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I have read through your query. Please find my observations below.
Breathing treatments like nebulizers will not hide any suspected lung cancer when given before taking a chest x-ray. Chest x-ray is a basic investigation used to screen the lungs field and heart and usually done as a routine investigation. A routine chest x-ray usually picks up any nodules or mass in the lung fields that might represent lung cancer and is not affected by the patient taking nebulization therapy.
If there are strong reasons to suspect a lung cancer, then a CT scan chest is the best investigation and gives a complete detailed overview of the lungs. In conclusion, breathing treatment will not hide lung cancer when given before taking a chest x-ray.