A dosimetric evaluation on applying RTOG-based and CT/MRI-based delineation methods to brachial plexus in radiotherapy of nasopharyngeal carcinoma treated with helical tomotherapy.

Objective: In radiotherapy of nasopharyngeal carcinoma (NPC) patients, the brachial plexus (BP) situated at both sides of the neck is often irradiated to high dose. This study was to evaluate different BP delineation methods and analyse the dosimetric consequences when applying BP dose constraints in radiotherapy planning of NPC. Methods: 15 NPC cases radically treated with helical tomotherapy were recruited. Apart from the original treatment plan (Plan A), two new plans (Plans B and C) with additional BP dose constraints were computed using the same planning CT images, structures and planning parameters. Plan B consisted of BP contours based on Radiation Therapy Oncology Group (RTOG)endorsed atlas: while those in Plan C were based on MR images registered with the planning CT images. Results: The mean BP volume by RTOG method was 19.04 +/- 3.50 cm(3) vs 10.44 +/- 2.00 cm(3) by CT/MRI method. The mean BP overlapping volume between the two contouring methods was 1.9 cm(3) (0.38-4.03 cm(3)). There was significant difference between two methods (p < 0.001). The average D-max, D-mean, D-5%, D-10% and D-15% of both sides of BP in Plan A were significantly higher than those in both Plan B and Plan C. There were no significant dose differences in the targets and organs at risk (OARs) after applying dose constraints in Plan B and Plan C. Conclusion: RTOG method was recommended since larger BP volume provided better protection. Applying BP dose constraints during tomotherapy plan optimisation for NPC patients could significantly reduce the BP dose (p < 0.05) without compromising the doses to the targets and other OARs. Advances in knowledge: This is the first study comparing the delineation method based on RTOG-endorsed atlas with the conventional CT/MRI delineation method for BP in tomotherapy of NPC patients. Our results showed that BP dose could be significantly reduced after applying the dose constraints without compromising the doses to the target volumes and other OARs. The RTOG method was more favoured as it gave a relatively larger BP volume and therefore offered better organ sparing.