The effect of alternative biological modelling parameters (α/β and half time of repair T ½ ) on reported EQD2 values in the treatment of advanced cervical cancer

Purpose: To evaluate the effect of different α/β and half-time of repair T(½) 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 D90 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 32×60cGy) and HDR (two applications of 2×7Gy). We assumed a low, standard and high value for the biological parameters: HR-CTV α/β=5/10/15Gy and T(½)=0.5/1.5/2.5h; OAR α/β=2/3/4Gy; T(½)=0.5/1.5/4.5h.

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 α/β has a significant impact on reported EQD2 values, whereas for PDR both α/β and T(½) are important. Generally, the ratio D(2cc)(bladder)/D90(HR-CTV) is more favourable for PDR, even considering the larger uncertainties in EQD2.