On-treatment Adaptation of Head and Neck Cancer Dosimetry Plans Using Novel Cone-beam CT and AI-driven Software

This study focuses on imaging and radiotherapy treatment plan modification for head & neck cancer patients using a high-performance cone beam CT (CBCT) image guidance system (HyperSight) and AI-driven adaptive planning software. The HyperSight imaging system is a forthcoming component of a radiation therapy (RT) treatment platform, called Ethos (Varian Medical Systems).

RT is used in the treatment of approximately 75% of all head and neck (H&N) cancers with either curative or palliative intent. External beam radiation therapy is typically delivered over 5-7 weeks in 25-35 daily treatments (or "fractions"). Image guided radiation therapy (IGRT) is now a standard of care for external beam radiotherapy. During an IGRT treatment session, a CBCT imaging system built into the treatment unit is used to acquire 3D images of the patient's anatomy immediately prior to, and, if required, at several time points during, the daily delivery of radiation. The CBCT image data are used by the treatment team to carefully align the patient to a treatment planning fan-beam CT (FBCT) images that was acquired 1-2 weeks prior to the start of treatment, and which was used to generate the dosimetry plan that determines the daily radiation delivery. Verification of the patient position and alignment with the treatment planning images increases the precision of radiation delivery.

Daily imaging is particularly important for accurate setup of patients treated for head and neck (H&N) cancer, as these patients often experience weight loss and target volume changes throughout the course of treatment. These volume changes can alter the expected dose distribution, as organs at risk (OAR) migrate to high dose regions or target volumes develop inhomogeneities. When volume changes are perceived by the treatment team to have a significant impact on radiation delivery, patients may be sent for a second planning FBCT and have a new treatment plan created for the remainder of their treatment. This process is referred to as offline replanning or offline adaptation. The radiation oncologist must decide whether to pause treatment or continue with the original treatment plan until a new plan is created. In either case, it can take several days between the point when an issue is identified and when a new plan is ready to be delivered. Offline replanning is not a regularly scheduled part of the clinical workflow and creates disruption of routine imaging, contouring and planning tasks for other patients. These tasks must be moved aside to free the necessary resources to create a new plan for the patient who is already receiving treatment and to permit the treatment team to start delivering the new plan as soon as possible.

The CBCT images acquired with the high-performance HyperSight CBCT imaging system have demonstrated Hounsfield Unit accuracy sufficient for accurate dosimetry calculations in phantoms. These images can therefore potentially be used to plan, or re-plan a radiation treatment delivery. The ability to use imaging that is acquired as part of the daily treatment delivery for re-planning could reduce the need to send patients for additional imaging and could also reduce the time and resources required to re-plan treatment delivery mid-course. This is of great importance for a cohort of patients already known to be vulnerable to the need for replanning.

In addition to the challenges involved in mid-treatment re-planning, another issue commonly encountered in H&N patients is the presence of imaging artifacts from high-density metal objects in their oral cavity such as dental fillings and implants. These artifacts can cause difficulties in visualization of target volumes and OARs on planning CT as well as perturbation of dose calculation, particularly for cancers in the oral cavity. Metal artifacts can also cause a significant reduction in CBCT image quality, limiting the usefulness of IGRT for patient positioning.