Abstract: The searches identified 7558 titles, and we excluded 7513 during title and abstract searches. We reviewed 45 papers in full, and excluded 39. We identified four studies on handsearching of the references, which, along with the six eligible papers from the database search, led to 10 studies being included. Four studies, three of which were RCTs and one prospective study, investigated the effect of elemental diet on GI symptoms; one RCT investigated the effect of dietary modification and elemental diet; and five RCTs investigated dietary modification. Studies were varied in terms of risk of bias. Data were dichotomised for presence and absence of diarrhoea at the end of radiotherapy for four trials evaluating dietary modification comprising modified fat, lactose, fibre or combinations of these dietary changes. A reduction in diarrhoea was demonstrated with nutritional intervention (risk ratio (RR) 0.66; 95% confidence interval (CI) 0.51 to 0.87, four studies, 413 participants, moderate quality of evidence) with low heterogeneity (Chi2 = 3.50, I2 = 14%). Two trials evaluating dietary modification on weight change (comparing baseline and end of radiotherapy) showed no difference between intervention or control (mean difference (MD) -0.57 kg; 95% CI -1.22 to 0.09) with low heterogeneity (Chi2 = 1.41, I2 = 29%). Generally adverse effects were poorly reported in included studies. Elemental diet in particular was poorly tolerated. GI symptoms or toxicity > 6 months after radiotherapy was not reported in included studies There have been benefits demonstrated with dietary modification during pelvic radiotherapy to reduce diarrhoea. Those diets included single interventions or combinations of modified fat, lactose-restriction, fat-restriction and fibre supplementation. We were unable to meta-analyse elemental diet, as data were not available. We considered some of the studies to be at high risk of bias. There have been recent advances in novel, more targeted radiotherapy techniques, such that the findings of older studies need to be interpreted with caution. In addition, there were problems with compliance and palatability with some of the interventions, particularly elemental diet, which limits its usefulness in clinical practice.

Plain-language summary: Searches of all relevant sources of medical information identified 7558 articles, and, after initial screening of all these articles, we selected 45 as being suitable for this review. On reading the summaries of these 45 trials, 10 were suitable to be included in this review. We included trials that looked at the effects of a nutritional intervention in adults aged 18 years or over having radical pelvic radiotherapy as part of anticancer treatment for a primary pelvic cancer. We excluded patients with stomas and a previous history of inflammatory bowel disease. Results showed that studies evaluating dietary changes, including changes in dietary fat, fibre and lactose, during radiotherapy reduced diarrhoea at the end of treatment. However, these dietary changes did not seem to improve patients' weights. While some changes to diet during radiotherapy may benefit patients by reducing diarrhoea, it is important to recognise that some of the studies reviewed were dated and there have been changes in radiotherapy technique since then that may give patients less diarrhoea anyway. In addition, many studies were of poor quality and, therefore, only a small number were able to be included in this review. Some interventions described, for example, having to take a liquid diet, were not acceptable as many patients found them difficult to tolerate and were unable to take them completely.