The effect of alternative biological modelling parameters (α/β and half time of repair T1/2) on reported EQD2 values in the treatment of advanced cervical cancer

Purpose: To evaluate the effect of different alpha/beta and half-time of repair T-1/2 on the assessment of clinical treatment plans for patients with cervical cancer. Materials and methods: We used EBRT and BT treatment plans of five patients, planned with MRI guided BT. We computed 3D EQD2 dose distributions of combined EBRT and BT treatments and calculated 090 of high-risk clinical target volume (HR-CTV) and D-2cc for bladder and rectum, and the ratio D-2cc(bladder)/D90(HR-CTV). BT was modelled as PDR (two applications of 32 x 60 cGy) and HDR (two applications of 2 x 7 Gy). We assumed a low, standard and high value for the biological parameters: HR-CTV alpha/beta = 5/10/15 Gy and T-1/2 = 0.5/1.5/2.5 h; OAR alpha/beta = 2/3/4 Gy; T-1/2 = 0.5/1.5/4.5 h. Results: The chosen variation in modelling parameters had a much larger effect on PDR treatments than on HDR treatments, especially for OAR, thus creating larger uncertainties. The relative mean range of the ratio D-2cc(bladder)/D90(HR-CTV) is 72% for PDR and 25% for HDR. Out of the 125 modelled combinations 48 PDR plans and 23 HDR plans comply with clinical objectives. Conclusion: For HDR brachytherapy, only alpha/beta has a significant impact on reported EQD2 values, whereas for PDR both alpha/beta and T-1/2 are important. Conclusion: Generally, the ratio D-2cc(bladder)/D90(HR-CTV) is more favourable for PDR, even considering the larger uncertainties in EQD2. (C) 2011 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 101 (2011) 337-342