Evaluation of Integrated Equivalent Dose in 2 Gy (EQD2) DVH of EBRT and HDR Plans for Cervical Cancer
Y Kim1*, C Basak2, C Mart1, A Sheybani1, K Cheung3, W Sun1, S Bhatia1, (1) Radiation Oncology Dept, The University of Iowa, Iowa City, Iowa, USA (2) Biomedical Engineering Dept, The University of Iowa, Iowa City, Iowa, USA (3) Computer Science Dept, The University of Iowa, Iowa City, Iowa, USASU-D-137-2 Sunday 2:05PM - 3:00PM Room: 137
Purpose: To evaluate equivalent dose in 2 Gy (EQD2) parameters from an integrated EQD2 DVH of EBRT and HDR plans and compare them with the estimated values per GEC-ESTRO recommendations.
Methods: EBRT plans (Pinnacle, Phillips) consist of 45Gy (1.8Gy/fraction) and, if necessary, a 5.4⁻9 Gy (1.8Gy/fraction) parametrial and/or paraaortic boost. An HDR brachytherapy plan (BrachyVision, Varian) was generated per each implant with 27.5Gy/5fractions ⁻ 28Gy/4fractions. Fifteen point A plans and six conformal (volume⁻optimization) plans were retrospectively analyzed. Rectum and bladder were contoured on T2-weighted MRI. An EQD2 DVH tool was developed in-house to convert initial dose maps into EQD2 maps for the generation of EQD2 DVHs. The integrated D2cc and D0.1cc EQD2 values were compared to those calculated per the recommendations: assuming that the OARs receive full prescription dose from the initial EBRT and paraaortic boost, but receive no additional dose from the parametrial boost due to its 4 cm central block.
Results: All cases having a parametrial boost showed D2cc and D0.1cc EQD2 doses to the rectum and bladder that were higher than those calculated via the recommended method. The impact of this integrated analysis technique on the D2cc EQD2 values for the rectum and bladder was noted to be larger for conformal plans than for point A plans. Up to 28.8% (22.3%) and 22.9% (16.3%) higher D2cc EQD2 values were recorded for the rectum and bladder, respectively, for conformal plans (point A plans). The integrated D2cc EQD2 values for the rectum and bladder were, on average, 9.4 ⁺⁻ 10.1% (7.3 ⁺⁻ 4.7%) and 10.0 ⁺⁻ 6.8% (5.1 ⁺⁻ 4.7%) for conformal plans (point A plans).
Conclusion: The integrated maximum dose to OARs could be considerably underestimated for the rectum and bladder when GEC-ESTRO recommended methodology is made.