EVALUATING THE RELIABILITY OF ORTHOGONAL IMAGE BASED PLANNING IN INTERSTITIAL BRACHYTHERAPY OF TONGUE USING CONE-BEAM CT

To compare target dose coverage, dose homogeneity and dose received by mandible in orthogonal image based interstitial brachytherapy using Cone beam CT.Materials and methods - 6 HDR interstitial implantations analysed. Conventional orthogonal and CBCT images were acquired after implantation. Orthogonal image based planning done in PLATO TPS. CBCT images were imported to Oncentra TPS. Target volumes and OAR (mandible) contoured. Catheters, source positions and dwell times based on conventional orthogonal images were superposed and co registered to the contoured CBCT images. Dosimetric outcomes evaluated qualitatively by visualizing isodose distribution on CT images and quantitatively using the dose volume parameters, like coverage relative dose homogeneity index (HI), overdose volume index (OI) , conformal index (COIN), dose non uniformity ratio (DNR) and natural dose ratio (NDR). Mandible 5cc volume dose was used as a parameter in evaluating the plans. Results - Out of 6 patients with a median follow up of 16-18 months. In regard to dosimetric indices good target coverage by the prescription dose was achieved with geometric optimization mean V100 of 79.97 (8 SD), with 150 and 200 of the target volume receiving 31.16 and 6.74 of prescription dose respectively. Conformity and homogeneity were good with mean COIN 0.61 (0.05) and mean HI 0.49 (0.10) providing better conformity and homogeneity to the target. Conclusion - Even though target coverage was adequate in orthogonal image based planning, the CBCT verification resulted in modification of dwell time and source positions to avoid excess dose to mandible in certain plans. We conclude that conventional orthogonal image based planning is equivalent to CT based planning in terms of target coverage but CBCT based planning can accurately predict the dose to the organs at risk (mandible).