain how MIA develops and why early detection matters.

• Atypical adenomatous hyperplasia (AAH) — A microscopic cluster of mildly abnormal cells lining the alveoli. AAH is considered a very early precancerous change and is not yet a cancer.
• Adenocarcinoma in situ (AIS) — A non-invasive cancer in which abnormal cells are confined strictly to the alveolar lining with no invasion into surrounding lung tissue. AIS is 3 centimeters or smaller. Learn more about adenocarcinoma in situ of the lung.
• Minimally invasive adenocarcinoma (MIA) — The next step in the spectrum. The tumor is predominantly lepidic but now contains a small focus of stromal invasion — a tiny area (5 millimeters or less) where cancer cells have broken through the alveolar lining and begun growing into the supporting tissue of the lung.
• Invasive adenocarcinoma — When the invasive focus exceeds 5 millimeters or the overall tumor exceeds 3 centimeters, the cancer is classified as fully invasive. Learn more about invasive adenocarcinoma of the lung.

The progression along this spectrum is not inevitable or rapid — many early lesions remain stable for years — but recognizing and treating MIA before it progresses to invasive adenocarcinoma is the goal of early detection and lung cancer screening.

How is the diagnosis made?

The diagnosis of minimally invasive adenocarcinoma can only be made after the entire tumor has been surgically removed and examined in full. A small biopsy taken before surgery — or a cytology sample of individual cells — is not sufficient to establish this diagnosis, because confirming MIA requires the pathologist to measure the full extent of any invasion across the entire tumor. A pre-surgical biopsy may show features suspicious for early adenocarcinoma, but the definitive distinction between AIS, MIA, and invasive adenocarcinoma is made only on the complete surgical specimen.

On imaging, MIA typically appears as a ground-glass nodule — a hazy, partially cloudy area on a CT scan — often with a small solid component at its center. The ground-glass component corresponds to the lepidic growth pattern, while the solid component corresponds to the area of invasion. A nodule with a small but measurable solid core on CT raises the possibility of MIA and is one of the main reasons early lung nodules are followed closely or referred for resection.

Under the microscope, the pathologist identifies the characteristic lepidic growth pattern — cancer cells spreading along the surface of the alveoli without destroying their underlying framework — and carefully searches for areas where the cancer cells have invaded the surrounding lung tissue (called stromal invasion). The pathologist measures the largest dimension of any invasive focus using a calibrated eyepiece. For the diagnosis of MIA to apply, the total size of the tumor must be 3 centimeters or smaller, and the invasive component must measure 5 millimeters or less. If multiple separate invasive foci are present, their measurements are added together. Immunohistochemistry stains — laboratory tests that use antibodies to detect specific proteins in the cells — may be used to confirm that the cells are of lung origin (typically positive for TTF-1) and to help distinguish MIA from other conditions that can look similar under the microscope.

Once the diagnosis is confirmed, your doctor will arrange follow-up imaging to monitor the remaining lung tissue for new lesions.

Histologic grade

Minimally invasive adenocarcinoma is classified as Grade 1 (well differentiated) per the 2021 WHO Classification of Thoracic Tumors. This reflects its predominantly lepidic growth pattern — the least aggressive pattern seen in lung adenocarcinoma — combined with only a minimal focus of invasion. Grade 1 tumors grow slowly and are associated with the most favorable outcomes among lung adenocarcinomas. Because MIA is defined in part by having limited invasion and a predominantly lepidic pattern, all tumors that qualify as MIA by definition fall into this lowest-grade category.

Lymphovascular invasion

Lymphovascular invasion (LVI) means that cancer cells have been found within blood or lymphatic vessels — the small channels that carry lymph — in or near the tumor. These vessels can act as pathways for cancer cells to travel to distant parts of the body.

• Lymphovascular invasion not identified — No cancer cells are seen within blood or lymphatic vessels near the tumor. This is the expected and favorable finding in MIA.
• Lymphovascular invasion present — Cancer cells are found within vessels. This finding is extremely unusual in MIA — its presence should prompt the pathologist to reconsider whether the tumor truly qualifies as minimally invasive or warrants additional sampling. If confirmed, it is associated with a higher risk of spread.

Surgical margins

Surgical margins are the cut edges of the tissue removed during the operation. The pathologist examines all margins under the microscope to determine whether the tumor was completely removed.

• Negative margin — No cancer cells are seen at the cut edge of the tissue. This is the most favorable result and indicates the tumor was fully excised.
• Close margin — Cancer cells are present very near the cut edge but do not reach it. Depending on the distance and the specific margin involved, further treatment may or may not be recommended.
• Positive margin — Cancer cells are present at the cut edge. This raises concern that some tumor remains and typically leads to discussion of further surgery or radiation therapy.

Lymph nodes

Lymph nodes are small structures that are part of the immune system and are distributed throughout the chest. During surgery for lung cancer, the surgeon typically removes lymph nodes from specific locations within the lung and central chest (called lymph node stations) and sends them separately to the pathologist.

The pathology report will describe the total number of lymph nodes examined, their station locations, and whether any contain cancer. In minimally invasive adenocarcinoma, lymph node involvement is extremely rare — by definition, if cancer is found in lymph nodes, the tumor cannot be classified as MIA and must be staged as a more advanced cancer. Nevertheless, lymph node sampling is routinely performed during surgery to confirm the absence of spread and to ensure accurate staging.

Pathologic stage (pTNM)

Minimally invasive adenocarcinoma is staged using the TNM system based on AJCC 8th edition criteria. The T category describes the tumor itself, the N category describes whether lymph nodes are involved, and the M category describes whether the cancer has spread to distant organs. The M stage is determined by imaging rather than the pathology specimen and is typically not reported in the surgical pathology report.

Tumor stage (pT)

• pT1mi — The tumor is 3 centimeters or smaller and has a lepidic-predominant growth pattern with an invasive component of 5 millimeters or less. All minimally invasive adenocarcinomas are classified as pT1mi by definition.

Nodal stage (pN)

• pNX — Lymph nodes were not examined.
• pN0 — No cancer cells found in any lymph nodes examined. This is the expected finding in MIA.
• pN1 — Cancer cells found in lymph nodes within the lung or near the main airway on the same side of the chest (stations 10–14).
• pN2 — Cancer cells found in lymph nodes in the central chest on the same side (stations 4–9).
• pN3 — Cancer cells found in lymph nodes on the opposite side of the chest or in the lower neck. This indicates advanced nodal disease.

A finding of pN1, pN2, or pN3 in a tumor otherwise appearing to be MIA would prompt review of the diagnosis, as nodal involvement is inconsistent with the MIA classification.

What is the prognosis?

The prognosis for minimally invasive adenocarcinoma is excellent. When the tumor is completely removed with negative surgical margins, the disease-specific five-year survival rate is essentially 100% — meaning that virtually no patients die from MIA itself after successful resection. This outcome is comparable to adenocarcinoma in situ and reflects the very limited invasive potential of MIA.

Factors that are important for prognosis include:

• Complete surgical removal — Achieving negative margins at all cut edges is the most important factor. A positive or very close margin may lead to a recommendation for additional surgery or radiation.
• Confirmation of the MIA classification — The excellent prognosis applies specifically when the invasive component measures 5 millimeters or less and the tumor is 3 centimeters or smaller. Tumors that exceed these thresholds are reclassified as invasive adenocarcinoma and carry a higher risk of recurrence.
• Absence of lymphovascular invasion and nodal disease — The presence of either would be unusual in true MIA and would alter the prognosis and staging.
• Risk of additional lung tumors — Patients who develop MIA are at a modestly increased risk of developing additional early lung adenocarcinomas over time, which is why ongoing surveillance imaging after treatment is important.

What happens after the diagnosis?

Surgery to completely remove the tumor is the primary and usually the only treatment needed for minimally invasive adenocarcinoma. Chemotherapy and radiation therapy are not indicated for MIA when it has been completely resected.

The type of surgery depends on the tumor’s size, location, and the patient’s overall lung function. For small, peripheral tumors, a wedge resection — removal of a wedge-shaped segment of lung containing the tumor — or a segmentectomy (removal of a defined anatomical segment of lung) is usually sufficient. For tumors in less favorable locations, a lobectomy (removal of an entire lobe of the lung) may be preferred to achieve adequate margins. Minimally invasive surgical techniques, including video-assisted thoracoscopic surgery (VATS), are commonly used and are associated with shorter recovery times compared to open surgery. The choice of surgical approach is made by a thoracic surgeon in consultation with your broader care team.

After surgery, regular chest CT imaging is typically recommended — usually annually — to monitor the remaining lung for new ground-glass nodules or other abnormalities. Because patients with MIA are at some risk of developing additional early lung tumors, long-term surveillance is considered important even after successful treatment. The duration and frequency of follow-up will be guided by your care team based on your individual circumstances.

Molecular biomarker testing — such as testing for EGFR mutations or ALK rearrangements — is not routinely performed for minimally invasive adenocarcinoma, because no adjuvant targeted therapy or immunotherapy is indicated at this stage. An exception is when a patient has multiple lung tumors at the same time: in that situation, comparing the molecular profiles of each tumor can help the pathologist and oncologist determine whether the tumors arose independently or represent spread from a single cancer, which affects staging and treatment decisions.

Smoking cessation, where applicable, is strongly encouraged. Quitting smoking reduces the risk of developing additional lung tumors and improves overall cardiovascular and respiratory health.

Questions to ask your doctor

• Was the tumor completely removed, and what were the surgical margins?
• Was the invasive component measured, and how large was it?
• Is the diagnosis confirmed as minimally invasive adenocarcinoma, or could it be reclassified as invasive adenocarcinoma?
• Were any lymph nodes examined, and did any contain cancer?
• Was lymphovascular invasion identified in my pathology report?
• What is my pathologic stage, and what does pT1mi mean?
• Do I need any additional treatment after surgery?
• What type of follow-up imaging is recommended, and for how long?
• Am I at risk of developing additional lung tumors, and what symptoms should prompt me to contact you?
• Should I be enrolled in a formal lung cancer surveillance program?
• Does smoking cessation affect my risk of a new lung tumor, and can you connect me with resources to help?