The present work describes an extremely sensitive apparatus based upon laser photoacoustic spectroscopy (LPAS) methods which can be used for the optical detection of breath ethylene concentration at patients treated by radiotherapy. We analyzed the breath air from patients, subjected to radiation treatment with a high dose fraction, depending on the cancer type and we have found out that patients treated by external radiotherapy suffer a slight increase in the generation of oxidants; however, in accordance with the clinical practice, this increase is sufficient to assess the body response to the treatment: the smaller the increase, the higher the radio-resistivity developed by the patient. With the relevant characteristics of high sensitivity and specificity, LPAS holds a great potential for medical diagnostics and future steps are to be taken in order to bring this method near the patient bad.