 Needle biopsy is a way of determining whether a lung mass is cancerous or benign. Its accuracy was established by research in which patients underwent fine needle aspiration, a kind of needle biopsy that yields samples for cytology similar to the way a pap smear or a fluid sample would be evaluated for malignant cells. This kind of needle biopsy may supplemented or replaced by core needle biopsy, which yields samples for histology similar to surgical tissue samples but on a smaller scale rather than for cytology. Core needle biopsy is believed to be helpful particularly in obtaining a diagnosis in patients who have a lung mass that their doctors think is cancerous but is, in fact, benign. In spite of the advances in needle biopsy, however, there are patients who do receive a pathology report indicating no evidence of cancer but whose lung mass actually is cancerous. The fraction of such patients, among all patients who have no evidence of cancer according to the biopsy, is called the false negative rate. It is approximately 25 for needle biopsies that consist of fine needle aspiration alone. The false negative rate for needle biopsies that include core biopsy samples is not known. 