Background and purpose: Variations in organ position, shape, and volume cause uncertainties in dose assessment for brachytherapy (BT) in cervix cancer. The purpose of this study was to evaluate uncertainties associated with bladder dose accumulation based on DVH parameter addition (previously called "the worst case assumption") in fractionated BT. Materials and methods: Forty-seven patients treated for locally advanced cervical cancer were included. All patients received EBRT combined with two individually planned 3D image-guided adaptive BT fractions. D2 and D0.1 were estimated by DVH parameter addition and compared to dose accumulations based on an in-house developed biomechanical deformable image registration (DIR) algorithm. Results: DIR-based DVH analysis was possible in 42/47 patients. DVH parameter addition resulted in mean dose deviations relative to DIR of 0.4 ± 0.3 Gy (1.5 ± 1.8%) and 1.9 ± 1.6 Gy (5.2 ± 4.2%) for D2 and D0.1 respectively. Dose deviations greater than 5% occurred in 2% and 38% of the patients for D2 and D0.1 respectively. Visual inspection of the dose distributions showed that hotspots were located in the same region of the bladder during both BT fractions for the majority of patients. Conclusion: DVH parameter addition provides a good estimate for D2 whereas D0.1 is less robust to this approximation.
Radiotherapy and Oncology, 2013, Vol 107, Issue 1, p. 52-57