Dose analysis of boost irradiation of parapharyngeal space in nasopharyngeal carcinoma

Parapharyngeal space (PPS) is one of the common spreads of nasopharyngeal carcinoma (NPC). Since early eighties boost treatment has been routinely given to patients with PPS involvement. The use of oblique facio-cervical field in this treatment posed difficulty in assessing the dose distribution before the 3-D computer systems are available. This study evaluated the variations of dose distribution by different boost techniques on fifteen patients with PPS involvement using 3-D computer planning system. Comparisons were made between different radiation types, photon energies and modes of collimation respectively. Results showed that the dose distributions in the boost treatment were acceptable, as they did not bring the total doses of the normal structures exceeding their tolerance. There was little difference between 6 and 10 MV photons, but significant differences were found between photon and electron beams. Photon beams were better than the electron beams in giving lower doses to the spinal cord and the TM joint with more homogeneous dose to the target volume, whereas electron beams gave better sparing of the lens and temporal lobe. For the photon beams, half-blocked beam was superior to symmetrical beam in reducing the brain stem dose, but there was no advantage to the rest of normal structures.