The degree and time-course assessment of radiation-induced trismus occurring after radiotherapy for nasopharyngeal cancer

Objectives/Hypothesis. The objectives were to measure the degree of trismus induced after radiation therapy for nasopharyngeal cancer and assess its progress over time. Study Design: A prospective, single-armed measurement study with long-term follow-up. Methods. Seventeen patients with nasopharyngeal cancer treated between 1997 and 1999 were studied. Patients were given radiation therapy with bilateral parallel-opposing ports of 45 Gy, 25 fractions, then with a reduced volume to 68.4 to 70.2 Gy. The end point was the degree of trismus, which was measured by serial changes of the maximal interincisal distance (MID) at various specified time points before, during, and after radiation therapy. Results. During the 9 weeks of radiation therapy there was no significant change of MID (normalized MID ranged from 99.8% to 97%). The rate of decrease during this period was 1.3% per month. After radiation therapy there was a rapid decrease of MID between 1 and 9 months (normalized MID values at 1 and 9 mo were 95.5% +/- 3.1% and 74.2% +/- 5.7%, respectively). The rate of decrease during this period was dramatic (2.4%/mo). One year after radiation therapy, the rate of decrease became slower but was still measurable (0.2%/dmo for the period from 12 to 24 mo). For the period from 24 to 48 months. the rate dropped to 0.1% per month. By the end of 48 months, normalized MID was 67.8% +/- 7.6%. Conclusion: By means of measurement over a period of time, it was found that patients with nasopharyngeal cancer had a mean decrease in initial interincisal distance of 32% at 4 years after radiotherapy. The trismus process evolved at different rates. It was rapid at 1 to 9 months after radiation therapy, then became slower and protracted over later years.