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A J Lomax

Publications and source records attributed to A J Lomax.

12 recordsLinked to original sources
PubMed2005

Monte Carlo dose calculations for spot scanned proton therapy.

Density heterogeneities can have a profound effect on dose distributions for proton therapy. Although analytical calculations in homogeneous media are relatively straightforward, the modelling of the propagation of the beam through density heterogeneities can be more problematical. At the Paul Scherrer Institute, an in-house dedicated Monte Carlo (MC) code has been used for over a decade to assess the possible deficiencies of the analytical calculations in patient geometries. The MC code has been optimized for speed, and as such traces primary protons only through the treatment nozzle and patient's CT. Contributions from nuclear interactions are modelled analytically with no tracing of secondary particles. The MC code has been verified against measured data in water and experimental proton radiographs through a heterogeneous anthropomorphic phantom. In comparison to the analytical calculation, the MC code has been applied to both spot scanned and intensity modulated proton therapy plans, and to a number of cases containing titanium metal implants. In summary, MC-based dose calculations could provide an invaluable tool for independently verifying the calculated dose distribution within a patient geometry as part of a comprehensive quality assurance protocol for proton treatment plans.

Algorithms↗
PubMed2004

First spinal axis segment irradiation with spot-scanning proton beam delivered in the treatment of a lumbar primitive neuroectodermal tumour. Case report and review of the literature.

Primary intraspinal primitive neuroectodermal tumour (PNET) is a rare tumour entity. The optimal therapeutic management is unclear but, in general, this tumour is treated with surgery followed by radiotherapy and chemotherapy. Proton beam radiation therapy (PT) offers superior dose distributional qualities compared with X- or gamma rays, as the dose deposition occurs in a modulated narrow zone called the Bragg peak. As a result, organs at risk are optimally speared. Here, we present a patient treated with the first spinal axis segment irradiation using spot-scanning PT with a single field, combined with conventional cranio-spinal axis radiotherapy after surgery and chemotherapy, and an extensive review of the literature outlining the clinical features and treatment modality of spinal PNET.

Adult↗
PubMed2001

A comparison of dose distributions of proton and photon beams in stereotactic conformal radiotherapy of brain lesions.

PURPOSE: Micromultileaf collimators (mMLC) have recently been introduced to conform photon beams in stereotactic irradiation of brain lesions. Proton beams and stereotactic conformal radiotherapy (SCRT) can be used to tailor the dose to nonspherical targets, as most tumors of the brain are irregularly shaped. Comparative planning of brain lesions using either proton or stereotactically guided photon beams was done to assess the institution's clinically available modality for three-dimensional conformal radiotherapy. METHODS AND MATERIALS: For the photon treatment, multiple stereotactically guided uniform intensity beams from a linear accelerator were used, each conformed to a projection of the planning target volume (PTV) by a mMLC. Proton beams were delivered from an isocentrically mounted gantry, using the spot-scanning technique and energy modulation. Seven patients were scanned in a stereotactic frame; target volumes and organs at risk (OAR) were delineated with the help of MR images. Four different lesions were selected: (1) concave, (2) ellipsoid isolated, (3) superficial and close to an organ at risk, and (4) irregular complex. Dose distributions in the PTV and critical structures were calculated using three-dimensional treatment-planning systems, followed by both a quantitative (by dose--volume histogram and conformity index) and qualitative (visual inspection) assessment of the plans. RESULTS: A high degree of conformation was achieved with a mMLC and stereotactic uniform intensity beams with comparable conformity indices to protons for 5 out of 7 plans, especially for superficial or spherical lesions. In the cases studied, the conformity index was better for protons than for photons for complex or concave lesions, or when the PTV was in the neighborhood of critical structures. CONCLUSION: The results for the cases studied, show that for simple geometries or for superficial lesions, there is no advantage in using protons. However, for complex PTV shapes, or when the PTV is in the vicinity of critical structures, protons seem to be potentially better than the fixed-field photon technique.

Brain Neoplasms↗
PubMed2001

Intensity modulated proton therapy: a clinical example.

In this paper, we report on the clinical application of fully automated three-dimensional intensity modulated proton therapy, as applied to a 34-year-old patient presenting with a thoracic chordoma. Due to the anatomically challenging position of the lesion, a three-field technique was adopted in which fields incident through the lungs and heart, as well as beams directed directly at the spinal cord, could be avoided. A homogeneous target dose and sparing of the spinal cord was achieved through field patching and computer optimization of the 3D fluence of each field. Sensitivity of the resultant plan to delivery and calculational errors was determined through both the assessment of the potential effects of range and patient setup errors, and by the application of Monte Carlo dose calculation methods. Ionization chamber profile measurements and 2D dosimetry using a scintillator/CCD camera arrangement were performed to verify the calculated fields in water. Modeling of a 10% overshoot of proton range showed that the maximum dose to the spinal cord remained unchanged, but setup error analysis showed that dose homogeneity in the target volume could be sensitive to offsets in the AP direction. No significant difference between the MC and analytic dose calculations was found and the measured dosimetry for all fields was accurate to 3% for all measured points. Over the course of the treatment, a setup accuracy of +/-4 mm (2 s.d.) could be achieved, with a mean offset in the AP direction of 0.1 mm. Inhalation/exhalation CT scans indicated that organ motion in the region of the target volume was negligible. We conclude that 3D IMPT plans can be applied clinically and safely without modification to our existing delivery system. However, analysis of the calculated intensity matrices should be performed to assess the practicality, or otherwise, of the plan.

Adult↗
PubMed1999

A treatment planning inter-comparison of proton and intensity modulated photon radiotherapy.

PURPOSE: A comparative treatment planning study has been undertaken between standard photon delivery techniques,b intensity modulated photon methods and spot scanned protons in order to investigate the merits and limitations of each of these treatment approaches. METHODS: Plans for each modality were performed using CT scans and planning information for nine patients with varying indications and lesion sites and the results have been analysed using a variety of dose and volume based parameters. RESULTS: Over all cases, it is predicted that the use of protons could lead to a reduction of the total integral dose by a factor three compared to standard photon techniques and a factor two compared to IM photon plans. In addition, in all but one Organ at Risk (OAR) for one case, protons are predicted to reduce both mean OAR dose and the irradiated volume at the 50% mean target dose level compared to both photon methods. However, when considering the volume of an OAR irradiated to 70% or more of the target dose, little difference could be shown between proton and intensity modulated photon plans. On comparing the magnitude of dose hot spots in OARs resulting from the proton and IM photon plans, more variation was observed, and the ranking of the plans was then found to be case and OAR dependent. CONCLUSIONS: The use of protons has been found to reduce the medium to low dose load (below about 70% of the target dose) to OARs and all non-target tissues compared to both standard and inversely planned photons, but that the use of intensity modulated photons can result in similar levels of high dose conformation to that afforded by protons. However, the introduction of inverse planning methods for protons is necessary before general conclusions on the relative efficacy of photons and protons can be drawn.

Algorithms↗
PubMed1999

Gaze patterns in laparoscopic surgery.

By understanding surgeons' patterns of gaze, and what visual information is being obtained during a procedure, one can improve the operation via new techniques or instrumentation. Part of a larger project on Remote Manipulation in Endoscopic Surgery, we analyzed eye patterns of surgeons from videotape annotation. Three categories of eye patterns were defined: 1) eyes on (gaze on monitor); 2) eyes down (gaze on external operative space); 3) eyes off (gaze away from monitor/hands). In the context of hierarchical decomposition of procedures we compared eye patterns and sequential dependencies (gaze as a function of previous gaze) by procedure, surgical steps and tasks. Timelines showed transitions in eye patterns during the procedure. We determined what visual information is available and what visual information is needed by the surgeons. By comparing these, we suggest technology that can provide these needs.

Feedback↗
PubMed1999

Virtual hand laboratory meets endoscopic surgery.

We describe two recent research projects: the Virtual Hand Laboratory, and Remote Manipulation in Endoscopic Surgery. The Virtual Hand Laboratory (VHL) is a prototype experimental tool for investigating human visuomotor coordination for object manipulation in augmented and virtual environments. The Remote Manipulation in Endoscopic Surgery (RMES) project examined surgeon's use of viewing and manipulating technologies in laparoscopic surgery, both in clinical and experimental settings. Current research brings together these two parallel research projects (VHL and RMES), for applications in planning and real-time execution of surgical procedures as well as for surgical training. We outline our research directions and detail current activities on superposition of display space on the workspace for the surgeon's hands.

Endoscopy↗
PubMed1993

A transputer based three-dimensional dynamic cardiac imaging system.

In Aberdeen, a single-section transverse emission scanner has been adapted to trigger off patient ECG signals, allowing for the acquisition of gated blood pool tomograms at a number of slice levels through the patient's heart. This paper describes a system for the routine generation and display of surface rendered images derived from this data using a transputer based hardware system. Surface rendering algorithms have been implemented to provide an indication of the distribution of the cardiac blood pool in three dimensions, whilst the additional use of colour and/or cine sequences provide a succinct method of representing the extra information provided by gated acquisition. The transputer provides sufficient computing power to produce rendered views at a rate of about 1 frame per second, thus putting view selection fully under operator control. The success of the system described is reflected in its routine use in a busy clinical department.

Algorithms↗
PubMed1983

Surgery in the aged.

The ageing process has changed little since the days of the psalmist. The number of elderly patients, however, is increasing. Old people seem more reluctant to undergo surgery than younger patients and more concerned with quality of life. The authors conducted a survey in a small community in British Columbia to determine the attitudes of old people towards surgery. The survey confirmed that they are reluctant to undergo surgery if there is an appreciable risk that they will be physically or mentally impaired afterwards. A substantial number also do not wish to undergo complicated procedures to prolong their lives. Quality of life and conservatism should be important considerations in the surgical management of elderly patients.

Aged↗