A 66-year-old man has developed a chronic cough over the past 2 months. There are no abnormal physical examination findings. A chest x-ray shows a 3 cm mass in the right upper lobe. After fine needle aspiration biopsy, the mass is resected. Gross and microscopic examination of the mass is performed and it is determined that a neoplasm is present. Which of the following findings regarding this mass is most likely to be irrelevant for determining further therapy and prognosis?