Analytical modeling of depth-dose degradation in heterogeneous lung tissue for intensity-modulated proton therapy planning

Highlights

• Heterogeneous lung tissue causes degradation of pristine proton pencil beams.

• A degradation model was implemented into a treatment planning system.

• For large tumor volumes, degradation was negligible.

• The mean lung dose was not negatively affected in any of the investigated cases.

• Small tumor volumes surrounded by lung tissue might get a local target underdose.

Abstract

Background and purpose

Proton therapy may be promising for treating non-small-cell lung cancer due to lower doses to the lung and heart, as compared to photon therapy. A reported challenge is degradation, i.e., a smoothing of the depth-dose distribution due to heterogeneous lung tissue. For pencil beams, this causes a distal falloff widening and a peak-to-plateau ratio decrease, not considered in clinical treatment planning systems.

Materials and methods

We present a degradation model implemented into an analytical dose calculation, fully integrated into a treatment planning workflow. Degradation effects were investigated on target dose, distal dose falloffs, and mean lung dose for ten patient cases with varying anatomical characteristics.

Results

For patients with pronounced range straggling (in our study large tumors, or lesions close to the mediastinum), degradation effects were restricted to a maximum decrease in target coverage (D₉₅ of the planning target volume) of 1.4%. The median broadening of the distal 80–20% dose falloffs was 0.5 mm at the maximum. For small target volumes deep inside lung tissue, however, the target underdose increased considerably by up to 26%. The mean lung dose was not negatively affected by degradation in any of the investigated cases.

Conclusion

For most cases, dose degradation due to heterogeneous lung tissue did not yield critical organ at risk overdosing or overall target underdosing. However, for small and deep-seated tumors which can only be reached by penetrating lung tissue, we have seen substantial local underdose, which deserves further investigation, also considering other prevalent sources of uncertainty.