Purpose: MR-guided online adaptive radiotherapy (MRgOART) allows iso-toxic treatments, i.e., adjusting fraction target dose to take advantage of daily anatomic and/or biologic changes while maintaining the same doses to organs at risk (OAR). This work describes a framework for iso-toxic planning in MRgOART using biophysical models.
Methods: This framework includes (1)converting fractional adaptive plan optimized based on the daily anatomy to biologically effective dose (BED) distribution, (2) calculating composite BED dose distribution for total or a portion of the fractions delivered, (3) converting the composite BED plan to the physical dose distribution with the original fractionation, and (4) calculating tumor control probability (TCP) and normal tissue complication probability (NTCP) using the dose volume histograms and the tumor/organ specific model parameters derived from published dose-response data. Data collected from selected pancreatic cancer cases treated with SBRT using MRgOART were used to demonstrate the utility of the proposed framework. The fractional target dose was rescaled such that the same OAR maximum doses (Dmax), or doses to 0.03 or 1.0 cm3 (D0.03cc or D1cc), were maintained. Comparison was made with respect to the treatment planning goals (TPGs) and the sum daily dose (SDD) of D0.03cc & D1cc and the ratio of SDD to TPGs was calculated.
Results: The proposed framework allowed to calculate BEDs for increased/decreased fraction doses based on daily anatomy change (e.g., OAR moved away from the target), and to estimate fraction doses for the remaining fractions considering total BED. For example, the ratio of SDD to TPGs were < 0.65 in one case, which would allow to increase fraction dose in subsequent fractions or to deliver an extra fraction.
Conclusion: A framework for iso-toxic adaptive planning is developed to account for biological effect of different fraction doses and to optimize the therapeutic ratio in MRgOART.