[A bi-isocentric irradiation technique in radiotherapy for nasopharyngeal carcinoma].

Background & Objective: Although the problem of fields gap in radiotherapy for nasopharyngeal carcinoma (NPC) was solved by mono-isocentric technique, the dose distribution was heterogeneous with "hot" and/or "cold" spots. This study was to explore the best way to solve the problems of both fields gap and dose distribution using bi-isocentric technique.

Methods: Mono-isocentric and bi-isocentric irradiation techniques were used to simulate irradiation, respectively. Mono-isocentric irradiation technique was used to design the beam-split facio-upper neck field and the tangential low neck-supraclavicular field at the match plane. Bi-isocentric irradiation technique was used to design the facio-upper neck field and the tangential low neck-supraclavicular field at the match plane, namely the upper border of the tangential field and the lower border of the facio-upper neck fields.

Results: Both mono-isocentric and bi-isocentric irradiation techniques were conformed well to QA at the junction of the neighboring portals, without "hot" or "cold" spots, by dosimetry. But the CADplan Three Dimensions Treatment Planning System showed that the dose distributional uniformity of sagittal plane in nasopharynx was much better in bi-isocentric technique than in mono-isocentric irradiation technique, without cold region in gross tumor volume (GTV) and/or hot region in normal tissue.

Conclusions: Bi-isocentric irradiation technique takes a perfect match-plane and a satisfied dose distributional uniformity.