11.10.3 - Consistency Of Contours During Treatment

Consistency of surface contours and source-surface distance

A patient’s shape may change between planning and treatment, or during treatment itself. This variation may be due to fluid accumulation (seroma or ascites), weight loss, or errors in patient immobilisation and setup.
The SSD is measured daily using the distance indicator, and compared to previous measurements and the treatment plan using a record and verify system. If the SSD has changed significantly the plan may need to be altered, but small changes are usually accepted. The tolerance of SSD shifts is highly dependent on the treatment location. For a highly immobilised patient, SSD measurements should remain similar between treatments. Patients with less or no immobilisation have a greater degree of tolerance.

Changes in normal tissue and tumour size during treatment

Some tumours, particularly large head and neck cancers and haemotological malignancies, may shrink rapidly after radiation is delivered. Alternatively, aggressive tumours may increase in size, particularly between planning and treatment. These changes may necessitate replanning after several weeks (in the case of a shrinking tumour), or urgently before treatment begins (in the case of a tumour that has rapidly grown between planning/treatment).
More deeply seated tumours are unlikely to cause changes in surface contours if they change in size. If this is expected to occur due to the nature of the treatment, replanning may be included in part of the treatment package after several weeks. Image guided radiotherapy may be able to pick up these changes on a daily basis (eg. with cone beam CT) and allow treatment changes to occur earlier.