Case Overview This case report describes a 63-year-old male with stage IVa pulmonary adenosquamous carcinoma (ASC) who achieved a complete response to immunotherapy-based treatment, with a progression-free survival of 46.5 months - an exceptionally durable outcome in advanced lung cancer.
What is Adenosquamous Carcinoma Adenosquamous carcinoma is a rare and aggressive lung cancer subtype representing 0.4-4% of all lung carcinomas. It contains both adenocarcinoma and squamous cell carcinoma components in the same tumor, each comprising at least 10% of the total. ASC carries a worse prognosis than pure adenocarcinoma or squamous cell carcinoma.
Molecular Complexity Next-generation sequencing (NGS) revealed an unusually complex molecular profile with co-occurring mutations in KRAS G12C (variant allele fraction 3.3%), BRAF G466R (VAF 4.7%), PIK3CA H1047R (VAF 3.5%), and FLT1 L1232F (VAF 2.1%). The low VAFs suggest these mutations may represent subclonal populations rather than dominant oncogenic drivers.
Clinical Significance This case contributes to the emerging literature on immunotherapy in ASC and raises important questions about how molecular heterogeneity, low-VAF subclonal mutations, and uncommon co-alterations affect therapeutic decision-making and treatment outcomes.
KRAS G12C KRAS G12C is the most common actionable KRAS mutation, present in approximately 13% of NSCLC. KRAS G12C inhibitors (sotorasib, adagrasib) are FDA-approved for previously treated NSCLC. However, in this patient the low VAF of 3.3% suggests KRAS G12C may be a subclonal event, complicating targeted therapy planning.
BRAF G466R BRAF mutations occur in approximately 3-4% of NSCLC. The V600E mutation is most common and responds to BRAF/MEK inhibitor combinations, but BRAF G466R is a class II kinase-activating mutation that does not respond as predictably to these agents. Its co-occurrence with KRAS mutation is unusual since these are typically mutually exclusive.
PIK3CA H1047R PIK3CA mutations occur in 3-5% of NSCLC and activate the PI3K/AKT/mTOR pathway. The H1047R variant is a hotspot activating mutation. PIK3CA mutations frequently co-occur with other driver mutations in NSCLC and can represent resistance mechanisms to upstream targeted therapies.
Low VAF Subclonal Architecture The finding of four mutations at VAFs below 5% suggests intratumoral heterogeneity, with these alterations potentially representing distinct tumor cell subclones rather than dominant truncal driver mutations. This has important implications for targeted therapy - treating a subclonal alteration may select for expansion of other clones while the target clone is suppressed.
PD-L1 and TMB Assessment PD-L1 tumor proportion score (TPS) was 18.11%, placing the patient in the moderate expression category (1-49% TPS). Tumor mutational burden (TMB) was 3.7 mutations/megabase, below the high-TMB threshold (10 mut/Mb). Neither biomarker individually would predict exceptional immunotherapy response.
Treatment Choice Given the molecular complexity, absence of a clear dominant actionable driver, and the presence of PD-L1 expression, the multidisciplinary team chose tislelizumab (an anti-PD-1 antibody) combined with paclitaxel plus carboplatin (TP chemotherapy). This approach aims to leverage both immunological and cytotoxic mechanisms.
Response Trajectory The patient achieved progressive tumor reduction across sequential imaging assessments, ultimately reaching complete response (CR) by imaging. This represents the disappearance of all measurable tumor lesions, a remarkable outcome in stage IV lung cancer.
Rationale for Chemotherapy-Immunotherapy Combination Platinum-based chemotherapy can induce immunogenic cell death, releasing tumor antigens and DAMPs that enhance the anti-tumor immune response. When combined with PD-1 inhibition, chemotherapy-induced antigenicity may prime T cells that are then released from checkpoint inhibition, potentially explaining the exceptional response in this case.
Dual Histology Implications The co-existence of adenocarcinoma and squamous cell carcinoma components creates challenges for molecular testing and treatment. Mutations and expression markers may differ between histological components within the same tumor, meaning a biopsy from one region may not represent the full molecular landscape.
Driver Mutation Prevalence in ASC Literature review by the authors found that EGFR mutations occur in 16-25% of ASC (lower than pure adenocarcinoma), KRAS mutations in 10-15%, and ALK rearrangements in 1-5%. The frequency of actionable drivers is generally lower in ASC than in pure adenocarcinoma, making molecular testing critically important but less often actionable.
Immunotherapy in ASC The published evidence for immunotherapy in ASC is limited to small retrospective series and case reports. Available data suggests response rates comparable to mixed-histology NSCLC cohorts, but robust prospective data is lacking. ASC patients are typically excluded or underrepresented in landmark ICI clinical trials.
Prognostic Factors ASC generally has a worse prognosis than pure adenocarcinoma or squamous cell carcinoma in early-stage disease. In advanced disease, outcomes have historically been poor, making the 46.5-month PFS in this case particularly noteworthy and suggesting that molecular profiling and modern immunotherapy may dramatically alter prognosis for some ASC patients.
Published Case Series The authors reviewed published cases and small series of immunotherapy-treated ASC patients. Objective response rates with single-agent PD-1/PD-L1 inhibitors ranged from 20-35% in retrospective cohorts, with some patients achieving prolonged responses beyond 24 months.
PD-L1 and Response Correlation Among reviewed cases, PD-L1 expression above 50% TPS was associated with higher response rates, consistent with the broader NSCLC literature. However, responses occurred across the PD-L1 expression spectrum, including in PD-L1-low patients like the presented case, suggesting other immune factors beyond PD-L1 contribute to outcomes.
Combination vs. Monotherapy Cases treated with immunotherapy plus chemotherapy combinations generally showed higher response rates and longer PFS than monotherapy, consistent with findings in pure adenocarcinoma and squamous cell carcinoma. The presented case supports combination therapy as a reasonable first-line approach for advanced ASC.
Biomarker Limitations The review highlights that standard biomarkers (PD-L1 TPS, TMB) have limited predictive accuracy in ASC, similar to the broader NSCLC population. Tumor microenvironment characterization, spatial immune profiling, and composite biomarker panels may be necessary to reliably predict immunotherapy benefit in this heterogeneous tumor type.
Comprehensive Molecular Profiling This case underscores the importance of comprehensive NGS testing in all ASC patients, even when multiple co-mutations are identified at low VAF. Multi-region sampling and liquid biopsy can better characterize the clonal architecture and identify dominant versus subclonal alterations that may guide sequential treatment planning.
ASC-Specific Clinical Trials Given the rarity of ASC and its historical exclusion from landmark trials, prospective trials specifically enrolling ASC patients with pre-defined molecular stratification are urgently needed. Alternatively, histology-specific substrata in NSCLC basket trials could generate prospective data on immunotherapy efficacy and biomarker correlates.
Serial Liquid Biopsy Monitoring The evolution of ctDNA during treatment could monitor clonal dynamics, detect minimal residual disease, and provide early warning of emerging resistance. In molecularly complex cases like this, tracking the trajectory of KRAS, BRAF, and PIK3CA mutant clones during therapy may reveal resistance mechanisms.
Long-Term Follow-Up Complete responses in advanced NSCLC require long-term follow-up to determine whether they represent true cures or inevitable late relapses. This case will be important to follow beyond 46.5 months to understand the durability of response and factors predicting long-term survival in immunotherapy-treated ASC.