Assessment of the Impact of Deformable Registration of Diagnostic MRI to Planning CT on GTV Delineation for Radiotherapy for Oropharyngeal Carcinoma in Routine Clinical Practice

Background: Aim of study was to assess impact of deformable registration of diagnostic MRI to planning CT upon gross tumour volume (GTV) delineation of oropharyngeal carcinoma in routine practice. Methods: 22 consecutive patients with oropharyngeal squamous cell carcinoma treated with definitive (chemo)radiotherapy between 2015 and 2016, for whom primary GTV delineation had been performed by a single radiation oncologist using deformable registration of diagnostic MRI to planning CT, were identified. Separate GTVs were delineated as part of routine clinical practice (all diagnostic imaging available side-by-side for each delineation) using: CT (GTV(CT)), MRI (GTV(MR)), and CT and MRI (GTV(CTMR)). Volumetric and positional metric analyses were undertaken using contour comparison metrics (Dice conformity index, centre of gravity distance, mean distance to conformity). Results: Median GTV volumes were 13.7 cm(3) (range 3.5–41.7), 15.9 cm(3) (range 1.6–38.3), 19.9 cm(3) (range 5.5–44.5) for GTV(CT), GTV(MR) and GTV(CTMR) respectively. There was no significant difference in GTV(CT) and GTV(MR) volumes; GTV(CTMR) was found to be significantly larger than both GTV(MR) and GTV(CT). Based on positional metrics, GTV(CT) and GTV(MR) were the least similar (mean Dice similarity coefficient (DSC) 0.71, 0.84, 0.82 for GTV(CT)–GTV(MR), GTV(CTMR)–GTV(CT) and GTV(CTMR)–GTV(MR) respectively). Conclusions: These data suggest a complementary role of MRI to CT to reduce the risk of geographical misses, although they highlight the potential for larger target volumes and hence toxicity.