[Creation and evaluation of a concave dose distribution by combining multiple arc fields].

To create a concave dose distribution, a partial shielding radiation technique or intensity-modulated radiation therapy (IMRT) is usually required. However, in the present study we focused on how to create a concave dose distribution using conventional irradiation techniques. A treatment plan was experimentally created using planning CT scans of the neck. Two target volumes were predefined: planning target volume (PTV) 1, which included macroscopic tumor volume, tonsil, and bilateral retropharyngeal node, and PTV2, which included macroscopic and microscopic tumor volume. The prescribed doses for PTV1 and PTV2 were 66 Gy and 50 Gy, respectively. Nine isocenters, 7 in PTV2 and 2 on the sides of PTV2 were arranged equally spaced. Seven of the 9 arcs were divided in two arcs in order to avoid irradiating the spinal cord and salivary glands. Thus, 9 arcs were used in combination with a field size of 4-5 cm x 9-13 fields. Sixteen Gy was given to each isocenter with 10 MV photons. The plan was compared with a conventional plan (lateral opposing fields with electron boost) by analyzing the dose-volume histogram and dose distributions. The horseshoe-like distribution exceeding 66 Gy becomes conformal to PTV1, and the V95 of PTV1 (volume receiving 95% of the prescribed dose) was compatible with the conventional plan. On the other hand, maximum spinal cord dose decreased from 51 Gy with the conventional plan to 40 Gy with the 9-arc plan, and parotid gland volume (%) irradiated with > 32 Gy was reduced from 99% with the conventional plan to 72% with the 9-arc therapy. Lower normal tissue doses to the spinal cord and salivary gland, while maintaining the target dose, are achieved using the multiple arc plan, and the technique presented may be convenient and useful for facilities that do not yet have full access to IMRT.