Contrast-enhanced CT of the lung
Follow-up examination of a rectal adenocarcinoma with known lung metastases receiving chemotherapy: response evaluation.
Previous medical history of rectum resection (adenocarcinoma) and lobectomy of the middle lobe and segmentectomy from the right upper lobe (metastasis).
Formation of a small endoluminal irregularity adherent to the left tracheobronchial wall (diameters 10 mm anteroposterior x 5 mm laterolateral x 8 mm craniocaudal), density 80 HU, review of a PET-CT 3 weeks prior already showed a smaller lesion.
Recurrence of a nodular lesion in the right hilum in the basal part of the RUL.
Plate-shaped compaction against the horizontal fissure with suture material and nodular component posteromedial: presumably atelectasis.
Unchanged volume of the right infrahilar lymph gland. No lymphadenopathies.
No skeletal metastases. No intra-abdominal recurrence.
CONCLUSION: Disease progression. Nodular recurrence in the RUL and formation of a endobronchial mass, both suspect for metastases until proven otherwise.
Bronchoscopy with biopsy was performed and pathologic examination showed well-differentiated intestinal type adenocarcinoma, appropriate for metastatic localisation of the known colorectal carcinoma.
Tracheobronchial tumours are rare (<0, 4% of all tumours, <1% of all thoracic tumours) and classified as primary or secondary. [1, 2] Ninety percent of primary tracheobronchial tumours are malignant.  The remaining 10% are primary benign tracheobronchial tumours which are predominantly small (<2cm), mostly from mesenchymal origin and have a smooth contour as a consequence of their submucosal location. 
Secondary malignant tumours are most commonly the result of direct invasion or secondly, by haematogenous spread. Most common primary malignancies are melanoma or kidney-, breast-, colorectal-, or hepatocellular cancer. 
Only when 50-75% of the luminal diameter is occluded symptoms of upper airway obstruction occur (dyspnoea, wheezing, stridor), possibly obscuring pathology for months or even years.  Other possible symptoms are cough and haemoptysis due to mucosal irritation or dysphagia or recurrent laryngeal nerve palsy due to invasion.
Squamous cell carcinoma is the most common primary tracheobronchial malignancy, presenting in the 6th & 7th decades of life.  Men are more affected and prior tobacco use is frequent.  An irregular contour is typical because it arises from the surface epithelium, mostly the posterior wall of the lower trachea. It co-occurs frequently with cancer of the oropharynx, larynx or lung in 40%. Lung- and mediastinal metastases at presentation are unfortunately frequent.
Adenoid cystic carcinoma of salivary origin is the second most common primary tracheobronchial malignancy, has equal sex distribution and occurs in the 4th & 5th decades of life. These also have a smooth contour as a consequence of their submucosal location.  Their submucosal circumferential and infiltrative growth can cause a remarkable craniocaudal extent, requiring multiplanar reconstructions for correct interpretation.
Mucoepidermoid carcinoma tends to occur in younger patients and more distally (in the lobar or segmental bronchi) as an intraluminal nodule, associated with atelectasis, mucus plugging and sometimes obstructive pneumonia. 
Carcinoid tumours are frequently of thoracic origin (25%) and are so-called 'iceberg' tumours (having a large extraluminal component and a smalller smooth intraluminal nodular component) with intense enhancement due to their intrinsically high vascularity.  Metabolic uptake on FDG-PET is comparatively lower than other thoracic malignancies. 
Secondary malignant tumours predominantly invade directly with an apparent extramural source.  Haematogenous metastatic disease manifests as solitary or multiple nodules, sometimes with the “finger-in-glove" appearance, or as eccentric wall thickening. [9-11] Most common presentation is atelectasis, with or without endoluminal lesion. Differential diagnosis with a primary bronchogenic carcinoma can be difficult. 
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