Date of Graduation
12-2013
Document Type
Dissertation (PhD)
Program Affiliation
Medical Physics
Degree Name
Doctor of Philosophy (PhD)
Advisor/Committee Chair
Radhe Mohan, Ph.D.
Committee Member
Laurence Court, Ph.D.
Committee Member
Sastry Vedam, Ph.D.
Committee Member
Narayan Sahoo, Ph.D.
Committee Member
Kenneth Hess, Ph.D.
Abstract
Protons as a source of therapeutic radiation can provide a substantial improvement over dose distributions that can be achieved with conventional sources of radiation such as high-energy photons. However, respiratory motion can significantly impact the delivered proton and photon dose distributions during lung cancer radiotherapy. The goals of this dissertation research were to evaluate the impact of respiratory motion and to estimate the benefit of respiratory gating for passively scattered proton therapy (PSPT) and intensity modulated photon therapy (IMRT).
The first aim of this project was to determine the impact of respiratory motion in PSPT and IMRT. Four dimensional dose distributions were calculated in both modalities for a cohort of 20 patients. The mean changes in normal tissue dose-volume indices were indistinguishable except proton therapy had a greater increase in lung V5, heart V5 and spinal cord maximum dose. The effects of respiratory motion on the calculated dose were not correlated to the tumor motion.
The second aim estimated the benefit of PSPT and IMRT respiratory gating by simulating end-exhale gated treatment plans. The results demonstrated that respiratory gating showed a benefit for a majority of proton and photon treatment plans. PSPT gating, compared to IMRT gating, allowed for larger reduction of all lung and intermediate esophagus dose-volume indices. The third aim attempted to correlate the benefit of respiratory gating to the extent of tumor motion. For the cohort, the benefit of respiratory gating in PSPT and IMRT cannot be predicted by the extent of tumor motion. This aim showed that the tumor motion was inadequate to predict the benefit of respiratory gating.
In an additional fourth aim, we proposed a new metric to quantify respiratory motion in proton therapy: the water equivalent thickness (WET). The change in WET between the inhale and exhale phases of respiration (∆WET) was significantly correlated to the change in dose during respiration. Additionally, ∆WET analysis was used to create treatment plans that were more robust to respiratory motion. The use of ∆WET gives a powerful new tool, especially in proton therapy, to quantify the anatomical variations of all irradiated tissues along the beam path.
Keywords
proton radiotherapy, lung cancer, respiratory motion, respiratory gating, 4D dose, IMRT, motion management