Adenosquamous carcinoma of the lung (ASC) is a rare and aggressive form of non-small cell lung cancer that contains both glandular (adenocarcinoma) and squamous cell components within the same tumor. Like most lung cancers, smoking is the dominant cause, though ASC patients tend to have lighter smoking histories than those with pure squamous cell tumors. Treatment follows many of the same principles as other non-small cell lung cancers, including surgery, chemotherapy, targeted therapy, and immunotherapy, but the dual nature of this tumor creates complications at nearly every step, from diagnosis to drug selection.
A Tumor With Two Faces
To qualify as adenosquamous carcinoma under World Health Organization criteria, a lung tumor must contain at least 10 percent of each component: glandular tissue (the type seen in adenocarcinoma) and squamous tissue (the flat, scale-like cells seen in squamous cell carcinoma). This is not a collision of two separate cancers growing side by side. Research using whole-exome sequencing has shown that the two components share a common origin, with strong evidence that the adenocarcinoma portion transforms into squamous tissue over time. One study found that loss of a tumor suppressor gene called STK11 and amplification of a gene called SOX2, changes previously linked to this kind of transformation in mouse models, were present in adenosquamous tumors.1PubMed. Deciphering the clonal relationship between glandular and squamous components in adenosquamous carcinoma of the lung using whole exome sequencing
A separate genomic study examining both components in surgical specimens found that 96 percent of patients had shared genetic variations between the adenocarcinoma and squamous cell portions, confirming their shared ancestry. Interestingly, when those tumors spread to lymph nodes, the metastases were never purely squamous, suggesting the glandular component drives the seeding of distant disease.2PubMed. Genomic origin and EGFR-TKI treatments of pulmonary adenosquamous carcinoma
This matters practically because the two tissue types can behave differently when it comes to drug targets and immune markers. A single tumor can harbor mutations typical of adenocarcinoma (like EGFR) while also expressing proteins more common in squamous cell carcinoma (like PD-L1). Treating one component effectively does not guarantee the other will respond the same way.
Smoking and Other Risk Factors
Smoking is far and away the leading cause. A study of 100 lung cancer patients that broke down smoking histories by tumor type found that adenosquamous cases had a mean of 48 pack-years of smoking, which was significantly less than squamous cell carcinoma (82 pack-years) and small cell carcinoma (78 pack-years) but still a substantial exposure.3PubMed. Present and past smoking history and other predisposing factors in 100 lung cancer patients The pattern fits with what researchers know about the two parent subtypes: pure squamous cell carcinoma is the lung cancer most tightly linked to heavy smoking, while adenocarcinoma shows up more often in lighter smokers and never-smokers. ASC, being a hybrid, falls somewhere in between.
Beyond smoking, the same environmental and genetic risk factors that raise the odds of any lung cancer apply here: radon exposure, occupational carcinogens like asbestos, air pollution, and family history of lung cancer. ASC does occur in never-smokers, particularly in East Asian populations where EGFR mutations are more common, but it remains less frequent in that group than pure adenocarcinoma. Studies that track demographics tend to find a male predominance, though the gap is narrower than it is for squamous cell carcinoma.
Why Diagnosis Is Frequently Missed on Biopsy
One of the biggest clinical headaches with ASC is that small biopsies often miss it entirely. A needle biopsy or bronchoscopic sample grabs a tiny piece of the tumor, and if that piece happens to come from the adenocarcinoma zone, the pathologist will call it adenocarcinoma. If it comes from the squamous zone, it gets labeled squamous cell carcinoma. The dual nature only becomes apparent when a larger surgical specimen is examined and both components are visible.
A study looking at poorly differentiated non-small cell lung cancers found that while immunohistochemistry staining allowed accurate classification in most biopsy cases, about 18 percent were reclassified after the full surgical specimen was reviewed. Among those reclassifications, two cases initially called squamous cell carcinoma on biopsy were changed to adenosquamous carcinoma after resection.4PubMed. Accuracy of classifying poorly differentiated non-small cell lung carcinoma biopsies with commonly used lung carcinoma markers This means a meaningful number of patients treated for “adenocarcinoma” or “squamous cell carcinoma” actually have adenosquamous disease and may be getting suboptimal treatment based on an incomplete picture.
The practical consequence is that molecular profiling should ideally be performed on both components when possible. If you are diagnosed with one subtype on a small biopsy and the tumor does not respond as expected, it is worth considering whether the tumor might actually be adenosquamous.
Prognosis Compared to Other Lung Cancers
ASC generally carries a worse prognosis than either pure adenocarcinoma or pure squamous cell carcinoma. A large Japanese study comparing surgical outcomes found five-year survival rates of about 23 percent for ASC, compared with 58 percent for adenocarcinoma and 41 percent for squamous cell carcinoma. The survival gap was most dramatic at earlier stages: for stage IA disease, five-year survival was roughly 42 percent for ASC versus 82 percent for adenocarcinoma.5European Journal of Cardio-Thoracic Surgery. Adenosquamous carcinoma of the lung: surgical results as compared with squamous cell and adenocarcinoma cases
This aggressive behavior likely stems from the tumor’s biological flexibility. Having two histologic components means the cancer has more tools to evade treatment. A separate surgical series found a five-year survival of about 19 percent for all resected ASC cases and identified nodal involvement as one of the strongest predictors of poor outcome, with most long-term survivors being patients who had no lymph node spread at the time of surgery.6PubMed. A clinicopathologic study of resected cases of adenosquamous carcinoma of the lung Another series reported a similar five-year rate of about 23 percent and confirmed a significantly higher incidence of lymph node metastasis in ASC compared with other non-small cell subtypes.7PubMed. Adenosquamous carcinoma of the lung. Surgical results compared with squamous cell and adenocarcinoma
Where It Tends to Spread
When ASC reaches an advanced stage, it spreads to the same organs as other lung cancers but with a pattern worth knowing. A population-level study of over 850 patients with metastatic ASC found that bone was the most common site, present in about 57 percent of cases. Lung-to-lung metastasis was next at roughly 38 percent, followed by liver (about 22 percent) and brain (about 21 percent). Median overall survival once the cancer had spread was just five months, dropping to three months for patients with disease at three or more sites.8Heliyon. Metastatic pattern and prognosis in patients with lung adenosquamous carcinoma: A surveillance, epidemiology, and end results-based population study
Brain metastases deserve special attention. While the rate was about one in five in the large registry study above, some case reports emphasize the central nervous system as a frequent site of recurrence, sometimes in unexpected patterns.9Cancer Treatment Communications. An unusual recurrence of adenosquamous carcinoma of the lung This has led many oncologists to push for baseline brain imaging in ASC patients even when there are no neurological symptoms.
Surgical Treatment
For patients with early-stage disease, surgery remains the most important treatment and the only approach that offers a realistic chance of cure. Complete resection, typically a lobectomy with mediastinal lymph node dissection, is the standard. The five-year survival data across multiple surgical series consistently land in the 19 to 23 percent range for all-comers, which sounds grim but is heavily dragged down by the high proportion of patients who already have lymph node involvement at the time of surgery.6PubMed. A clinicopathologic study of resected cases of adenosquamous carcinoma of the lung
The key takeaway from the surgical literature is that node-negative disease is the strongest predictor of long-term survival. In one series, five of the seven patients who survived five years had no nodal involvement at surgery. Gender also appeared to influence outcomes, with women tending to do somewhat better, though sample sizes in most ASC surgical studies are small enough that this finding should be interpreted cautiously.
Imaging Features
On CT scans and PET scans, ASC tumors tend to appear as peripheral masses with lobulated, spiculated, or ill-defined margins. One study of 26 patients found the average diameter was about 3.8 cm, with roughly 80 percent located peripherally in the lung. Internal low attenuation and heterogeneous density were universal findings, reflecting the mixed tissue composition.10Clinical Nuclear Medicine. Adenosquamous Carcinoma of the Lung On PET scanning, these tumors are consistently metabolically active; a separate study of 41 patients found an average SUVmax of about 11.5, which was linked to tumor size, location, and stage.11PubMed. (18)F-FDG PET/CT in lung adenosquamous carcinoma and its correlation with clinicopathological features and prognosis None of these imaging features are unique enough to distinguish ASC from adenocarcinoma or squamous cell carcinoma before biopsy, which circles back to the diagnostic challenge discussed earlier.
Targeted Therapy When EGFR Mutations Are Present
Because the adenocarcinoma component can carry EGFR mutations, a subset of ASC patients are candidates for EGFR-targeted drugs. One microdissection study of Western patients found EGFR mutations in about 13 percent of ASC cases, a rate similar to conventional adenocarcinoma in that population. The same study found KRAS mutations at a similar rate but noted significant variability in EGFR amplification between the glandular and squamous portions of the same tumor.12American Journal of Clinical Pathology. Adenosquamous Carcinoma of the Lung: A Microdissection Study of KRAS and EGFR Mutational and Amplification Status in a Western Patient Population In East Asian populations, EGFR mutation rates in ASC are considerably higher, often above 40 percent.
For patients whose tumors do harbor EGFR mutations, tyrosine kinase inhibitors produce meaningful responses, though outcomes are generally a step below what is seen in pure adenocarcinoma. A retrospective study and pooled analysis found that EGFR-mutant ASC patients treated with these drugs had a response rate of about 63 percent, disease control in roughly 86 percent, a median time before progression of around 10 months, and median overall survival of about 21 months.13PubMed Central. Efficacy of epidermal growth factor receptor-tyrosine kinase inhibitor for lung adenosquamous cell carcinoma harboring EGFR mutation: a retrospective study and pooled analysis A smaller prospective study using the third-generation drug aumolertinib reported a response rate of about 58 percent and median progression-free survival of roughly six months, though median overall survival had not yet been reached at the time of reporting.14Journal of Clinical Oncology. First-line treatment of EGFR mutation adenosquamous carcinoma of the lung with aumolertinib: A multicenter, single-arm, prospective study (ARISE Study)
For patients with recurrent disease after surgery, EGFR-targeted drugs also appear to help. One study found a disease control rate of about 74 percent among ASC patients treated with these drugs, and those with confirmed EGFR mutations had a median progression-free survival of 15 months.15PubMed Central. Clinical outcomes of epidermal growth factor receptor tyrosine kinase inhibitors in recurrent adenosquamous carcinoma of the lung after resection The message is clear: molecular testing is not optional for ASC. Missing an EGFR mutation in this tumor means missing one of the most effective treatment options available.
Immunotherapy and PD-L1 Expression
Immune checkpoint inhibitors have become central to lung cancer treatment, and ASC presents an interesting picture. PD-L1, the protein that checkpoint inhibitors target, was expressed in about 39 percent of ASC tumors in one study. But the expression was unevenly distributed: the squamous component expressed PD-L1 at roughly 39 percent, while the glandular component expressed it at only about 11 percent. In other words, the squamous part of the tumor looked immunologically “hot” while the glandular part looked “cold.”16Scientific Reports. PD-L1 expression in lung adenosquamous carcinomas compared with the more common variants of non-small cell lung cancer
Clinical outcomes with checkpoint inhibitors reflect this complexity. In a study of advanced ASC patients treated with immune checkpoint inhibitors, those who were PD-L1-positive had a response rate of about 36 percent and disease control in all cases, while PD-L1-negative patients had a zero percent response rate and only 50 percent disease control. Median progression-free survival was about five months for PD-L1-positive patients versus under two months for PD-L1-negative patients.17PubMed Central. Clinical outcomes of immune checkpoint inhibitor therapy for advanced lung adenosquamous carcinoma PD-L1 testing matters even more in ASC than in garden-variety lung cancer, because the lopsided expression between components makes negative results especially predictive of poor immunotherapy response.
Radiation and Its Limits for Mixed Histology
Radiation therapy, particularly stereotactic body radiation (SBRT), is used for early-stage patients who cannot tolerate surgery and for palliative control of metastatic disease. There is a wrinkle for ASC that deserves mention. A study of SBRT outcomes by histology found that squamous cell tumors failed locally at a significantly higher rate than adenocarcinomas, with a three-year local failure rate of about 19 percent for squamous histology versus roughly 9 percent for adenocarcinoma. Squamous histology was the strongest predictor of local failure on multivariable analysis.18Journal of Thoracic Oncology. A Histologic Basis for the Efficacy of SBRT to the lung Since ASC contains a squamous component, there is a theoretical concern that it may be more radiation-resistant than pure adenocarcinoma, though dedicated studies of SBRT in ASC specifically are lacking.
Neoadjuvant Chemoimmunotherapy Before Surgery
Giving chemotherapy and immunotherapy before surgery, known as neoadjuvant treatment, has gained ground for resectable lung cancers in general. Early case-level data suggest it can work for ASC as well. One report described a patient with stage IIIA ASC who received two cycles of the checkpoint inhibitor sintilimab combined with chemotherapy before surgery. Imaging showed stable disease in the primary tumor but partial shrinkage of mediastinal lymph nodes. After lobectomy, pathology revealed a complete response in the primary lesion with only minimal residual cancer cells in surrounding lymph nodes.19PubMed Central. Effective neoadjuvant immunotherapy and chemotherapy in stage IIIA adenosquamous carcinoma of the lung with a complete response and surgical success: A case report This is a single case and cannot be generalized, but it is encouraging because stage IIIA disease has very poor outcomes with surgery alone in ASC. Larger trials specifically enrolling ASC patients for neoadjuvant regimens are needed.
Liquid Biopsy for a Heterogeneous Tumor
Given the diagnostic challenges and internal genetic variability of ASC, liquid biopsy, which analyzes fragments of tumor DNA circulating in the blood, is a particularly appealing tool. Traditional tissue biopsies sample a single spot in the tumor and may miss the genetic profile of the other component entirely. Circulating tumor DNA represents a composite signal from multiple tumor sites, making it better suited to capture the genetic diversity within an adenosquamous tumor. This can help detect targetable mutations that a tissue biopsy from the wrong part of the tumor might miss, and it can track how the tumor’s genetics evolve during treatment.20Breathe. Liquid biopsy in lung cancer
For ASC specifically, liquid biopsy could help resolve a persistent clinical problem: when a biopsy calls the tumor adenocarcinoma but EGFR-targeted therapy underperforms, circulating DNA analysis might reveal the presence of squamous-associated molecular features that explain the resistance. The technology is still maturing in terms of sensitivity, but its advantage for genetically heterogeneous tumors is hard to overstate. As treatment decisions become increasingly driven by molecular profiling, getting a complete picture of the tumor’s genetics becomes more and more consequential for patients with this uncommon but difficult cancer.