Background

CBCT scans acquired using integral kV imaging facilities with the patient in the treatment position can provide a full 3D data-set of the tissue structures. These can be imported into the treatment planning system (TPS) to determine dosimetric changes within the patient during curative schedules where acute reactions can result in marked weight loss as well as tumour shrinkage. If significant, the treatment plan could potentially be revised to adapt for these. As part of an ethics approved single centre study, the potential of CBCT adaptive radiotherapy for patients with HNCa was assessed.

Method

15 patients receiving curative treatment were recruited into the study. Weekly CBCT images were acquired using the Varian on-board imager. Clinical target volumes (CTVs) on the original scan (CTV_wk0) and week-6 CBCT (CTV_wk6) were defined by an experienced radiation oncologist. The CBCT was imported into the TPS to estimate dosimetric changes within CTV_wk6. The original treatment plan was recalculated on the planning CT and CBCT scans without heterogeneity correction to remove potential errors due to CBCT Hounsfield Units inaccuracies.

Results

3 of 5 patients analysed had > 4% absolute reduction in CTV_wk6 D₉₅ (dose delivered to 95% of volume) compared to CTV_wk0. Changes in D95 were: - 9% in low risk elective CTV in one patient, -5.3% in the intermediate risk CTV and -4.5% in low risk elective CTV in another patient, and -4.5% in the high risk CTV in a third patient.

Conclusion

Using CBCT in this small study, we found that in 3 of 5 patients there were dosimetric changes of > 4% over their treatment course and that these patients may benefit from re-planning during their treatment. CBCT is a useful tool for adaptive radiotherapy but further work is required to evaluate the clinical impact of these dosimetric changes.