Browsing by Author "Alshreef, Abdullah"

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    Brachytherapy Using Permanently Implanted Cs 131 Seeds For The Treatment Of Brain Tumors: Dosimetry, And Dose In Heterogeneity
    (2024-05) Alshreef, Abdullah
    The number of patients with brain tumors has been increasing over the past 30 years. According to the American Brain Tumor Association, 90.000 people are diagnosed with primary brain cancer yearly. Twenty to forty percent of these patients will go on to develop brain metastases. Furthermore, 16.4% of primary brain tumors is glioblastoma multiforme (GBM). The median overall survival (OS) of individuals diagnosed with GBM is low and the likelihood of local recurrence is typically very high. The standard of care for GBM consists of maximal surgical resection (MSR), followed by radiotherapy, and concomitant and adjuvant chemotherapy with temozolomide. Yet, most GBM progression or recurrence happens locally, in areas next to the resection cavity. The brain metastases, also, is treated by different options including surgery, radiotherapy, and chemotherapy. Commonly, external beam radiation therapy (EBRT) has been used in conjunction with tumor resection, however, the EBRT is typically delivered three to five-week after tumor resection to allow for wound healing. Permanent low-energy brachytherapy (BT) seeds are an alternative form of radiation therapy that can be implanted in the tumor bed immediately after tumor resection without the need to wait for wound healing. It is hypothesized that immediate start of radiation therapy after tumor resection can improve local tumor control. Cs-131 has been widely used in brachytherapy to treat tumors. In 2019, GammaTile (GT) was cleared by the US Food and Drug Administration to be used clinically. Each GammaTile consists of four Cs-131 seeds embedded in a collagen tile. Several tiles can be used to provide coverage to the entire tumor bed. GT is used to treat meningiomas, GBM, and brain metastases. Current treatment planning systems (TPS) used clinically calculate the dose delivered to patients using the TG-43 formalism. The TPS assumes all tissues are homogeneous (water), and it does not account for any heterogeneities in the patient composition, such as bone, brain, air, and skin. Hence, the actual dose delivered can be substantially different from the dose calculated by the TPS. The aim of this project was to accurately determine the dose deposited in patients implanted with Cs-131 low energy rate (LDR) seeds using GAMOS Monte Carlo (MC) simulation. To estimate the doses, CT images from patients treated at the University of Minnesota Twin Cities were used (IRB STUDY00010486). To validate the MC code, GammaTiles containing Cs-131 seeds were simulated in both water and heterogeneous phantoms and compared with results measured experimentally using film dosimetry. In this work, the simulated dose in the brain tissue was shown to be approximately 20% less than the dose calculated by the clinical TPS at a distance of 5 mm from the implant. Moreover, the simulated dose in bone was 3.5 times higher than the dose calculated by the clinical planning system near the brain/bone interface. In addition, the dose to skin was 2.4 times less than the dose to water at the bone/skin interface. The dose differences depended considerably on the medium, i.e. bone, brain, and skin and the distances from the implant. It is of paramount importance to quantify these differences to help the brachytherapy field move towards a more accurate dose calculation and future implementation of model-based dose calculation algorithms (MBDCA).