Development of a Patient Machine Time Model to Evaluate Dose Perturbation by Respiratory Tumor Motion in Pencil Beam Scanning Proton Radiation Therapy

The pencil beam scanning (PBS) modality for delivering intensity modulated proton radiation therapy is being adopted quickly. Drawing from the dosimetric advantages provided by the Bragg Peak, PBS proton therapy has been shown to produce dose distributions with improved healthy tissue sparing.

Although PBS proton therapy is very promising, lung cancer treatment is not without its challenges. Rapid tissue density changes and respiratory tumor motion present a particularly difficult treatment geometry. The tumor moves continuously within the lung as the patient breathes.

In this project, the dose perturbation of a PBS proton therapy lung plan is evaluated and time based models of respiratory cycle and radiation delivery of a pencil beam scanning treatment are created. The combined model of the patient and machine is referred to as the patient machine time model (PMTM). The PMTM is used to calculate the respiration rate at which the treatment machine dose delivery and patient respiration rate produce frequency matching (FM).

Frequency matching between the respiratory cycle and radiation delivery is demonstrated to reduce intra-fraction dose perturbation. The use of the PMTM to produce FM provides an advanced tool to mitigate respiratory dose perturbation with minimal impact on the patient or the treatment delivery time.