Thanks for the interesting question. I would want to know: on what basis are we saying that the patient does not have radiation pneumonitis (RP)? I am going to assume that this is being stated since the imaging findings are underwhelming in this case. Assuming that this is the case, I would answer: ...

I’ll echo @Nitin Ohri’s recommendation to look for other causes of dyspnea if the patient doesn’t fit a radiation pneumonitis picture, especially PE. A CTA will often allow you to rule out the big 3- PE, RP, and infection. 

This is an interesting question, and I am not aware of large studies that have addressed it.In practice, if I see a patient with worsening dyspnea after thoracic radiotherapy and without radiation pneumonitis/fibrosis or a large effusion on CT, I worry about other causes of dyspnea (anemia, cardiac ...

Yes. There’s no need to see any radiographic changes to derive this working diagnosis. I inform all my patients they might develop asthma-like symptoms 2-3 mo after XRT (any dose) and to still let me know if they do so we can develop a proper DDx to make sure we don’t miss anything more ...