Session Item

Particle treatment planning for tumors in or near lung is compromised by the inhomogeneous tissue causing a degradation of the integrated depth dose (IDD), leading to under-dosage of the target and to unwanted dose distal to the target.
This effect is independent of the algorithm used for dose calculation but is caused by insufficient information about sub-CT-resolution structures. Previous studies showed that the degradation may be modeled via a “modulation power” assigned to lung-tissue. We investigate the impact of this degradation on physical and biological effective (RBE-weighted) dose distributions for clinical patient treatment plans for proton, helium, and carbon ions.

Patient treatment plans were calculated and optimized using the treatment planning system (TPS) matRad. matRad’s pencil beam algorithm was used in combination with the analytical probabilistic modeling (APM) framework. APM substitutes the IDD with a sum of up to 13 Gaussian functions. This enables an efficient integration of lung degradation effects through analytical convolution with Gaussian modulation kernels while maintaining compatibility with matRad’s biological models.
Proton and carbon ion treatment planning relies on validated base data against the Syngo TPS from the Heidelberg Ion Therapy Center (HIT). For helium ions, generic machine data was fitted to MC simulations.
Each plan was optimized on both physical and RBE-weighted dose with similar objectives and dose per fraction. The plans were recalculated including heterogeneity effects induced by lung-tissue. The resulting dose distributions were compared to the uncorrected dose in terms of homogeneity D₅-D₉₅, dose coverage ΔD₉₅, and dose difference in the planning target volume (PTV).

In Figure 1, heterogeneity-corrected RBE-weighted dose distributions of one selected patient are shown for all particles, and the relative difference to the uncorrected dose distributions which reveals deviations up to 11.3% for helium ions.
Figure 2 shows selected quality indicators for all patients inside the PTV. When heterogeneity is considered, the mean dose decreases inside the PTV. The correction yields a larger D5-D95 (less homogeneous) and a lower D95 (less dose coverage). This holds true for both physical and RBE-weighted dose, yet no systematic differences can be identified. However, the difference is larger for helium and carbon ions than for protons.

For the facilitated degradation model based on an analytical convolution within a pencil beam algorithm, we did not observe large differences caused by degradation between physical and RBE-weighted dose. Yet, large local differences were observed. While their effect might be smaller than other uncertainties in particle lung treatments, the systematic nature may still require mitigation. This study shows that future analysis with MC simulations on treatment plans is worthwhile, especially in combination with 4D data to compare the magnitude of degradation to motion induced effects.