Medical physics

A detailed dosimetric comparison between Inverse Planning by Simulated Annealing (IPSA) and Dose Points (DP) optimized treatment plans has been performed
for High Dose Rate (HDR) brachytherapy of skin lesions using Freiburg Flap applicator
in order to find out whether or not IPSA offers better clinical dosimetric outcomes for
lesions categorized into four different curvatures. Without compromising target coverage,
IPSA reduced the volume of Planning Target Volume (lesion) that received at least 125%
of the prescription dose on average by 41%. It also reduced the volume of the healthy
skin surrounding the lesion that receives at least 100% of the prescription dose on
average by 42%. IPSA did not show any advantage over DP in sparing normal structures
underlying the lesions treated. Although DP optimization algorithm has been regularly
used at Lynn Cancer Institute for HDR brachytherapy of skin lesions, recent upgrades in IPSA software have made IPSA more amenable to rapid treatment planning and therefore
IPSA can be used either in place of DP or as its alternative.

Common methods for commissioning linear accelerators often neglect beam data for small fields. Examining the methods of beam data collection and modeling for commissioning linear accelerators revealed little to no discussion of the protocols for fields smaller than 4 cm x 4 cm. This leads to decreased confidence levels in the dose calculations and associated monitor units (MUs) for Intensity Modulated Radiation Therapy (IMRT).
The parameters of commissioning the Novalis linear accelerator (linac) on the Eclipse Treatment Planning System (TPS) led to the study of challenges collecting data for very small fields. The focus of this thesis is the examination of the protocols for output factor collection and their impact on dose calculations by the TPS for IMRT treatment plans. Improving output factor collection methods, led to significant improvement in absolute dose calculations which correlated with the complexity of the plans.

The design and construction of a tri-cable, planar robotic device for use in neurophysical rehabilitation is presented. The criteria for this system are based primarily on marketability factors, rather than ideal models or mathematical outcomes. The device is designed to be low cost and sufficiently safe for a somewhat disabled individual to use unsupervised at home, as well as in a therapist's office. The key features are the use of a barrier that inhibits the user from coming into contact with the cables as well as a "break-away" joystick that the user utilizes to perform the rehabilitation tasks. In addition, this device is portable, aesthetically acceptable and easy to operate. Other uses of this system include sports therapy, virtual reality and teleoperation of remote devices.

A Monte Carlo model has been developed using MCNP5 to simulate the Nucletron Ir-192 HDR source in order to investigate the influence of tissue heterogeneities on dose calculations compared to the dose in homogeneous water media, as it is typically calculated by brachytherapy Treatment Planning Systems (TPS). Validity of the simulation was verified in water medium in comparison with peer reviewed results using the dosimetric parameters recommended by AAPM, Task Group-43. The dose-rates in simulated prostate, bladder and rectum were compared to those obtained in the homogeneous water phantom. Based on the resulting dose differences, it is inferred that TPS algorithms for brachytherapy dose calculations overestimate the dose to tissues like prostate and bladder by up to 49%. A clinically relevant dose underestimation of 5.5% to the rectum was also found. We recommend that further investigation using actual patient CT data as input to the Monte Carlo simulation be performed.

The Thesis explores additional applications of LAP's Aquarius external laser alignment verification Phantom by examining geometric accuracy of magnetic resonance images commonly used for planning intracranial stereotactic radiation surgery (ICSRS) cases. The scans were performed with MRI protocols used for ICSRS, and head and neck diagnosis, and their images fused to computerized tomographic (CT) images. The geometric distortions (GDs) were measured against the CT in all axial, sagittal, and coronal directions at different levels. Using the Aquarius Phantom, one is able to detect GD in ICSRS planning MRI acquisitions, and align the external LAP patient alignment lasers, by following the LAP QA protocol. GDs up to about 2 mm are observed at the distal regions of the longitudinal axis in the SRS treatment planning MR images. Based on the results, one may recommend the use of the Aquarius Phantom to determine if margins should be included for SRS treatment planning.