How do you approach adaptive planning in lung cancer?

The role of adaptive radiotherapy in lung cancer is still under investigation.

I do daily CBCTs on my definitive NSCLC and small cell lung cancer patients. I only adapt plans if there is a change in anatomy such as opening of a collapsed lung or tumor growth outside of the PTV.

I do not shrink volumes if there is significant tumor shrinkage. I do not believe that lung cancers shrink concentrically. It probably is more like a swiss cheese effect and so I worry that micoscopic disease would be missed. The only time I would consider shrinking tumor volumes in responding tumors, is if the original PTV was so large that the normal tissue constraints, such as the lung, were not met and potentially unsafe. But this would be a rare event.

I only use adaptive thoracic RT for non-small cell lung cancer when faced with a large tumor associated with a (subjectively judged) high lung V20 (for example, >30-35%). Tumors treated with concurrent chemo-radiotherapy have a reasonable chance of shrinking (15-20%), so I replan around 36-40 Gy. I do not worry about microscopic disease left behind, since my reason for adaptive RT in such case is to minimize the risk of radiation pneumonitis as an overriding concern.

Although in general I treat limited stage small cell lung cancers with concurrent chemoradiotherapy (both starting day 1), sometimes for large small cell tumors which did not shrink after one induction chemotherapy cycle (a rare event), I would try an adaptive RT, rescanning at...