Why would someone with lung cancer only have one month of radiation therapy?

Intent of treatment. Intent of treatment and dose fractionation. Intent of treatment is palliative or curative. Dose fractionation means how big of a dose over what period is given. For example in cyberknife steriotactic body radiotherapy we can deliver radiation over 3-4 days to cure early stage lung cancer. With smaller dose/ fraction it takes about 7weeks of radiation to cure. 2-4 weeks of radiation for palliation.
For Palliation. We treat cancer for cure or palliation (releive a symptom). Its usually 2 to 2 and 1/2 weeks for pallliation and for cure its usually 5 to 7 weeks depending on type of lung cancer. So am not sure why only a month or 4 weeks unless the doctor was trying to do a better job in palliation or was just dragging it out for another reason. I would definitely ask my doctor though knowing the info i gave.
Treatment Intent. There are case treated with curative intent, and some of these are ultra-short, and others that are treated to relieve a symptom. For many years the duration of treatment has moved from 6 - 8 weeks. Short course/ high dose/fraction radiotherapy is quite effective, but it cannot be used to big fields.