Predicting the severity and prognosis of trismus after intensity-modulated radiation therapy for oral cancer patients by magnetic resonance imaging

Abstract

To develop magnetic resonance imaging (MRI) indicators to predict trismus outcome for post-operative oral cavity cancer patients who received adjuvant intensity-modulated radiation therapy (IMRT), 22 patients with oral cancer treated with IMRT were studied over a two-year period. Signal abnormality scores (SA scores) were computed from Likert-type ratings of the abnormalities of nine masticator structures and compared with the Mann-Whitney U-test and Kruskal-Wallis one-way ANOVA test between groups. Seventeen patients (77.3%) experienced different degrees of trismus during the two-year follow-up period. The SA score correlated with the trismus grade (r = 0.52, p<0.005). Patients having progressive trismus had higher mean doses of radiation to multiple structures, including the masticator and lateral pterygoid muscles, and the parotid gland (p<0.05). In addition, this group also had higher SA-masticator muscle dose product at 6 months and SA scores at 12 months (p<0.05). At the optimum cut-off points of 0.38 for the propensity score, the sensitivity was 100% and the specificity was 93% for predicting the prognosis of the trismus patients. The SA score, as determined using MRI, can reflect the radiation injury and correlate to trismus severity. Together with the radiation dose, it could serve as a useful biomarker to predict the outcome and guide the management of trismus following radiation therapy.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1. Describe the trismus related muscles and structures that were targeted by colors in one of enrolled patients.

Light blue: left side medial pterygoid muscle; Pink: left side lateral pterygoid muscle; Light orange: left side temporalis; Forest green: left side masseter; Steel blue: left side temporomandibular joints. Sky blue: right side medial pterygoid muscle; Yellow: right side lateral pterygoid muscle; Yellow green: right side temporalis; Orange: right side masseter; Purple: right side temporomandibular joints.

Figure 2. Illustrated T1W MRI post-contrast enhancement of trismus patient after RT.

(a) Coronal view with fat saturation showed radiation effects in right lateral (white arrows), medial (black arrows) pterygoid muscles and masseter muscle (white arrowhead) with increased enhancement. (b) Coronal view showed atrophic change of the right lateral (white arrows), medial (black arrows) pterygoid muscles and masseter muscle (white arrowhead). (c) Axial view with fat saturation showed remarkable fibrotic tissue (white arrow) occupying right maxillary sinus and pterygoid space. (d) Axial view with fat saturation showed increased enhancement with atrophic change of the right parotid gland. All these findings were rated as point 2 according to our Likert scoring system of signal abnormality (SA score).

Figure 3. Comparison of signal abnormality scores between different groups of trismus severity.

Significant difference in the SA scores between groups is noted (p = .03). The grade 3 group (most severe one) has the highest score and the grade 1 (least severe one) group has the lowest score.

Figure 4. Correlation of SA score with radiation dose and the time trend of trismus development.

(a) Correlation of the received radiation dose of individual masticator structures with their two year SA scores summation (r = 0.48, p

Figure 5. Comparison of radiation doses and SA scores between good prognosis and poor prognosis trismus groups.

A significant difference is noted in the mean dose of masticator structures (p = .04; Fig. 5a), mean dose of masticator muscles (p = .05; Fig. 5b), mean dose of lateral pterygoid muscles (p = .04; Fig. 5c), mean dose of parotid glands (p = .01; Fig. 5c), the product of the six month SA score with mean masticator muscles dose (p = .04; Fig. 5d) and the twelve month SA score (p = .01; Fig. 5e). (MP: medial pterygoid muscle; LP: lateral pterygoid muscle; Temp: temporalis muscle; TMJ: temporomandibular joint; PG: parotid gland; SA score: signal abnormality score; GP: good prognosis; PP: poor prognosis).