Strategies for CBCT-guided patient setup of stereotactic radiotherapy and assessments of their dose consequences

Date(s) - 12/03/2012
2:00 pm

Bo Lu, BME-Medical Phsyics PhD Student

Stereotactic radiotherapy (SRT) technique has been widely utilized for various treatments sites over the past two decades. 3D conformal radiation therapy, intensity-modulated radiation therapy (IMRT) and arc therapy have been routinely applied for various SRT treatments in order to escalate target dose while keeping normal tissue dose under toxicity threshold.  However, the full potential of these technologies in radiation treat¬ment can be achieved only if the patient can be positioned accurately and reproducibly during every session of the entire course of treatment delivery. The need for more precise patient positioning has increased the interest in developing 3D imaging techniques that can verify the patient setup immediately before and after treatment. The avail¬ability of large area flat-panel detectors has facilitated the development of integrated cone-beam computed tomography (CBCT) systems on linear accelerators. Until now, kilovoltage (kV) CBCT systems integrated into the gantries of linear accelerators have been pervasively employed as an advanced image-guided radiotherapy (IGRT) modality to acquire high-resolution volumetric images of patients for treatment localization purposes. Using on-line kV CBCT software and hardware, a patient’s position can be determined with a high degree of precision and, subsequently, setup parameters can be adjusted to deliver the intended treatment.  However, in the current research regime, it lacks specific strategies for various sites on how to achieve the optimal patient setup though the guidance of CBCT system and pertinent assessment of their dose consequence. Most of the registration strategies are based upon the methodology of intensity matching between reference and localization images for 6 degree freedom shift, which is not always practical for all the clinical scenarios and even possess intrinsic flaws for some specific site. Meanwhile the relationship between the patient’s dose received from the treatment (beam delivery and CBCT scan itself) and setup alignment strategies had not been sufficiently addressed. This work will provide specific strategies for CBCT-guided setup procedures to achieve optimal dose effects for various sites as well as assess the dose consequence of those strategies. We will start with the assessment of the patient’s dose generated by CBCT scan itself. It involved with the investigation of the effect of CBCT on registration accuracy and image qualities with a reduced number of planar projections used in volumetric imaging reconstruction as well as ultimately seeking for the optimal projection-number needed for reconstruction to balance between the dose and registration accuracy. Then we will address the impact of small rotational errors on the magnitudes and distributions of spatial dose variations for intracranial stereotactic radiotherapy (SRT) treatment setups. Furthermore, a quick online evaluation method to assess the dose consequence due to small rotational error will be introduced. Eventually, in lieu of intensity matching based approaches, we will develop a novel method to facilitate the lung SBRT setup for translational-only scenarios in order to achieve optimal tumor dose coverage. The thorough evaluation and efficiency improvement of this method will also be addressed using a phantom study.