Re-treatment after full-course radiotherapy: is it a viable option?

Re-irradiation of previously treated areas may become necessary for recurrent cancer, new primary tumours (common in head and neck cancer patients), or nodal and metastatic disease. Factors that should be taken into account in the decision to re-treat include: 1) previously treated volume (how much overlap is there with new treatment fields) and dose fractionation schedule; 2) which critical tissues or organs are at risk; 3) how much time has elapsed since first treatment; 4) whether there are any practical alternatives to re-irradiation? Rapidly proliferating tissues generally recover well from the initial radiotherapy and will tolerate re-irradiation to almost full doses. Some slowly proliferating tissues are also capable of partial proliferative and functional recovery, although this takes several months and some residual damage remains. Preclinical data demonstrate that re-irradiation with reduced doses is possible in lung and spinal cord after intervals of 3-6 months. Other slowly proliferating organs, e.g. the kidneys, do not appear to be capable of recovery, even after low, subtolerance doses. The largest clinical experience of re-irradiation is for head and neck cancers. A review of this literature reveals that the most frequent normal tissue complication seen is trismus (lockjaw), which occurs in 16 to 30% of re-treated cases, with lower incidences of soft tissue or bone necrosis and fibrosis. Myelitis is rarely reported, even in the re-treatment situation. In general the highest incidence of local control for the lowest incidence of serious complications is achieved for combinations of external beam and brachytherapy, and for small, well-differentiated, new primary tumours rather than recurrent disease. Re-treatment with total doses < 55 Gy gives very poor local control rates. Re-treatment schedules with curative intent require a high re-treatment dose, which is accompanied by an increased risk of normal tissue damage. To minimize serious complications, re-irradiation schedules require the best possible treatment planning (conformal therapy where possible). Hyperfractionation or a combination of external beam and brachytherapy could also be beneficial.