Subpectoral lymph nodes sit in a gray zone that depends heavily on which classification system you consult and exactly where beneath the pectoral muscles a given node lives. Under the widely used Berg system, axillary lymph nodes are divided into three levels based on their relationship to the pectoralis minor muscle, and level II nodes lie directly behind that muscle, which places them in a subpectoral position by definition. Yet imaging reports and some surgical literature treat “subpectoral” nodes as a separate category from classic axillary nodes, and interpectoral nodes lodged between the two pectoral muscles are sometimes grouped with axillary drainage and sometimes not. The practical answer matters most in breast cancer management, where whether a node counts as “axillary” can change staging, treatment planning, and surgical approach.
The Berg Classification and Why It Creates Confusion
The Berg system divides the axillary basin into three levels using the pectoralis minor muscle as a landmark. Level I nodes sit lateral and inferior to the muscle’s outer edge, level II nodes lie directly behind (deep to) the pectoralis minor, and level III nodes are medial to the muscle’s inner border, extending toward the collarbone. This muscle-based categorization has been used extensively by pathologists and surgeons to describe the extent of axillary node dissection in breast cancer patients.1American Journal of Clinical Oncology. Is the Berg Axillary Lymph Node Categorization Useful in the 3D Environment? Because level II nodes are defined by being behind the pectoralis minor, they are by definition subpectoral, and they are also by definition axillary. So a node that is both subpectoral and axillary is not an oddity; it is simply a level II axillary node.
The confusion arises because “subpectoral” is an anatomical description of position (beneath a pectoral muscle), not a formal nodal station in any staging system. A radiologist reading a PET/CT scan may label a node “subpectoral” based on what they see on the image, leaving the clinician to decide whether that node fits within the axillary levels or represents something outside the standard axillary basin. In practice, some subpectoral nodes are clearly level II axillary nodes, some are interpectoral (Rotter’s) nodes sitting between the pectoralis major and minor, and a few may be infraclavicular nodes near the top of the axilla. These are related but distinct locations, and their clinical significance differs.
Interpectoral Nodes and Their Relationship to the Axilla
Interpectoral lymph nodes, often called Rotter’s nodes, lie in the fat pad between the pectoralis major and pectoralis minor muscles. They are technically subpectoral in the sense that they are beneath the pectoralis major, but they are not behind the pectoralis minor the way level II axillary nodes are. Anatomic studies show that these interpectoral nodes receive lymphatic drainage from the deep breast tissue. Retromammarian lymphatics arise from the breast lobules, run along the surface of the pectoral fascia, and follow penetrating blood vessels through the pectoral and intercostal muscles to reach these nodes.2PubMed Central. Lymphatic Drainage Patterns From the Breast
Whether interpectoral nodes count as axillary has never been fully settled. Some anatomists and surgeons treat them as part of the axillary group because they drain into axillary chains. Others consider them a separate extra-axillary station because they are physically separated from the axillary fat pad by muscle. In surgical practice, a standard axillary dissection that clears levels I and II may miss Rotter’s nodes entirely unless the surgeon specifically opens the space between the two pectoral muscles. One surgical approach described in the literature involves first dissecting below the lateral edge of the joined pectoralis muscles (the subpectoral approach) and then sequentially opening the space between the muscles (an interpectoral approach) to ensure more complete clearance.3PubMed. Radicality effect of adding an interpectoral to a subpectoral approach for dissection of level III axillary lymph nodes in breast cancer The fact that these are treated as separate surgical steps underscores that interpectoral and axillary nodes are anatomically adjacent but not always reached by the same dissection.
How Imaging Reports Label These Nodes
One of the biggest sources of confusion for patients and referring clinicians is the way imaging reports handle subpectoral nodes. Radiologists reading PET/CT scans sometimes report “subpectoral” or “infraclavicular” nodes as a distinct finding from axillary nodes, even when the node may technically fall within the Berg level II zone. In one study of PET/CT imaging in breast cancer, researchers identified extra-axillary lymph node involvement that conventional techniques had missed; two of those cases involved hypermetabolic lymph nodes specifically described in the “subpectoral and infraclavicular regions.”4International Journal of Radiation Oncology*Biology*Physics. Effect of 18F-FDG PET/CT Imaging in Patients With Clinical Stage II and III Breast Cancer In that context, subpectoral nodes were flagged as extra-axillary findings worthy of note precisely because they had not been captured by standard staging.
A separate PET/CT study looking at inflammatory breast cancer found hypermetabolic uptake in the ipsilateral axillary nodes in about 90% of patients and in the ipsilateral subpectoral nodes in about 44%.5Journal of Nuclear Medicine. Retrospective Study of 18F-FDG PET/CT in the Diagnosis of Inflammatory Breast Cancer: Preliminary Data That the two groups were reported as separate categories tells you something about how radiology practice handles the distinction: axillary and subpectoral are tracked as different lines on the report, even when the underlying anatomy may overlap. For the clinician, this creates a task of translation, mapping the imaging label back to a surgically and oncologically meaningful nodal level.
Why the Classification Matters in Breast Cancer Staging
Whether a node is classified as axillary or extra-axillary directly affects a patient’s cancer stage, which in turn drives treatment decisions. In the TNM staging system used worldwide, axillary lymph node involvement is categorized by level (and number of nodes involved), while involvement of certain extra-axillary basins, such as internal mammary or supraclavicular nodes, changes the staging in different ways. Nodes that fall outside the recognized axillary levels can shift a patient into a more advanced stage or trigger the addition of regional radiation therapy.
Research on sentinel node biopsy has shown that nodes outside the standard axillary levels I and II are not uncommon. In one large series, roughly 27% of patients had a sentinel node outside the standard axillary basin, including internal mammary nodes in 86 patients, other non-axillary sentinel nodes in 44, and both in 19 patients. Staging improved in about 13% of those patients with non-axillary sentinel nodes, and the treatment plan was changed in 17%.6PubMed Central. Impact of non-axillary sentinel node biopsy on staging and treatment of breast cancer patients The “other non-axillary” category in that study would include interpectoral and subpectoral nodes that did not map neatly onto the axillary levels. For those patients, finding and biopsying these nodes meant a more accurate picture of how far the cancer had spread.
Skip Metastasis and the Interpectoral Node
One clinical scenario that highlights the ambiguity of subpectoral nodes is axillary skip metastasis. This is defined as cancer spreading to level II or III lymph nodes without involving any level I nodes.7PubMed Central. A Case of Invasive Ductal Carcinoma with Axillary Skip Metastasis Confined to the Interpectoral (Rotter’s) Lymph Node It is considered rare, and the published incidence among all invasive breast cancers sits around 2.6%, rising to about 7% among cancers with nodal metastases detected by ultrasound-guided biopsy.8PubMed. Breast Cancer Skip Metastases: Frequency, Associated Tumor Characteristics, and Role of Staging Nodal Ultrasound in Detection
Skip metastases can land in axillary level III, the supraclavicular basin, the contralateral axilla, or distant organs. Rotter’s interpectoral nodes are another recognized landing site. When cancer skips to an interpectoral node without touching level I, it reinforces the idea that the drainage pathways between the breast and pectoral muscles are anatomically real and clinically important. Because standard sentinel lymph node biopsy typically targets level I, a skip to the interpectoral space can be missed unless the surgeon or imaging protocol specifically looks for it. The deep lymphatics from the dorsal part of the breast that drain toward interpectoral and internal mammary nodes offer a plausible anatomical explanation for why cancer sometimes bypasses the lower axillary nodes entirely.2PubMed Central. Lymphatic Drainage Patterns From the Breast
Radiation Planning and Subpectoral Node Coverage
Radiation oncologists face a very concrete version of the subpectoral question: when drawing treatment fields on a CT scan, should the area behind the pectoralis minor be included, and how far should coverage extend? Contouring atlases developed by groups such as ESTRO and RTOG provide guidelines for outlining the axillary levels on a planning scan. A study mapping sentinel lymph node locations using SPECT imaging found that subpectoral sentinel nodes localized on axillary level II were usually situated very close to the thoracic wall, underscoring the need to carefully cover the anterior surface of the upper ribs during radiation planning.9PubMed Central. Atlas of sentinel lymph nodes in early breast cancer using single-photon emission computed tomography: implication for lymphatic contouring If a radiation field is drawn too conservatively and misses the region hugging the chest wall behind the pectoralis minor, a subpectoral node that harbors residual disease could be left untreated.
Contouring the axilla is further complicated by patient positioning. Standard radiation planning is often done with the patient supine, but some centers treat breast cancer patients in the prone position to reduce dose to the heart and lungs. A contouring atlas developed for the prone position found that modifications to the standard RTOG guidelines were needed, with changes to the borders of both level I and level II to account for how gravity shifts the pectoral muscles and surrounding landmarks when a patient lies face-down.10Practical Radiation Oncology. A Radiation Therapy Contouring Atlas for Delineation of the Level I and II Axillae in the Prone Position: A Single-Institution Experience These adjustments matter because a subpectoral node that is comfortably inside a supine radiation field might drift outside it when the patient is repositioned.
Subpectoral Implant Placement and Lymphatic Disruption
The term “subpectoral” crops up in another breast cancer context that has nothing to do with node classification but everything to do with lymphatic health: breast reconstruction. When a breast implant is placed beneath the pectoralis major muscle (the subpectoral approach), the surgery necessarily involves dissection in the same tissue planes where subpectoral and interpectoral lymph nodes reside. A comparative analysis of breast cancer-related lymphedema after different reconstruction methods found that the overall rate of lymphedema was similar across autologous tissue reconstruction, prepectoral implant placement, and subpectoral implant placement, at roughly 9%, 15%, and 16% respectively. However, over two years of follow-up, the subpectoral group showed faster progression of lymphedema and greater limb volume increase, and the least improvement in tissue compliance, compared with both the prepectoral and autologous groups.11Journal of Plastic Surgery and Hand Surgery. Comparative analysis of breast cancer-related lymphedema after autologous and implant-based reconstruction: emphasizing differences between prepectoral and subpectoral placement of implant
The reason likely involves the additional disruption of lymphatic channels in the subpectoral space during the creation of the implant pocket. When the surgeon lifts the pectoralis major off the chest wall, small lymphatic vessels running through or beneath the muscle can be severed. Over time, this may impair drainage from the arm and contribute to swelling. Interestingly, a separate study comparing implant-based and autologous reconstruction found that edema, axillary cording, and limb girth measurements were similar between the two groups.12PubMed. Postoperative Upper Extremity Function in Implant and Autologous Breast Reconstruction The discrepancy between studies probably reflects differences in how lymphedema was measured, how long patients were followed, and whether the axillary dissection itself was the dominant driver of swelling. The point for patients weighing reconstruction options is that the subpectoral space is not just a passive pocket for an implant; it is a lymphatic neighborhood, and surgery there can have downstream consequences for drainage.
When “Subpectoral” Means Different Things to Different Specialists
Part of the reason this question persists is that surgeons, radiologists, radiation oncologists, and pathologists each use slightly different mental maps of the same anatomy. A breast surgeon performing an axillary dissection thinks in terms of Berg levels and uses the pectoralis minor as a physical landmark during the operation. A radiologist reading a CT or PET scan sees nodes in cross-section and labels them by their spatial relationship to the muscle on the image, which may or may not align perfectly with what the surgeon would call level II. A radiation oncologist draws contours on a planning scan using atlas-defined boundaries that approximate but do not perfectly replicate the surgical levels. And a pathologist receiving a specimen labeled “subpectoral lymph node” has to decide whether to categorize it as level II axillary or as something else in the final report.
These differences are not just academic. If a PET scan shows an active subpectoral node and the referring oncologist interprets that as axillary level II disease, the patient may be staged and treated one way. If the same node is interpreted as extra-axillary, staging and treatment could change. The 2007 PET/CT study that flagged subpectoral nodes as extra-axillary findings led to changes in radiation fields for some patients.4International Journal of Radiation Oncology*Biology*Physics. Effect of 18F-FDG PET/CT Imaging in Patients With Clinical Stage II and III Breast Cancer In multidisciplinary tumor boards, where surgeons, radiologists, and oncologists review cases together, these naming discrepancies are usually hashed out case by case. But outside that setting, the terminology can lead to genuine miscommunication.
For anyone reading their own imaging report and wondering whether a “subpectoral lymph node” is inside or outside the axilla, the honest answer is that it depends on the exact location of the node relative to the pectoralis minor muscle. Nodes directly behind the pectoralis minor are almost universally considered axillary level II. Nodes between the two pectoral muscles, the interpectoral or Rotter’s nodes, are anatomically adjacent to the axilla and share its drainage, but whether they are counted as axillary varies by institution and by the clinical question being asked. Nodes well medial to the pectoralis minor or up near the clavicle start to shade into infraclavicular or supraclavicular territory. The label “subpectoral” by itself does not resolve the question; it just tells you the node is beneath a pectoral muscle, and the rest requires anatomical context that the reporting physician can usually clarify.