Determining DVH parameters for combined external beam and brachytherapy treatment: 3D biological dose adding for patients with cervical cancer

Purpose To compare two methods of DVH parameter determination for combined external beam and brachytherapy treatment of cervical cancer. Materials and methods Clinical treatment plans from five patients were used in this study. We simulated two applications given with PDR (32×60cGy per application, given hourly) or HDR (4×7Gy in two applications; each application of two fractions of 7Gy, given within 17h) standard and optimised treatment plans, all combined with IMRT (25×1.8Gy). Additionally, we simulated an external beam (EBRT) boost to pathological lymph nodes or the parametrium (7×2Gy). We determined D90 of the high-risk CTV (HR-CTV) and D 2cc of bladder and rectum in EQD2 in two ways. (1) ‘Parameter adding': assuming a uniform contribution of the EBRT dose distribution and adding the values of DVH parameters for the two brachytherapy insertions, and (2) ‘distributions adding': summing 3D biological dose distributions of IMRT and brachytherapy plans and subsequently determining the values of the DVH parameters. We took α/β=10Gy for HR-CTV, α/β=3Gy otherwise and half-time of repair 1.5h. Results Without EBRT boost, ‘parameter adding' yielded a good approximation. With an EBRT boost to lymph nodes, the total D90 HR-CTV was underestimated by 2.6 (SD 1.3)% for PDR and 2.8 (SD 1.4)% for HDR. This was even worse with a parametrium boost: 9.1 (SD 6.2)% for PDR and 9.9 (SD 6.2)% for HDR. Conclusion Without an EBRT boost ‘parameter adding', as proposed by the GEC-ESTRO, yielded accurate results for the values for DVH parameters. If an EBRT boost is given ‘distributions adding' should be considered.