External evaluation of the Radiation Therapy Oncology Group brachial plexus contouring protocol: Several issues identified

Radiation-induced brachial plexus toxicity after SBRT of apically located lung lesions

Purpose: To evaluate the rate and dose response of brachial plexus toxicity post stereotactic body radiation therapy (SBRT) of apically situated lung lesions. Material/methods: We retrospectively identified all patients with apically located tumors, defined by the epicenter of the tumor being located superiorly to the aortic arch, and treated with SBRT between 2008 and 2013. Patients with a shorter follow-up than 6 months were excluded. Primary aim was to evaluate radiation-induced brachial plexopathy (RIBP). Dose to the plexus was assessed by a retrospective delineation of the brachial plexus on the CT used for treatment planning. Then, D_max, D_0.1cc, D_1cc and D_3.0cc of the brachial plexus were collected from the dose-volume histograms (DVH) and recalculated to the biologically effective dose (BED) using α/β = 3 Gy. A normal tissue complication probability (NTCP) model, based on four different dose-volume parameters (BED_3,max, BED_3,0.1cc, BED_3,1.0cc, BED_3,3.0cc) was fitted to the data. Results: Fifty-two patients with 56 apically located tumors were identified. Median prescription dose per fraction was 15 Gy (range 6-17) and median number of fractions was 3 (3-10). With a median follow-up of 30 months (6.1-72) seven patients experienced maximum grade 2 (scored 3 times) or 3 (scored 4 times) RIBP after a median of 8.7 months (range 4.0-31). Three patients had combined symptoms with pain, sensory and motor affection and four patients had isolated pain. Median BED_3,max for the patients experiencing RIBP was 381 Gy (range 30-524) versus BED_3,max of 34 Gy (range 0.10-483) for the patients without RIBP. The NTCP models showed a very high predictive ability (area under the receiver operating characteristic curve (AUC) 0.80-0.88). Conclusion: SBRT of apically located lung lesions may cause severe neurological symptoms; for a three-fraction treatment, we suggest that the maximum dose to the plexus should be kept ≤30 Gy (130 Gy BED₃).

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 cm³ vs 10.44 ± 2.00 cm³ by CT/MRI method. The mean BP overlapping volume between the two contouring methods was 1.9 cm³ (0.38-4.03 cm³). 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.

Uncertainties in volume delineation in radiation oncology: A systematic review and recommendations for future studies

BACKGROUND AND PURPOSE

Volume delineation is a well-recognised potential source of error in radiotherapy. Whilst it is important to quantify the degree of interobserver variability (IOV) in volume delineation, the resulting impact on dosimetry and clinical outcomes is a more relevant endpoint. We performed a literature review of studies evaluating IOV in target volume and organ-at-risk (OAR) delineation in order to analyse these with respect to the metrics used, reporting of dosimetric consequences, and use of statistical tests.

METHODS AND MATERIALS

Medline and Pubmed databases were queried for relevant articles using keywords. We included studies published in English between 2000 and 2014 with more than two observers.

RESULTS

119 studies were identified covering all major tumour sites. CTV (n=47) and GTV (n=38) were most commonly contoured. Median number of participants and data sets were 7 (3-50) and 9 (1-132) respectively. There was considerable heterogeneity in the use of metrics and methods of analysis. Statistical analysis of results was reported in 68% (n=81) and dosimetric consequences in 21% (n=25) of studies.

CONCLUSION

There is a lack of consistency in conducting and reporting analyses from IOV studies. We suggest a framework to use for future studies evaluating IOV.