Dosimetric impact of iodinated contrast agent on planning CT for volumetric-modulated radiotherapy of head and neck cancer

Purpose

Volumetric-modulated arc therapy for head and neck cancers requires accurate delineation of target volumes and organs at risk. Iodinated contrast agents enhance visualization, but their impact on dose distribution remains a concern. Traditionally, dosimetry is performed on non-contrast CT scans to minimize dose calculation discrepancies. This study evaluates the dosimetric impact of planning directly on contrast-enhanced CT scans.

Material and methods

Twelve patients with head and neck cancers (oral cavity, oropharynx, pharyngolarynx, ethmoid, thyroid) treated using volumetric-modulated radiation technique were included. Target and organs at risk delineation and dose calculation were performed on contrast-enhanced CT scans and subsequently transferred to non-contrast CT scans without renormalization. Prescribed doses for high-risk planning target volume were 60 to 70 Gy, for low-risk planning target volume 54 Gy, in 30 to 33 fractions. Volumetric-modulated arc therapy planning was performed on Eclipse® (version 15.6), with simultaneous integrated boost, using the AAA dose calculation algorithm. Dosimetric variations in organs at risk and planning target volumes were assessed.

Results

Mean variations of mean dose for organs at risk mandible, larynx, parotid glands, and pharyngeal constrictor muscle ranged from 0.02 Gy (0.10 %) to 0.16 Gy (0.38 %). Mean D2 % variations for spinal canal, brainstem, brachial plexus, and carotid arteries ranged from 0.05 Gy (0.17 %) to 0.16 Gy (0.46 %). None of the variations for organs at risk were found to be statistically or clinically significant. For planning target volumes, variations in coverage by the 95 % isodose of the prescribed dose were always less than 1 %. The maximum variation of maximum dose was a decrease of 0.6 Gy, representing 0.8 %.

Conclusion

Performing volumetric-modulated arc therapy planning directly on contrast-enhanced CT scans leads to minimal dosimetric variations (less than 1 %) compared to non-contrast CT scans. This approach could streamline clinical workflows by eliminating the need for dual CT acquisitions and optimizing treatment preparation.