Discussion

Primary tracheal tumours are rare, and 10%–15% are due to ACC, which comprises of tubular, cribiform and solid subtypes. All variants have the propensity for longitudinal, submucosal and perineural invasion.1 Lymphatic and haematogenous metastases are even rarer at 18% and 7%, respectively.1 2 Local recurrence and/or metastasis can develop up to 94 months after diagnosis,2 thereby highlighting the indolent nature of ACC, and the need for prolonged surveillance. Our patient developed local invasion of thyroid, lymphatic and haematogenous metastases 8 years later. Surgery is first-line therapy for tracheal ACC, and promising results have been demonstrated using autologous composite tissue replacement when end-to-end anastomosis is impossible.3

Radiotherapy is recommended in patients who have positive resection margins, perineural and lymphatic invasion, or are inoperable as ACC is radiosensitive.1 2 Good local control is observed at radical doses in excess of 60 Gy. Radiotherapy with neutrons, photons or photon/neutron mixed beam can aid in dose escalation for disease control while minimising toxicity.2

ACC shows slow growth kinetics and poor response to single agent or combination chemotherapy.4 Tumour expression of c-KIT and other growth factor receptors have led to clinical evaluation with imatinib (c-KIT inhibitor) and tyrosine kinases against epidermal growth factor receptor (EGFR), which have also yielded dismal results.4

Translocation between chromosomes 6q and 9 p that juxtaposes the genes for MYB and nuclear factor 1/B (NF1B) transcription factors is specific to and found in 86% of ACC. MYB:NF1B gene fusion leads to over-expression of MYB oncoprotein, and deregulated MYB is central to tumour pathogenesis. Downstream effectors of MYB may herald future drug development against these novel targets.4

Five and 10-year survival rates of ACC are over 70% and 50%, respectively.2 Since good local disease control is achieved through advanced surgical techniques and adjuvant radiotherapy in excess of 60 Gy, distant metastasis from hematogenous and lymphatic spread become more apparent consequent to longer survival. Surgery for solitary pulmonary metastasis should be considered if the local disease is under control. Bronchoscopy plays an important role not only for tissue diagnosis but also in re-establishing the airway necessary for surgery with, electrosurgical snare, argon plasma coagulation, laser or cryotherapy. In addition, bronchoscopy offers airway surveillance and palliation.5 Our patient was offered radiotherapy and chemotherapy, but elected for expectant management of metastastic ACC and died 12 years after ACC was first diagnosed.