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Multi-Target Stereotactic Brain Radiation Therapy

Skarholt, Ingrid
Master thesis
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URI
http://hdl.handle.net/11250/2494321
Date
2018
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Abstract
Metastasis of the brain is one of the most common and severe neurological complications of cancer. Stereotactic radiation therapy (SRT) characterized by a high dose of radiation, delivered in a single or few fractions, with conformal target coverage and steep dose-falls, has been shown to be effective in treating brain metastases and recurrence gliomas. Historically, Gamma Knife (GK) has been superior to other SRT-techniques.

However, the increased flexibility of the new generation of linear accelerators (linac), has made linac-SRT

to a competing method. Traditionally, treatment plans for multiple-brain metastases were made with 1

isocenter pr.target. Recent studies using volumetric arc therapy (VMAT) showed about the same target

coverage when optimized with single isocenter.

The aim of this study was to investigate the influence of different linear accelerator models and treatment

planning strategies on irradiated volume of brain normal tissue (V12Gy and V3Gy), conformity (Paddick

Conformity Index(PCI)) and gradient index (Dose Gradient Index(DGI)). The sensitivity of the location

and number of GTVs was also studied.

Treatment plans for different linac-models and treatment planning strategies were made for 4 different

multi-target brain cases. These plans were supported by the software RayStation (RaySearch Laboraties

AB, v.5.99.0.5 Research). All plans were optimized with the same planning objectives, with some individual

corrections for some cases, until they reached the same acceptable plan criterias

Reducing the leaf width of multi-leaf collimator (MLC) reduced the V12Gy, V3Gy and increased the dose

gradient (DGI). However, the conformity (PCI) was reduced. Reducing the tip gap, for 5 mm leaf width,

showed about the same impact on the volumes treated. It also showed higher PCI compared to the fine-leaf

plans. The linac at St.Olavs, Elekta Versa HD Agility, was often the least favourable.

Single isocenter, especially when combined with new-functionalities in Raystation, showed less sensitivity

to location of targets on V12Gy. 1 ioscenter pr.target treated a small volume for distant GTVs, but were

less favourable for close GTVs. The same sensitivity for location was seen PCI. Overall, the PCI for most

treatment plans was higher than 0.75. For 1 isocenter pr.target on close GTVs PCI was lower than 0.55.

The DGI was 0.2-0.3 for most plans, with some single isocenter plans as exceptions. Single isocenter treated

a large V3Gy, compared to the other treatment planning strategies for distant GTVs.

Based on the result of this project, decreasing the leaf width and leaf tip gap improves the flexibility of

the linacs. Due to low PCI and less stability for the plans using-fine leaf, the leaf tip gap, rather than the

leaf width might be of most importance. Single isocenter might be an attractive and time-effective alternative

to the multiple-isocenter technique, especially when combined with new functionalities in the treatment

planning system. However, when used for targets located distant to the isocenter, different methods should

be tested in hope to reduce the dose spillage. This study was only based on simple cases, the trends observed

must be studied further for more complex cases.
Publisher
NTNU

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