Optimal radiation field to design a uniform radiation dose distribution within the planning target volume of early glottic cancer

The International Commission on Radiation Units and Measurements (ICRU) Report 50 have recommended that one has to force a certain degree of heterogeneity, which today in the best technical and clinical conditions should be kept within +7% and -5% of prescribed dose. To follow this recommendation closely and assure the planning target volume within the 95%-isodose line in applying radiotherapy to early glottic cancer, the author studied how to determine the appropriate radiation field size in the two lateral parallel opposing fields technique. The author defined the superior boundary of the planning target volume at the superior margin of the thyroid cartilage, the interior boundary at the superior margin of the anterior portion of the cricoid cartilage, the anterior boundary at the posterior surface of the thyroid cartilage, and the posterior boundary at the anterior surface of the cricoid cartilage, respectively. First the mean size of the planning target area for 25 patients was measured. The average size of 2.8 x 3.6 cm2 was disclosed. Then phantom model experiments were conducted to clarify the appropriate radiation field size for keeping a heterogeneity of dose distribution of the planning target volume within +7% and -5% of central axis depth dose. A radiation field 5 cm in length and 6 cm in height seemed appropriate for a target area 2 to 3 cm in width and 2.7 to 3.7 cm in height. However, a smaller or larger target area required a radiation field 4 cm or 6 cm in width, and 5 cm or 7 cm in length. When the distance between the anterior neck surface and the posterior surface of the anterior portion of the cricoid cartilage was less than 5 mm, a low-dose area appeared in the anterior portion of the planning target volume. In such patients, the anterior border of the field should be placed more than 10 mm anteriorly to the anterior neck skin surface. (author)