A study of the factors affecting radiation-induced temporomandibular joint changes in post-radiotherapy nasopharyngeal carcinoma patients

Radiotherapy of nasopharyngeal carcinoma (NPC) often delivers considerable radiation dose to the temporomandibular joint (TMJ). This may lead to radiation-induced trismus, which can have impact on the quality of life of post-radiotherapy (post-RT) patients. However, the exact cause of trismus and its relationship with dose has not been fully understood. This study aimed to evaluate the quantitative relationship between severity of post-RT trismus and TMJ doses and other patient characteristics. Through ultrasound examination, this study also aimed to identify factors affecting morphological changes of TMJ. Ninety-seven NPC patients treated by radiotherapy for more than 4 years were recruited. The doses to both sides of the TMJ were calculated from the treatment planning system. The severity of trismus was assessed by measuring the maximum inter-incisal distance (MID) with a caliper. Ultrasonography examination of the both sides of TMJ was performed in both transverse and longitudinal directions. For the scan of each TMJ, the MID, irregularity of mandibular condyle and the echogenicity of the joint disc ligament and pterygoid muscles were assessed. The median age of all the subjects was 56 years, with median post-RT duration of 14 years and median mean TMJ dose of 64.15 Gy. The MID (median, 3.7 cm, range, 1.3-5.2 cm) demonstrated strong negative correlations with post-RT duration (r (p) = -0.441, p < 0.0005) and mean doses to the TMJ (r (p) = -0.262, p = 0.009). The negative relationship between TMJ dose and maximum disc thickness (MDT) was also found in transverse scan (r (p) = -0.218, p = 0.002). However, there is no significant relationship between TMJ dose and MDT in longitudinal scan, irregularity of condyle and echogenicity of pterygoid muscle. In post-RT NPC patients, the severity of trismus, expressed in MID, increased with the doses to the TMJ and post-RT duration. In addition, the joint disc thickness decreased with increased dose to TMJ. Keeping a reasonably low TMJ dose in treatment planning and longer-term monitoring of the trismus of post-RT NPC patients are suggested to ensure their quality of life.