There are no studies assessing the optimal surveillance interval, but an early study found tumour doubling time of 4.2 years for head and neck PGLs24 and 5.8 years in a series of SDHB and SDHD patients with head and neck PGLs, whereas abdominal and thoracic PGLs grew more slowly, doubling at 6.94 and 11.8 years, respectively.25 This evidence concerns the gene SDHB and neoplasm.