Aim:

            To compare the target dose coverage, dose homogeneity and dose received by the mandible in orthogonal image based interstitial brachytherapy using Cone beam CT.

Materials and methods

           Six HDR interstitial implantations done at our institute were taken in this study. Both conventional orthogonal and CBCT images were acquired after implantation. Orthogonal image based planning was done in PLATO treatment planning system. CBCT images were imported to Oncentra treatment planning system. Target volumes; organ at risk (mandible) were contoured. Catheters, source positions and dwell times based on conventional orthogonal images were superposed and co registered to the contoured CBCT images.Dosimetric outcomes were evaluated qualitatively by visualizing isodose distribution on CT images and quantitatively using the dose volume parameters, which includes coverage relative dose homogeneity index (HI), overdose volume index (OI) , conformal index (COIN), dose nonuniformity ratio (DNR) and natural dose ratio (NDR).Mandible 5cc volume dose was also used as a parameter in evaluating the plans.

Results:           

               Out of 6 patients with a median follow up of 16–18 months, all of them developed mucositis post treatment lasted for 6–8 wks. One patient had local reccurence and underwent salvage glossectomy. Another patient had ipsilateral level II nodal recurrence which was FNAC positive and underwent ipsilateral radical neck dissection. Remaining 4 patients had prophylactic superior homohyoid neck dissection (SOHND). In regard to dosimetric indices good target coverage by the prescription dose was achieved with geometric optimization (mean V100 of 82.52% (14 SD), with 150% and 200% of the target volume receiving 36.7% and 17.99% of prescription dose, respectively. Conformity and homogeneity were good with mean COIN = 0.005 and mean HI = 0.48 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).