Adenoid cystic carcinoma (ACC) is a rare, slow-growing but relentless cancer that most often arises in the salivary glands, though it can appear in the breast, lung, skin, and other sites throughout the body. What makes it unusual among cancers is its paradoxical nature: tumors grow slowly enough that patients often survive for many years, yet ACC has a stubborn tendency to recur decades after treatment and to spread along nerves in ways few other cancers do. It accounts for a small fraction of all head and neck malignancies, but its biology and behavior have drawn intense research interest precisely because it breaks the rules that apply to most tumors.
Where ACC Develops and Who It Affects
ACC originates in glandular tissues, and the salivary glands are its most common home. A large analysis of the National Cancer Database found that the major salivary glands accounted for about 45% of head and neck ACC cases, with the average age at diagnosis being roughly 59 years.1PubMed. Demographic and prognostic indicators of Adenoid cystic carcinoma in the head and neck: A National Cancer Database analysis But ACC is not exclusively a salivary gland cancer. An updated analysis of the SEER cancer registry identified over 4,200 ACC cases spanning multiple organ systems: about a third arose in major salivary glands, another third in minor salivary glands scattered throughout the mouth and sinuses, and the remaining third in non-salivary sites including the breast, skin, lung, and eye socket.2PubMed. Demographics, clinical features, and survival of patients with adenoid cystic carcinoma in major and minor salivary glands versus non-salivary sites: an updated SEER analysis
The disease does not strongly favor one sex over the other, and it can appear at almost any age, though the late fifties is a common window. Pediatric ACC is genuinely rare, but when it does appear in children or adolescents, the prognosis tends to be better than in adults, possibly because younger patients’ tumors more often have less aggressive features under the microscope.3PubMed. Adenoid cystic carcinomas of the lacrimal gland in childhood and adolescence
Three Growth Patterns and Why They Matter
Under a microscope, ACC presents in three recognizable patterns. The cribriform type, which looks like Swiss cheese with tumor cells forming rounded spaces, carries the best prognosis. The tubular pattern, with cells arranged in small tube-like structures, falls in the middle. The solid type, in which tumor cells pack together in dense sheets without those characteristic open spaces, is the most aggressive and is linked to significantly worse outcomes.4Oral Oncology. Adenoid cystic carcinoma: A rare clinical entity and literature review Most ACC tumors contain a mixture of these patterns, and pathologists typically classify the tumor by whichever pattern predominates.
A systematic review pooling data from over 17,000 ACC cases confirmed that the solid subtype, along with advanced tumor stage, positive lymph nodes, distant spread, and positive surgical margins, all predict poorer survival.5PubMed. Survival Outcomes in Adenoid Cystic Carcinoma of the Head and Neck: A Systematic Review of 17 497 Cases and Meta-Analysis This is why pathology reporting in ACC is especially important: the histologic subtype can shape not just the prognosis but the intensity of follow-up and decision-making around additional treatment after surgery.
The Genetic Engine Behind ACC
For decades, researchers puzzled over what drives ACC at the molecular level. The breakthrough came with the discovery that nearly all ACC tumors harbor rearrangements involving either the MYB gene or its close relative MYBL1. These genes encode transcription factors, proteins that switch other genes on and off. In ACC, chromosomal rearrangements fuse MYB or MYBL1 to a partner gene (most often NFIB), producing an abnormally active version of the protein that drives tumor growth. Research has shown that the vast majority of ACC tumors carry some type of MYB or MYBL1 rearrangement, making this the dominant “driver” mutation in the disease.6Cancer Discovery. Recurrent Fusions in MYB and MYBL1 Define a Common, Transcription Factor–Driven Oncogenic Pathway in Salivary Gland Adenoid Cystic Carcinoma
A second important genetic player is NOTCH1. Mutations that activate the NOTCH1 signaling pathway define a distinct, more aggressive subset of ACC. Patients with NOTCH1-mutated tumors are significantly more likely to present with the solid subtype, advanced-stage disease, and metastases to the liver and bones. Their survival is dramatically shorter: one study found median overall survival of roughly 30 months in NOTCH1-mutated cases compared to over 10 years in patients without the mutation.7PubMed Central. Activating NOTCH1 Mutations Define a Distinct Subgroup of Patients With Adenoid Cystic Carcinoma Who Have Poor Prognosis, Propensity to Bone and Liver Metastasis, and Potential Responsiveness to Notch1 Inhibitors NOTCH1 mutation rates vary widely depending on the tumor subtype being sampled, ranging from under 10% in primary tumors to over 60% in solid-subtype cases.8PubMed Central. Deciphering NOTCH1 as a Biomarker in Adenoid Cystic Carcinoma: Insights From a Systematic Review With Meta‐Analysis This makes NOTCH1 status a potentially valuable prognostic marker and a tempting therapeutic target.
How ACC Spreads Along Nerves
If there is one behavior that defines ACC more than any other, it is perineural invasion: the cancer’s remarkable ability to grow along and around nerves, sometimes tracking for centimeters beyond the visible tumor boundary. This is why ACC in the head and neck so often causes numbness, pain, or facial weakness; the tumor uses nerve pathways as highways for spread. This nerve-tracking behavior also makes complete surgical removal exceptionally difficult, because tumor cells can be traveling along nerve fibers well beyond what the surgeon can see or feel.
Recent research has begun to uncover the molecular mechanisms behind this tropism. One line of investigation found that ACC tumors express high levels of the β2-adrenergic receptor, with roughly 80% of salivary ACC specimens showing moderate to strong expression. The research demonstrated that sympathetic nerve fibers actively grow toward ACC tumors, and that blocking this receptor slowed tumor progression in experimental models.9Journal of Experimental Medicine. Sympathetic axonogenesis promotes adenoid cystic carcinoma progression Another study identified a protein called Apolipoprotein D that is dramatically overexpressed in ACC and appears to promote perineural invasion from multiple angles: it enhances cancer cell migration, stimulates nerve growth toward the tumor, and may even trigger a bizarre phenomenon in which tumor cells begin to resemble neurons, contributing to nerve formation within the tumor itself.10British Journal of Cancer. Apolipoprotein D is crucial for promoting perineural invasion in salivary adenoid cystic carcinoma
The practical implication of all this is that ACC is not merely growing near nerves by coincidence. The tumor and the nervous system engage in a two-way molecular conversation that actively draws them together, and understanding these signals could eventually open doors to treatments that interrupt the process.
Surgery and the Margin Problem
Surgery remains the primary treatment for localized ACC. The goal is to remove the tumor with a margin of healthy tissue around it, but ACC’s perineural invasion habit makes clean margins notoriously hard to achieve. Even when pathology reports say the margins are clear, microscopic tumor cells may already be tracking along nerve fibers beyond the resection edge. An international collaborative study noted that the presence of perineural invasion in specimens with nominally negative margins is often treated as an independent reason to add radiation therapy after surgery.11PubMed Central. Defining the surgical margins of adenoid cystic carcinoma and their impact on outcome: An international collaborative study
Because ACC in the head and neck frequently involves critical nerves, surgery can carry significant functional consequences. Tumors of the parotid gland, for example, may require sacrifice of the facial nerve, leading to paralysis on one side of the face. In some cases, nerve grafting techniques can restore partial function; one approach uses the great auricular nerve as a graft to reconnect branches of the facial nerve, with some patients recovering meaningful facial movement within a year.12PubMed Central. Adenoid cystic carcinoma of the parotid gland: Anastamosis of the facial nerve with the great auricular nerve after radical parotidectomy Balancing oncologic completeness against quality of life is one of the most difficult judgment calls in ACC surgery.
Radiation and the Case for Particle Therapy
Radiation therapy plays a major role in ACC, both as an addition to surgery (adjuvant treatment) and as the primary approach when a tumor cannot be fully removed. Conventional radiation using photon beams is widely available, but ACC’s tendency to sit near the skull base, brain, eyes, and spinal cord creates a dilemma: the dose needed to control the tumor can damage nearby critical structures.
This is where particle therapy, using protons or carbon ions instead of photon beams, has generated significant interest. Particle beams can deposit their energy more precisely, sparing surrounding tissue. Reviews of the evidence suggest that particle therapy achieves five-year local control rates of roughly 65% to 90% for ACC, compared to approximately 26% to 61% reported with conventional photon-based radiation. Severe acute side effects also appear lower with particle therapy: around 10% to 15% experience grade 3 or higher toxicity with protons and carbon ions, versus rates of 50% to 100% for severe mucositis with some photon regimens.13PubMed Central. The Role of Particle Therapy in Adenoid Cystic Carcinoma and Mucosal Melanoma of the Head and Neck An early report on pencil beam scanning proton therapy for head and neck ACC found a two-year local control rate above 92%.14PubMed. Clinical outcomes of head and neck adenoid cystic carcinoma patients treated with pencil beam-scanning proton therapy
Access remains a practical barrier: proton and carbon-ion centers are expensive and geographically limited, so many ACC patients still receive conventional radiation. Researchers acknowledge that randomized trials comparing the two approaches are unlikely to happen because ACC is simply too rare to enroll enough patients.
The Long Game of Recurrence and Metastasis
One of the most disorienting aspects of ACC is its timeline. Unlike many cancers where recurrence, if it happens, tends to show up within the first few years, ACC can return astonishingly late. A study of airway ACC documented local recurrences as far out as 29 years after surgery.15PubMed. Adenoid cystic carcinoma of the airway: thirty-two-year experience This means that a patient who has been cancer-free for a decade is not necessarily out of the woods, and lifelong surveillance is standard practice for ACC.
When ACC does metastasize, the lungs are the most common destination. Lung metastases can grow so slowly that some patients live for years with them, sometimes without symptoms. The question of whether to treat isolated lung metastases with surgery or other local therapies, rather than systemic treatment, is an active one. A study of 219 ACC patients with lung metastases found that those who received local treatment of their pulmonary lesions were more likely to have a longer interval between their original diagnosis and the appearance of spread, and were more likely to have only a few (oligometastatic) lesions.16PubMed Central. Survival outcomes of pulmonary metastasis‐directed local therapy in adenoid cystic carcinoma Patient selection matters enormously here: a few slow-growing nodules are a very different clinical picture from widespread metastatic disease.
Why Immunotherapy Has Largely Disappointed
Immunotherapy has transformed outcomes in many cancers, so its failure in ACC is a natural question. The answer lies in the peculiar immune environment within ACC tumors. Multiple studies have found that ACC is an immunologically “cold” tumor: it has very few infiltrating immune cells capable of attacking it, and very low expression of PD-L1, the molecule most commonly targeted by checkpoint inhibitor drugs.17PubMed. Immune microenvironment and evasion mechanisms in adenoid cystic carcinomas of salivary glands A randomized phase II trial of pembrolizumab (a PD-1 inhibitor), with or without radiation, in patients with recurrent or metastatic ACC confirmed low PD-L1 expression and sparse immune infiltration, and the drug showed limited activity.18PubMed Central. A Randomized Phase 2 Study of Pembrolizumab With or Without Radiation in Patients With Recurrent or Metastatic Adenoid Cystic Carcinoma
Research has begun to explain why the immune system seems blind to ACC. Tumor cells appear to have very low surface expression of the molecules (HLA class I proteins) that immune cells use to recognize and target cancer. This low expression may not be a permanent genetic loss but rather a reversible, epigenetic state. Laboratory work has shown that treatment with immune-activating agents, particularly STING agonists, can restore HLA expression on ACC cells, and at least one patient with metastatic ACC showed a promising response to this approach.19Journal for ImmunoTherapy of Cancer. Reversible downregulation of HLA class I in adenoid cystic carcinoma If this finding holds up, it raises the possibility that immunotherapy might work in ACC, but only after a priming step that makes the tumor visible to the immune system.
Systemic Therapies for Advanced Disease
For patients with ACC that has spread beyond what surgery and radiation can address, systemic treatment options remain limited compared to more common cancers. Traditional cytotoxic chemotherapy has modest activity in ACC. The most promising systemic agents so far have been multi-targeted kinase inhibitors, drugs that block several growth-signaling pathways simultaneously. A review of registered clinical trials found that axitinib, apatinib, and lenvatinib showed the best results among the small-molecule inhibitors tested, though the overall time to disease progression remained modest.20PubMed. Drug-based therapy for advanced adenoid cystic carcinoma: Current landscape and challenges based on an overview of registered clinical trials
Lenvatinib, a multitargeted kinase inhibitor, demonstrated enough antitumor activity in a phase II trial of recurrent or metastatic ACC to meet its primary endpoint.21PubMed Central. Phase II Study of Lenvatinib in Patients With Progressive, Recurrent or Metastatic Adenoid Cystic Carcinoma A separate phase II trial combined axitinib with avelumab (a PD-L1 inhibitor), achieving a confirmed overall response rate of 18% and a median progression-free survival of about seven months.22PubMed Central. Phase II Clinical Trial of Axitinib and Avelumab in Patients With Recurrent/Metastatic Adenoid Cystic Carcinoma An 18% response rate might sound unimpressive compared to immunotherapy results in other cancers, but in a disease where conventional chemotherapy achieves very little, it represents meaningful progress. Whether the immunotherapy component added anything beyond the kinase inhibitor alone remains unclear.
Targeting the MYB Driver Directly
Since MYB activation is the central genetic event in nearly all ACC, there is strong motivation to find drugs that can shut it down. Transcription factors like MYB have historically been considered “undruggable” because they lack the binding pockets that small-molecule drugs typically latch onto. Researchers have been looking for indirect routes. One approach found that blocking the IGF1R signaling pathway with a drug called linsitinib reduced expression of the MYB-NFIB fusion protein in ACC cells, regardless of the specific type of MYB rearrangement present. The drug selectively suppressed the abnormal fused version of MYB without affecting the normal copy.23JNCI: Journal of the National Cancer Institute. Targeting the Oncogenic Transcriptional Regulator MYB in Adenoid Cystic Carcinoma by Inhibition of IGF1R/AKT Signaling
Another line of research identified a kinase called BUB1 that is directly controlled by MYB in ACC cells. When researchers blocked BUB1 with a drug, it caused growth arrest and cell death specifically in MYB-driven ACC cells and organoids, a concept known as synthetic lethality, where disabling a downstream target is lethal only in cells that depend on the upstream driver.24PubMed Central. The mitotic checkpoint kinase BUB1 is a direct and actionable target of MYB in adenoid cystic carcinoma These approaches are still in early stages, but they represent a shift from the generic kinase inhibitors currently available toward therapies tailored to ACC’s specific biology.
New Ways to Track the Disease
Two emerging diagnostic tools are beginning to change how ACC is monitored and staged. The first is PSMA PET imaging, better known for its role in prostate cancer. ACC tumors often express PSMA on their surface, and PSMA-targeted PET scans have been able to detect ACC lesions that standard imaging missed.25MDPI / Cancers. PSMA PET Imaging and Therapy in Adenoid Cystic Carcinoma and Other Salivary Gland Cancers: A Systematic Review Beyond imaging, PSMA expression also opens the door to theranostics, where the same molecule used for scanning can also be loaded with a radioactive payload to treat the tumor. Early-phase work on PSMA-directed radioligand therapy for ACC is underway.
The second tool is liquid biopsy, specifically the analysis of tumor DNA fragments circulating in the blood. A methylation-based assay called METER was able to detect ACC-derived DNA in about 65% of blood samples from ACC patients. The test also distinguished between two molecular subtypes of the disease (ACC-I and ACC-II), with higher detection rates in the more aggressive ACC-I subtype. Patients who tested positive had a substantially higher risk of disease progression.26Journal of Clinical Oncology. Detection, quantification, and subtyping of adenoid cystic carcinoma (ACC) using methylation from liquid biopsies For a cancer that can recur decades later and requires lifelong monitoring, a blood test that can flag growing disease burden without repeated scans would be a real practical advance.
ACC Outside the Salivary Glands
Because ACC arises in glandular tissue, it can and does appear in unexpected places. The breast is one of the more notable non-salivary sites, and breast ACC behaves quite differently from the common forms of breast cancer most people have heard of. Breast ACC is typically triple-negative, meaning it lacks the estrogen, progesterone, and HER2 receptors that define most breast cancer subtypes and guide their treatment. In the broader world of breast cancer, triple-negative status usually signals an aggressive tumor with fewer targeted-therapy options. But breast ACC is an exception: despite its triple-negative label, it generally carries a favorable prognosis, and treatment decisions remain a matter of debate since the disease does not fit neatly into standard breast cancer protocols.27PubMed Central. Adenoid cystic carcinoma: triple negative breast cancer with good prognosis
ACC of the lung and trachea, while rare, is another well-recognized presentation. Airway ACC tumors tend to grow slowly and can be managed with surgical resection, often with good long-term palliation even when the tumor cannot be completely removed. The tradeoff is that late recurrences remain possible over very long timescales, as the 29-year recurrence data cited earlier illustrates. ACC has also been reported in the skin, the eye socket, and the cervix, among other sites. The underlying genetic driver, MYB or MYBL1 activation, appears to be shared across anatomic sites, which supports the idea that ACC is fundamentally one disease that happens to arise in different locations rather than a collection of separate cancers that merely look alike under the microscope.