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Doctor of Philosophy (PhD)
Dr. Damodar Pokhrel
Stereotactic body radiation therapy (SBRT) of lung tumors uses high doses of radiation to deliver high biological effective doses (BED) in very few fractions (1-5). With the use of highly conformal fields to cover the tumor without depositing large doses to non-cancerous structures, this technique has proven time and again to be successful at achieving high local control. However, frequently patients receiving SBRT are elderly with multiple medical comorbidities who may not tolerate long treatment times. Furthermore, many patients present with oligometastatic or multiple primary lung tumors. The success of SBRT on oligometastatic lung disease relies on physician experience with precise patient positioning and immobilization, not available in all clinics. Likewise, there is no standard framework to guide radiation oncology clinics experienced in SBRT with planning and treating multiple lung tumors synchronously. This dissertation explores the treatment planning methods available for the SBRT of multiple lung lesions and presents innovative solutions to the challenges in current practice.
To begin, two treatment planning methods for multiple lesion SBRT are compared: treating each lesion individually with separate isocenters and treating all lesions at the same time with a single isocenter. Treating multiple lesions with multiple isocenters will increase the patient’s imaging and treatment time and the number of instances a radiation therapist must enter the treatment room, thus increasing the chances a patient will move from the setup position. Using an individual isocenter placed between the tumors and volumetric arc therapy (VMAT) to treat all tumors at the same time can reduce the treatment time, increasing patient comfort and decreasing the chance of movement from the treatment position. However, there is a chance of decreased target coverage and reduced BED due to small setup errors in the SBRT of synchronous lesions using a single-isocenter. The dissertation continues by quantifying this loss in target coverage using a novel simulation method. Simulations yielded average deviations of 27.4% (up to 72% loss) (p < 0.001) from planned target coverage. The largest deviations from planned coverage and desired BED were seen for the smallest targets (< 10 cc), some of which received < 100 Gy BED, which is suboptimal for SBRT. Patient misalignment resulted in a substantial decrease in conformity and increase in the gradient index, violating major characteristics of SBRT. To minimize coverage loss due to small setup errors, a novel Restricted Single-Isocenter Stereotactic Body Radiotherapy (RESIST) treatment method was developed to provide efficient and effective treatments without substantially increasing treatment time. Lastly, RESIST was automated in the treatment planning system to allow for efficient and accurate treatment planning for two lung lesion SBRT. Automation includes beam geometry, algorithm selection, and an in-house trained dose volume histogram estimation model to improve plan quality. Automated planning significantly improves treatment planning time and decreases the chance of planning errors. This treatment delivery framework allows all patients who are to be treated with SBRT to multiple lung lesions to be treated efficiently and effectively. Further development of RESIST for > 2 lesions and multi-site SBRT merits further investigation.
Critchfield, Lana Catherine, "DEVELOPMENT OF A ROBUST TREATMENT DELIVERY FRAMEWORK FOR STEREOTACTIC BODY RADIOTHERAPY (SBRT) OF SYNCHRONOUS MULTIPLE LUNG LESIONS" (2020). Theses and Dissertations--Radiation Medicine. 1.
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