Abstract
1224
Aim: Radeation therapy (RT) is an important component of cancer treatment for both early stage and locally advanced lung cancers. Photon and proton therapies are both utilized for RT but have different physical characteristics. Photon RT actively exposes healthy tissues along the beam path in front of and beyond the tumor to incidental irradiation. However, due to a central-axis depth-dose distribution pattern known as the spread-out Bragg peak (SOBP), proton RT generally has a more optimal dose delivery in comparison to 3D conformal photon therapy and intensity-modulated radiation therapy (IMRT). The SOBP may potentially allow for improvements in clinical outcomes in select cases by more safely and efficiently applying dose escalation/acceleration or RT with other treatment modalities like chemotherapy and surgery without increasing side effects such as radiation pneumonitis in lung cancer. Although proton RT is a promising technique on a clinical and scientific level, some challenges exist including target determination and motion effects. Proton doses are very sensitive to anatomical changes and motion effects, intrafractional tumor motion and normal tissue motion need to be taken into account for each patient, particularly in customized proton compensator design. Breathing causes lung and other thoracic normal tissues as well as cancer lesions to move during treatment. With the use of respiratory-gated 4D-positron emission tomography (PET) and 4D-computed tomography (CT), volumetric image data can be acquired at many different respiratory phases, where organ and lesion motion can then be characterized for treatment planning. However, the use of 4D-CT and 4D-PET data taken only at the beginning of the proton therapy course does not eliminate all mobility-induced errors, as tumor and normal anatomy can change significantly owing to daily positioning uncertainties and anatomic changes during the course of treatment secondary to non-rigidity of the body, tumor shrinkage, and weight loss. Also, an individual lung cancer patient's breathing patterns can be complex and can exhibit considerable variation. If tumor motion increases between fractions, even a shrinking tumor volume can cause an enlarged target volume. Proton RT also has higher cost and limited availability relative to other RT modalities, and is not yet approved by all insurance carriers. As such, there is a more limited clinical experience of proton RT for showing improved efficacy and decreased radiation-induced toxicities in lung cancer patients relative to photon RT. In recent years, 18F-fluorodeoxyglucose (FDG)-PET/CT is increasingly being used in RT treatment planning, risk stratification, prognostication, and response monitoring for patients with lung cancer. Importantly, FDG-PET/CT for RT planning has added biological information in defining the gross tumor volume as well as involved nodal disease sites. Furthermore, there has been meaningful progress in incorporating metabolic information from FDG-PET/CT imaging in RT planning strategies such as radiation dose escalation based on standardized uptake value (SUV) thresholds as well as using respiratory-gated PET and CT planning for improved target delineation of moving targets. PET/CT-based follow-up after RT allows for the early detection of local as well as distant tumor recurrences after treatment. Moreover, it has an emerging but integral role in assessing the extent of radiation pneumonitis after thoracic RT, which manifests as increased pulmonary FDG uptake. To date, there is no established means to predict the future onset of radiation pneumonitis. Therefore, there is a high demand for establishing an effective, reliable, noninvasive, and quantitative imaging method to assist in the determination of at-risk patients.