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

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.