DOSIMETRY OF STANDARD OVOID APLICATORS WITH DIFFERENT DIAMETERS AND NORMALISATION TECHNIQUES

AIM: Investigating the effect of normalization techniques (NT) on dosimetry of rectal and vaginal reference points (RPs) using different cap sizes of ovoids. METHODS: HDR Ir192 microselectron unit and standard ovoid applicator set with four different cap sizes used, Orthogonal localization films were obtained for the four different cap sizes. ICRU38 rectum reference point (R, and four more rectum points were defined. The vaginal cuff (Vc) reference point was defined at 5 mm depth in the point normalization. In the OCNT 5 vajinal reference points were defined in the distal part of each active source at 5 mm depth transvaginal surface (total: 10 points). Using two different NT's, eight treatment plans were formed in the TPS. The w, h, h', t, f of the reference isodoses were measured and v, v' was calculated. RESULT: R1 always received the max dose and increased as the cap size increased in both NT's. The percent dose difference of the mean rectum doses (R mean) was 31 %- 4.4% with the same trend. The percent dose of OCNT approached to 100% as the cap size increased. In VCNT, the percent mean dose of OCNT also became closer to 100% with the increase of cap size. In both NT's, the w showed the most significant increase as the cap size enlarged. As the cap size increased both v and v' values rose. CONCLUSION: As the cap size increased, R1, as well as R mean doses for each normalization technique approached to their counterparts. Also VCNT and OCNT vaginal reference doses, closed to defined 100% reference dose. These results are in accordance with 'inverse square law'. Thus the selection of the normalization techniques loses its importance as the cap size increases.

Keywords: brachytherapy, normalization techniques, rectum reference doses