Dosimetric effects of air cavity on target volume and organs at risk during intensity-modulated radiation therapy for nasopharyngeal carcinoma

Objective To evaluate the dosimetric effects of air cavity on primary tumor and organs at risk (OARs) during intensity-modulated radiation therapy (IMRT) for nasopharyngeal carcinoma patients. Methods Nine patients with nasopharyngeal carcinoma had CT simulation before treatment and on the 25 fraction of radiotherapy. Radiotherapy plan1 was first created by delineating the target volumes and OARs on the first CT image, which was then copied and merged with the second CT image. Air cavity that was formed following tumor volume regression was delineated on the first CT image, and the density of air cavity was set to zero. A new gross target volume (GTV) was formed by subtracting the air cavity from the original GTV, and the new CT image was named CTAir. Plan2 was then created by calculating the dose distribution on CTAir using the same portals and parameters as Plan1. Assuming that Plan1 and Plan2 were both used throughout the course of radiotherapy, the dosimetric parameters of nasopharyngeal tumor and OARs in the absence and presence of air cavity were then compared using the paired t-test. Results Dmean, D95, D90, D10, and D5 of tumor were significantly higher in plan2 than in plan1(P=0.000, 0.001, 0.001, 0.001, and 0.005, respectively), with a<0.5 Gy increase in dose. A dose build-up effect was observed within the tumor region posterior to the air cavity, where the highest dose was 1.4 cm posterior to the air cavity, resulting in a dose difference of 0.36 Gy. In addition, D2cc and D1cc of the brain stem were significantly higher in plan2 than in plan1(P=0.036 and 0.044, respectively). Dmax of the optical chiasm, left optical nerve, and right optical nerve were also increased in the presence of air cavity (P=0.438, 0.434, and 0.477, respectively), but the change in dose was<0.12 Gy. Conclusions Air cavity induces a small but negligible increase in the tumor and OARs dose in patients with nasopharyngeal carcinoma during IMRT. However, closer monitoring should be conducted for patients with OARs that is close to or has surpassed tolerance prior to radiotherapy. Key words: Nasopharyngeal neoplasms/intensity-modulated radiotherapy; Air cavity effects; Organs at risk