Simple DVH parameter addition as compared to deformable registration for bladder dose accumulation in cervix cancer brachytherapy.

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. D(2cm(3)) and D(0.1cm(3)) 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(αβ3) (1.5±1.8%) and 1.9±1.6 Gy(αβ3) (5.2±4.2%) for D(2cm(3)) and D(0.1cm(3)), respectively. Dose deviations greater than 5% occurred in 2% and 38% of the patients for D(2cm(3)) and D(0.1cm(3)), 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 D(2cm(3)), whereas D(0.1cm(3)) is less robust to this approximation.