Study on delineation of gross tumor volume (GTV) of primary locally advanced nasopharyngeal carcinoma after induction chemotherapy

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Objective To investigate the delineation primary lesions of the gross tumor volume (GTVnx) in locally advanced nasopharyngeal carcinoma (LANC). Methods During 2012–2013, 52 patients with LANC treated with two-three cycles of induction chemotherapy accepted CT simulation and MRI scans in the same treatment position. All the images of CT and T1W1 sequence of MRI were collected and transmitted to the treatment planning system. The gross target volume (GTVnx) was delineated through CT and MRI images. Meanwhile, metastatic lymph nodes, CTV1, CTV2 and organs at risk(OAR) were delineated through CT images alone. The images from MRI and CT were then fused as MRI/CT fusion images by radiotherapy system, and the same prescription dose and limitation dose of OAR were given as CT images. The changes of GTVnx, the radiation volume and dose, and dose coverage of normal organs from two different images were compared. Results The delineation volume in fused MRI/CT image [(43.14±28.40 cm3)] was larger than CT [(40.09±27.04) cm3] (t=3.791, P<0.001). And there was statistical difference between the two images in the GTV changes before and after IC, with MRI/CT (27.90±11.86) and CT [(30.64±11.86) cm3](t=3.948, P<0.001), respectively. The radiation volume in MRI/CT fusion image [(41.71±26.86) cm3] was larger than CT [(38.65±25.66) cm3] (t=4.098, P<0.001). But there were no statistical differences in target volume dose and dose coverage of normal organs between the two images. Conclusions The technique of MRI/CT fusion image can increase the radicalized GTV and may decrease the possibility of leaving out the target volume definition. Key words: Locally advanced nasopharyngeal carcinoma; Induction chemotherapy; MRI/CT fusion image; Target volume delineation