Session Item

Curatively intended radiotherapy is predominantly delivered as a fractionated treatment lasting one to several weeks. The daily treatment is based on the CT or MR scanning obtained prior to treatment. However, anatomical changes may occur during the treatment course. The changes can be monitored by the cone-beam CT (CBCT) images used for patient positioning. In case of suspicion of target dose deterioration or normal tissue overdosage, the treatment plan can be adapted for the remaining fractions  to reflect these changes.

Adaptive radiotherapy may be performed either online just before each treatment delivery or offline after treatment delivery. In many patients, offline adaptation is sufficient to secure precise dose delivery during the treatment course and offline adaptation is less resource-demanding than online adaptation.

However, offline adaption requires extra resources in the clinical routine. The daily CBCT scans need to be checked for deviations. This may be done by visual inspection using e.g. geometric measures either online by the RTTs before treatment delivery or offline with a selected schedule. Additionally, the daily dose delivered can be calculated based on the CBCT images. This requires validation of the structure propagation from the CT scan to the CBCT scan and calibration of the CBCT images or the creation of synthetic CT images based on the CBCT images. In patients requiring plan adaption, rescanning, re-delineation and re-planning must be performed. All parts of this workflow will be prone to uncertainties, which must be investigated before large-scale offline adaptation can be implemented in daily clinical routines. Strict criteria for plan adaption will aid the decision workflow. With all of these tasks in place, an offline adaptive radiotherapy strategy may be possible for every curatively intended patient.

A clinical benefit in terms of significantly increased overall survival and reduced toxicity has been shown for lung cancer patients after the implementation of adaptive radiotherapy. In lung cancer patients, the most commonly observed anatomical changes are atelectasis, pleural effusion, pneumonia, tumour shrinkage, and differential shifts between tumour and lymph nodes. Up to 25% of the patients may benefit from treatment adaptation due to these anatomical changes. Most of the anatomical changes are systematic and thus, a single plan adaptation during the treatment course will be sufficient.

In head and neck cancer patients, tumour shrinkage may lead to overdosage of organs at risk which may be counteracted by plan adaption. Additionally, shrinkage and irreproducible patient fixation may lead to target dose depletion. Retrospective data does appear to suggest a benefit of adaptive radiotherapy in terms of higher loco-regional control.

In the pelvic and abdominal regions, the anatomical changes are more likely random variations depending on e.g. bladder and rectum filling. A plan selection strategy has been used in both bladder and cervical cancers, leading to reduced toxicity. Different treatment plans are created based on different fillings and at the daily treatment, the most appropriate plan is selected. However, these patients may benefit more from online adaptive radiotherapy.