Comparison between CT- and MRI-derived head and neck cancer target volumes using an integrated MRI-tri- 60 Co teletherapy device

Abstract

Purpose

Radiation treatment planning is typically based on the identification of a gross tumor volume (GTV) using computed tomography (CT). The clinical implementation of an integrated MRI-radiation therapy delivery unit allows for a strict comparison of CT- and MRI-derived GTVs for head and neck cancer.

Materials and methods

Twenty-six consecutive patients with squamous cell carcinoma of the head and neck were selected and planned for intensity-modulated radiotherapy (IMRT) on a novel tri-⁶⁰Co teletherapy system equipped with a 0.35 T MRI (ViewRay Incorporated, Oakwood Village, OH). All patients had measurable disease. Pre-treatment MRIs were imported into a contouring interface where the primary GTV were assessed and compared to those obtained from a registered CT with the patient in the identical position and immobilization apparatus.

Results

The median GTV as derived from the CT and MRI was 27.2 cm³ (range 3.8 to 155.0 cm³) and 34.9 cm³ (range, 5.0 to 189.5 cm³), respectively (p = 0.01). The MRI-derived GTV was larger than the CT-derived GTV in 21 of the 26 cases and was smaller in the remaining 5 cases. Among the 21 cases where the MRI-derived GTV was larger, the median difference in absolute GTV per individual patient was 6.9 cm³ (range 2.1 to 33.4 cm³), representing a 25% difference on average. The median concordance index for patients with de novo versus recurrent disease was 0.83 and 0.66, respectively (p = 0.03).

Conclusion

Significant differences in GTV extent were noted between MRI- and CT-derived ViewRay images. The implications for treatment planning are discussed.

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