Medical Physics Seminar – Tuesday, February 27, 2018
Investing New Planning Strategies: Online Dose Restoration for Proton Therapy and Integration of Microscopic Disease Spread with Geometric Errors into a Single PTV Margin
Prof. Dr. Ir. Edmond Sterpin
Associate Professor at Université catholique de Louvain, Belgium
The first part investigates an adaptive therapy workflow that will hopefully solve most specific issues linked to Proton therapy (PT) delivered with pencil beam scanning. What makes PT so appealing, i.e. the sharp Bragg-peak at the end of the range of the protons, is also its greatest challenge: the position of the Bragg-peak must be accurately estimated to ensure adequate tumor coverage and patient safety. However, such objective is challenging to achieve in PT because of the variability of the anatomy. We propose here an approach that was designed from the hypothesis that most adaptations in PT are due to their sensitivity to density modifications. Thus, by reoptimizing automatically the treatment plan using new imaging data acquired on-line but only to reproduce exactly the planned dose distribution (using rigidly projected contours), the effect of density modifications can be almost entirely compensated for.
The second part of the presentation will focus on a new approach to compute PTV margins. In external beam radiotherapy, the definition of several target volumes follows a sequential GTV->CTV->PTV margin expansion formalism. The first expansion accounts for the probability of microscopic extension, corrected for anatomical barriers. The second accounts for the probability of geometric errors. Such approach is fundamentally statistically biased. We suggest here a statistically sounder approach that combines both probabilities into a direct and single expansion from the GTV to the PTV, which, for a given confidence interval, reduces excess planning target volumes compared to the conventional approach.