Robustness evaluation of single- and multifield optimized proton plans for unilateral head and neck targets

Purpose:

To compare 4 different proton pencil beam scanning (PBS) treatment approaches for unilateral head and neck cancer (HNC) targets in terms of robustness, including anatomical changes during the treatment course.

Material and methods:

Eight patients with unilateral HNC treated with double scattered proton therapy were selected. Each patient dataset consists in a planning CT and several control CTs acquired by an in-room CT scanner during the treatment course. Four different proton PBS plans with simultaneous integrated boost and dose prescriptions of 50.3 Gy(RBE) to the low-risk CTV and 68 Gy(RBE) to the high-risk CTV in 34 fractions were calculated: conventional PTV-based single-field (SFO) and multifield optimization (MFO), and robustly optimized SFO and MFO plans on CTV level, considering ±3 mm and ±3.5% of setup and range uncertainty, respectively.
The treatment plans were recalculated on the registered control CTs and the cumulative doses calculated and compared with the nominal plan.
For robustness evaluation, perturbed doses using a probabilistic scenario-wise approach obtaining random setup shifts through Gaussian sampling, and range uncertainties of 0, +3,5% and -3,5% were calculated, using planning and control CTs, considering both anatomic changes and uncertainties. Cumulative doses from 30 different perturbed treatment courses were generated for each plan.

Results:

The target coverage for the four nominal plans was similar, fulfilling the clinical specification of D98≥95% of the prescribed dose (range 96.9-100.5% for low-risk CTV, 97.4-100.8% for high-risk CTV), being slightly lower on the robust optimized plans. The doses to the organs at risk were similar for all plans; however, for the ipsilateral parotid, higher median doses up to 5 Gy were found on the SFO approaches (Table 1), whereas the contralateral parotid is completely spared. The target coverage throughout the treatment course with slightly changing anatomy remains in general constant.
In terms of robustness evaluation, PTV-based MFO showed reduced robustness against both anatomical changes and uncertainties, i.e. wider DVH bands and a disagreement between planned and summed dose, whereas the robust MFO is less influenced. Both SFO approaches resulted in robust plans on the CTVs (Figure 1).

Conclusion:

The PTV-based MFO approach showed less robustness against uncertainties in setup and range, as well as for anatomical changes during the treatment course. Both SFO plans are robust in terms of CTV coverage; however, they present higher doses to the ipsilateral parotid gland. Robust MFO approach presents the lowest doses to the ipsilateral parotid and more robustness against uncertainties.
The dose to more organs at risk and the difference in normal tissue complication probabilities for the 4 planning approaches will be presented as well.