Spinal Stereotactic Body Radiotherapy (SBRT) Planning Techniques

Stereotactic body radiotherapy (SBRT) delivers a highly conformal and hypofractionated radiation dose to a small target with minimal radiation applied to the surrounding areas. The spine is an ideal site for SBRT owing to its relative immobility, the potential clinical benefits of high-dose delivery to this area, and the presence of adjacent critical structures such as the spinal cord, esophagus, and bowel. However, with the potential for radiation myelopathy if the dose is delivered inaccurately or if the spinal cord dose limit is set too high, proper treatment planning techniques for SBRT are important. Intensity modulation techniques are useful for spinal SBRT because of a rapid dose falloff and spinal cord avoidance. In this chapter, various planning techniques will be discussed and reviewed

Novel dosimetry verification solutions for advanced radiation therapy

Improved optimisation of radiation dose delivery to tumours with improved sparing of normal tissues is an ongoing goal of radiotherapy practice. Advanced radiotherapy techniques are constantly improving to achieve this goal. However, these techniques are more complex to deliver. Hence verifying the source of dose errors is increasingly challenging. Accurate verification of treatment delivery for advanced radiotherapy becomes increasingly important in mitigating dose delivery errors which may compromise clinical outcome. This dissertation investigated treatment dosimetry verification for two different radiotherapy delivery systems i) Open gantry linear accelerator and ii) Helical TomoTherapy HI-ART ® (HT). Part (i) Open gantry linear accelerator - novel prototype hybrid EPID based dosimeters were developed for treatment verification to combine geometric and dosimetric verification in a single system. Initial work on dose response for standard EPIDs provided a more consistent understanding of EPID under-dose response for small monitor units (MUs). The dose response linearity of a standard a-Si EPID was evaluated for different combinations of linac, image acquisition settings and imaging data processing methods. EPID nonlinear response was demonstrated to be primarily due to gain ghosting affects in the a-Si photodiodes. This work has resolved some of the inconsistencies in the literature regarding EPID dose response and proposes a simple yet robust pixel-to-dose calibration method for EPID-based IMRT dosimetry..

Special stereotactic radiotherapy techniques: procedures and equipment for treatment simulation and dose delivery

Stereotactic radiotherapy (SRT) is a multi-step procedure with each step requiring extreme accuracy. Physician-dependent accuracy includes appropriate disease staging, multi-disciplinary discussion with shared decision-making, choice of morphological and functional imaging methods to identify and delineate the tumor target and organs at risk, an image-guided patient set-up, active or passive management of intra-fraction movement, clinical and instrumental follow-up. Medical physicist-dependent accuracy includes use of advanced software for treatment planning and more advanced Quality Assurance procedures than required for conventional radiotherapy. Consequently, all the professionals require appropriate training in skills for high-quality SRT. Thanks to the technological advances, SRT has moved from a “frame-based” technique, i.e. the use of stereotactic coordinates which are identified by means of rigid localization frames, to the modern “frame-less” SRT which localizes the target volume directly, or by means of anatomical surrogates or fiducial markers that have previously been placed within or near the target. This review describes all the SRT steps in depth, from target simulation and delineation procedures to treatment delivery and image-guided radiation therapy. Target movement assessment and management are also described.

Impact of intrafraction motion on post-mastectomy TomoTherapy of the chest wall

Purpose: The present work investigates the impact of intrafraction motion of the chest wall due to respiration on Post Mastectomy Radiotherapy (PMRT) with TomoTherapy. The hypothesis of this work is that the impact of intrafraction motion on TomoTherapy PMRT will be insignificant as (1) the largest intrafraction movement of the chest wall (CW) in the medial-lateral, anterior-posterior, and superior-inferior dimensions will not exceed 1 cm and (2) that 95% of in-vivo CW point doses on the patient surface will be within 5% of calculated dose and all doses within 10% of calculated dose. Methods: 4DCT scans were acquired and intrafraction motion of the CW near mastectomy scar was analyzed for 5 PMRT TomoTherapy patients. In-vivo patient CW dose measurements, acquired for clinical purpose using TLD were analyzed. Measured dose was compared to the TomoTherapy calculated dose. Daily MVCT images were collected and the correlation between the amount of air cavity between CW skin and the bolus and the dose difference between TLD measured and calculated dose was studied for each patient. Surface dose measurement using a CW anthropomorphic phantom was performed to add confidence to the patients’ data. Results: The maximum anterior posterior (ant-pos) CW movement of our five patients did not exceed 0.15 cm. 28% of the TLD measured doses differed from the calculated dose by more than 5%, and 2% of all data differed from the calculated dose by more than 10%. Slight positive correlation between air cavity between bolus and the CW surface and measured dose difference was observed for both patients’ and phantom data. Conclusions: The result of this work indicates that the impact of intrafraction motion on TomoTherapy PMRT will be insignificant. Discrepancies between TLD measured CW point dose and calculated dose, but overall, the average dose differences were within 5%. Air cavities created between the bolus and the CW may impact on cause underdosing of the CW surface