Impact of intravenous contrast used in computed tomography (CT) scan on dose to the carotids and thyroid in IMRT planning for nasopharyngeal carcinoma (NPC)

Abstract

PURPOSE/OBJECTIVE(S): IMRT is the standard treatment for NPC. Intravenous contrast during planning CT is usually injected for better delineation of structures, in particular carotid arteries and thyroids. However patients are actually treated without contrast. We do not know if dose to these structures change after contrast, especially when there is a concern of radiation-induced carotid stenosis and 2nd malignancy. We wish to investigate if there is a difference of dose to these structures after contrast. MATERIALS/METHODS: 22 patients with stage I to IVB NPC (AJCC Staging Manual 6th Edition) who received radical IMRT +/- chemotherapy were included in this study. Non-contrast enhanced (NCE-CT) followed by contrast-enhanced computed tomography (CE-CT) scans with the same immobilization were performed in one session. 100ml of iopamidol containing 300mg iodine/ml was injected at a rate of 2ml/sec by injector during CE-CT, which was then used for contouring and planning. Carotid arteries and thyroids were outlined in both sets of CT images. Treatment plans were generated by computer planning systems for CE-CT images. CE-CT images were then co-registered manually (displacement discrepancy ≤2mm) with their NCE-CT images. The treatment plans for CE-CT were then copied to NCE-CT preserving the same beam shapes, angles and energies of CE-CT. Dose distributions on NCE-CT were re-calculated using this re-normalized plan so that differences in dose will be due to effect of contrast and displacement discrepancy. Volumes of the carotid arteries and thyroids on NCE-CT and CE-CT together with dosimetric parameters including D95, D50, D05, D01 and minimum, mean, maximum doses were compared by paired t-tests. RESULTS: For carotid arteries: (a) volume, maximum dose, D05 and D01 of right common carotid artery are significantly higher in NCE-CT (p value 0.000 - 0.001); (b) maximum and mean dose and D50 of right external carotid artery are higher in NCE-CT (p value 0.000); (c) volume, mean dose, D05 and D01 of right internal carotid artery are higher in NCE-CT (p value 0.000 - 0.002); (d) volume, maximum dose, D05 and D01 of left common carotid artery are higher in NCE-CT (p value 0.000-0.047); (e) minimum dose and D05 of left external carotid artery are higher in NCE-CT (p value 0.002 - 0.048); (f) maximum and mean dose, D50, D05 and D01 of left internal carotid artery are higher in NCE-CT (p value 0.000). For thyroids: both the volumes of right and left thyroid contoured on NCE-CT are higher as compared with CE-CT (p value 0.001 - 0.017). CONCLUSIONS: Dose and volumes of these structures are significantly higher without contrast. The volumes of these structures may be over-estimated on NCE-CT leading to an apparent increase in radiation dose. CE-CT is still preferred for better target delineation.