Volume 31, Issue 6, June 2004
Index of content:
- PH. D. THESES ABSTRACTS
31(2004); http://dx.doi.org/10.1118/1.1738171View Description Hide Description
Accurate tumor delineation is crucial to achieve successful radiotherapy (RT). While CTs are clinically used to plan 3D–RT, multimodality imaging (MMI) promises to aid accurate delineation of the tumor. This thesis examined delineated MMI volumes and studied their role in RT planning. A prostate modeling study evaluated the dose reductions in conformal and intensity-modulated radiotherapy (CRT and IMRT) due to interfractional gross tumor volume (GTV) motion and variability in delineation. In both CRT and IMRT, a 10 mm margin was adequate to account for interfractional GTV movement. However, 10 mm (CRT) and 5 mm (IMRT) GTV misdelineation produced unacceptable dose distributions. IMRT plans were more affected by delineation errors when compared to CRT plans due to their steep dose gradients. Methods were developed to use MRI only in RT planning. Water and bone electron density values were assigned to the image and a low-distortion MR-sequence was used. MR-delineated prostate volumes were in general smaller than those on CT by 10%. Patient data were separated into those that were affected by organ motion from those that were not. An independent-organ registration technique was developed to relate CT- and MR-delineated volumes that were affected by organ motion between the scans. The dosimetric study showed that unacceptable CRT was planned using planning target volume delineated using CT when volumes were assumed to be true. However, the dosimetric benefit was small as a 1–2 mm marginal increase on would deliver adequate dose to both PTVs and insignificant dose reductions were observed in rectum and bladder when plans were compared to plans. The dosimetric effect of in neuroblastoma CRT and IMRT was studied. Reduction in tumor control probability due to incorrectly delineated neuroblastoma was observed and a new treatment was planned using with acceptable doses to OARs the organs-at-risk. IMRT delivered more conformal dose distributions to complex-shaped PTVs compared to CRT. Methods required to add currently acquired diagnostic to thoracic lymphoma RT planning were developed. The addition confirmed the inferior disease extension in 60% of the cases studied and in 20% of cases, presented a large inferior difference. The addition of FDG–PET would affect the field sizes and the extension of lead blocking. A theoretical CRT study showed that greater than 50% decrease in lung dose was achievable with when compared to the original plan. This thesis highlighted not only the importance of using MMI in RT planning but developed methods to cope with the additional information. Though not all the imaging modalities added a significant dosimetric benefit, they added new information regarding GTV determination and decreased the subjectivity of GTV delineation with minimal technical difficulty, which will benefit current RT planning.
31(2004); http://dx.doi.org/10.1118/1.1738962View Description Hide Description
The dosimetry of small width) x-ray beams, such as those used in stereotactic radiotherapy, is much more complex than that of those used in routine clinical treatments. A thorough understanding of the properties of both small beams and small detectors is necessary to determine the optimum detector to use in each measurement situation. Accurate and reproducible experimental methods must also be developed to measure absolute and relative doses, obtain precise beam data and subsequently verify the delivered treatment dose. This work is an investigation of the above aspects of small beam dosimetry, with particular reference to the types of small fields used for stereotactic radiotherapy in the Edinburgh Cancer Centre. These are fields formed by circular stereotactic collimators (12.5 to 40 mm diameter), used in conjunction with arc therapy for the treatment of small brain lesions. Several detectors were compared in the measurement of percentage depth doses, tissue maximum ratios, off axis ratios, head scatter and relative output factors, on a 6 MV linac. This included the testing of three new, commercially available detectors. Clinical beam data were obtained via detector comparison and recommendations made as to the best methodology for each measurement parameter. The most accurate and reproducible technique for head scatter factors was extended to smaller stereotactic collimators (5 to 10 mm diameter) and square fields with widths of 10 to 20 mm. These were shaped with both the movable linaccollimators and the multileaf collimator and the results will be applied to the measurement of the small subfields used in intensity modulated radiotherapy(IMRT). A verification phantom was designed to be compatible with the stereotactic head frame and the properties of various small detectors were investigated for use in the phantom to measure point doses in both single arcs and multiple non-coplanar plans. The recommendations on beam data acquisition and dose verification were applied to two additional linacs. On all machines, the dose to the isocenter was verified in several typical treatment plans to within 2% of the calculated dose, for all clinical collimators. The results confirm the accuracy of the measurement processes used. The verification technique also provides the basis for a proposed audit of dosimetry in all stereotactic centers in the UK.
31(2004); http://dx.doi.org/10.1118/1.1753111View Description Hide Description
Magnetic resonance(MR) coronary imaging is susceptible to artifacts caused by motion of the heart. The purpose of this thesis was to study the respiratory motion of the coronary arteries and to use the results to develop strategies for improved MRimaging. The first section of the thesis describes a MR motion correction technique for objects undergoing a 3D affine transformation. The remainder of the thesis focuses on measuring the respiratory motion of the heart from free breathing x-ray angiograms. Stereo reconstruction methods are used to generate 3D models of the arteries from biplane angiograms. A method for tracking the motion of the arteries in a sequence of biplane images is presented next. The algorithm uses 3D regularizing constraints on the length changes of the arteries and on the spatial regularity of their motion. The algorithm was validated using a deforming vascular phantom. RMS 3D distance errors were measured between centerline models tracked in the x-ray images and gold-standard models derived from a gated 3D MR acquisition. The mean error was for four different orientations of the x-ray system. The motion field recovered from free breathing angiograms is a combination of the cardiac contraction and respiratory motion of the heart. A cardiac respiratory parametric model is formulated to decompose the field into independent cardiac and respiratory components. Results are presented for ten patients imaged during spontaneous tidal breathing. For all patients, the heart translated caudally (mean, and rotated in a cranio-dorsal direction (mean, during inspiration. In eight patients, the heart also translated anteriorly (mean, and rotated in a caudo-dextral direction (mean, Anatomic landmarks were used to compare results across patients. Three dimensional displacements and velocities were compared, and quiescent periods in the respiratory and cardiac cycles were measured. Finally, respiratory motion was analyzed using three linear motion models that correspond to available MR motion correction techniques: translation, rigid body, and affine. Calculations indicate that a two-to-four-fold increase in scan efficiency is attainable, resulting in reduced scan times while maintaining image quality.