Treatment of a stage IV NSCLC is based on chemotherapy and requires before each session to check clinically, biologically and radiologically absence of infection. This biological evaluation is based on the CRP rate. Lots of infections may occur in this situation and could reduce by three the patient's life duration. However it is very important to make as early as possible a correct diagnosis of infection. CRP rate is classically higher for those patients even without any infection. In comparison with CRP, Procalcitonin (PCT) is well-known for its better sensibility and specificity for the infection diagnosis but has never been study in case of active cancer. The aim of this study is to evaluate the Procalcitonin basal rate for those patients suffering from NSCLC. It could be a simple and reliable method to use. So the investigators decided to include each new patient with NSCLC stage IV coming for his first-line chemotherapy without any infection sign, and to realize a Procalcitonin dosage before the start of the treatment. This sample will be analyzed at the chemical laboratory and the result will not be communicated to the patient physician. A new clinical evaluation will be realized either at the hospital or by phone at day 7 to search any sign of infection. In absence of infection, patient will be reevaluated at day 14, 21 and 30. In case of infection, a new Procalcitonin (and CRP) dosage will be performed.