Tissue compensation and verification of dose uniformity

The problem of nonuniform dose distribution due to irregular and sloping surfaces is frequently encountered during treatment planning whose ultimate goal is to deliver uniform dose to a specific target volume. In order to overcome this problem, various tissue compensation systems have often been employed. The purpose of the present study was to evaluate the computerized tissue compensation system, Autocomp, manufactured by Nuclear Associates, for its ability to obtain dose uniformity both in phantoms and in clinical use. For the phantom studies, a film was placed below a tissue-equivalent phantom with a 45 degrees sloping surface and exposed to either a compensated or an uncompensated beam of 60Co. The field profiles scanned by a computerized film scanning densitometer showed significant improvement in dose uniformity when the compensator was used. For the clinical study, a left buccogingival sulcus carcinoma treated with 60Co gamma ray was chosen to verify the dose uniformity at a depth over the entire field of irradiation. A comparison of two isodose distributions, obtained with and without tissue compensation, indicated the elimination of the nonuniform dose distribution.