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Background  In intensity modulated radiotherapy (IMRT) of nasopharyngeal carcinoma (NPC), accurate delineation of the gross tumour volume (GTV) is important. Image registration of CT and MRI has been routinely used in treatment planning. With recent development of positron emission tomography (PET), the aims of this study were to evaluate the impact of PET on GTV delineation and dosimetric outcome in IMRT of early stage NPC patients.    Methods  Twenty NPC patients with T1 or T2 disease treated by IMRT were recruited. For each patient, 2 sets of NP GTVs were delineated separately, in which one set was performed using CT and MRI registration only (GTV CM ), while the other set was carried out using PET, CT and MRI information (GTV CMP ). A 9-field IMRT plan was computed based on the target volumes generated from CT and MRI (PTV CM ). To assess the geometric difference between the GTV CM  and GTV CMP , GTV volumes and DICE similarity coefficient (DSC), which measured the geometrical similarity between the two GTVs, were recorded. To evaluate the dosimetric impact, the D max , D min , D mean  and D 95  of PTVs were obtained from their dose volume histograms generated by the treatment planning system.    Results  The overall mean volume of GTV CMP  was greater than GTV CM  by 4.4 %, in which GTV CMP  was slightly greater in the T1 group but lower in the T2 group. The mean DSC of the whole group was 0.79 ± 0.05. Similar mean DSC values were also obtained from the T1 and T2 groups separately. The dosimetric parameters of PTV CM  fulfilled the planning requirements. When applying this plan to the PTV CMP , the average D min  (56.9 Gy) and D 95  (68.6 Gy) of PTV CMP  failed to meet the dose requirements and demonstrated significant differences from the PTV CM  ( p  = 0.001 and 0.016 respectively), whereas the doses to GTV CMP  did not show significant difference with the GTV CM .    Conclusion  In IMRT of early stage NPC, PET was an important imaging modality in radiotherapy planning so as to avoid underdosing the PTV, although its effect on GTV delineation was not significant. It was recommended that PET images should be included in the treatment planning of NPC patients.

radiation oncology

The impact of positron emission tomography on primary tumour delineation and dosimetric outcome in intensity modulated radiotherapy of early T-stage nasopharyngeal carcinoma