Study Characteristics
The characteristics of the studies included in this systematic review are summarised in table 1. The studies were published between 2002 and 2021 and originate from centres in the USA (51,53,55,57), Europe (50,52,56,64), Asia (58,59,61–63) and Australia (54). There were no eligible studies from the UK. One of the studies was based on extraction of data from a national cancer database (51), while the remainder were single institution or multi-centre studies from a single geographical region. Three studies were based on prospective databases (53–55) that were retrospectively analysed, and the remainder were all retrospective studies. All the studies identified patients based on diagnosis and reported outcomes with respect to treatments given, and as such, were observational cohort studies. None of the studies directly compared ND alone to radiotherapy alone in a prospective or matched cohort manner, although some retrospectively commented on outcomes for both groups.