(Monday, 7/30/2018) 9:30 AM - 10:00 AM
Room: Exhibit Hall | Forum 4
Purpose: American Brachytherapy Society has recently recommended EQD2 ≥48Gy to 90% of the CTV for brachytherapy alone or ≥65Gy for EBRT plus HDR with dose limits of 70-75 Gy to 2cc of the recto-sigmoid, 80-100 Gy to the bladder and 65 Gy to the bowel. Composite plans are not available for anatomic changes from applicator insertions and therefore summed doses from each HDR plan are used for treatment decisions. We want to assess the actual doses in composite plans.
Methods: The CTV, CTV minus applicator (CTV-app), bladder, bladder-wall and recto-sigmoid were defined in 40 HDR plans plus 10 EBRT plans for eight patients. DICOM image, dose, and structure data sets were transferred to a deformable image registration system (Velocity). Rigid applicator matching was performed followed by deformable registrations using contract-on and off as well as structure-guidance until the anterior recto-sigmoid, posterior bladder, applicator and uterus were closely matched. The final deformation was applied to the dose matrix and associated structures. DVHs of the matched structures were created from the composite plans.
Results: Summed EQD2 of 90% CTV and D2cc of organs at risk to individual patients were about 5% higher than the composite doses. Both summed and composite EQD2 of 90% CTV is 2-8% higher than that of EQD2 of 90% CTV-app. This difference increases to 16% for HDR alone using multiple channel cylinders. Interestingly, the summed D10cc to the bladder-wall is generally lower than the composite dose.
Conclusion: We recommend using CTV-app for tumor control evaluation and recognize that the summed EQD2 to the CTV is approximately 10% higher than the composite EQD2 to CTV-app. Summed D2cc to the OARs are up to 5% higher than the composite plan D2cc. Users should also know that the bladder and rectal walls have a summed D10cc lower than their composite plans.
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