Constipation is a common side effect of chemotherapy and radiotherapy. The cause is often related to pain control medications, along with the impact of certain treatment drugs. A starting point can be to look at your diet and experiment with building up your fibre intake. It is important to balance fibre intake between soluble and insoluble types. I often say these fibres are like a marriage and the best results for the bowel come when they are consumed together.
For example wholegrain cereal + fruit, wholegrain pita bread + salad, jacket