Date of Graduation
8-2018
Document Type
Thesis (MS)
Program Affiliation
Medical Physics
Degree Name
Masters of Science (MS)
Advisor/Committee Chair
Oleg N. Vassiliev
Committee Member
Stephen Kry
Committee Member
Shouhao Zhou
Committee Member
Dershan Luo
Committee Member
Mary Frances McAleer
Abstract
Of the patients that are diagnosed with metastatic disease, up to 40% will develop vertebral osseous metastases. These metastases tend to be located in close proximity to the spinal cord itself, making it difficult to achieve the recommended minimum dose of 14 Gy for single fraction SBRT or 21 Gy1 for three fraction SBRT while maintaining acceptable doses to the cord and cauda equina. This proximity of the target to critical structures has the potential to compromise the efficacy of the radiation treatment plan in favor of reducing normal tissue dose, resulting in poor local control and tumor recurrence at follow-up. Flattening Filter Free (FFF) photon beams have been shown to have lower out-of-field dose and sharper dose gradients when compared with conventionally flattened (FF) photon beams of similar energy; this sharp dose fall-off could potentially prove beneficial in cases where greater precision is required, such as for high-dose hypofractionated radiation treatments of vertebral metasases. The purpose of this project was to compare the physical properties, namely penumbral width and penumbral and out-of-field dose of FFF and FF photon beams as well as determine the clinical effects of these beams on vertebral osseous tumors. It was hypothesized that FFF beams would show a definitive improvement in target coverage while maintaining acceptable normal tissue doses when compared with FF beams. To test this hypothesis, penumbral width and dose were measured for FF and FFF beam profiles at various depths and field sizes using the Varian Standard Beam Data by examining the treatment plans for twelve patients with spine metastases using both FF and FFF beams. There was a statistically significant reduction in penumbral width for FFF plans when compared to FF plans; however, this difference was in effect quite small and may not translate into better treatment plans. There was no demonstrable difference between treatment plans developed using FF or FFF beams in terms of minimum dose to the GTV. However, there was significant reduction in treatment delivery time for FFF plans, which may lead to reduced intrafractional variation from patient motion and a more positive patient experience.
Keywords
flattening filter free, medical physics, radiation, spine, spinal, tumor