Biomedical subjects

J H Mott

Publications and source records attributed to J H Mott.

4 recordsLinked to original sources

PubMed2001

Clinical implementation of dynamic multileaf collimation for compensated bladder treatments.

BACKGROUND AND PURPOSE: To describe the clinical implementation of dynamic multileaf collimation (DMLC). Custom compensated four-field treatments of carcinoma of the bladder have been used as a simple test site for the introduction of intensity modulated radiotherapy. MATERIALS AND METHODS: Compensating intensity modulations are calculated from computed tomography (CT) data, accounting for scattered, as well as primary radiation. Modulations are converted to multileaf collimator (MLC) leaf and jaw settings for dynamic delivery on a linear accelerator. A full dose calculation is carried out, accounting for dynamic leaf and jaw motion and transmission through these components. Before treatment, a test run of the delivery is performed and an absolute dose measurement made in a water or solid water phantom. Treatments are verified by in vivo diode measurements and real-time electronic portal imaging. RESULTS: Seven patients have been treated using DMLC. The technique improves dose homogeneity within the target volume, reducing high dose areas and compensating for loss of scatter at the beam edge. A typical total treatment time is 20 min. CONCLUSIONS: Compensated bladder treatments have proven an effective test site for DMLC in an extremely busy clinic.

Carcinoma↗

PubMed2000

Requirements for leaf position accuracy for dynamic multileaf collimation.

Intensity modulated radiation therapy can be achieved by driving the leaves of a multileaf collimator (MLC) across an x-ray therapy beam. Algorithms to generate the required leaf trajectories assume that the leaf positions are exactly known to the MLC controller. In practice, leaf positions depend upon calibration accuracy and stability and may vary within set tolerances. The purpose of this study was to determine the effects of potential leaf position inaccuracies on intensity modulated beams. Equations are derived which quantify the absolute error in delivered monitor units given a known error in leaf position. The equations have been verified by ionization chamber measurements in dynamically delivered flat fields, comparing deliveries in which known displacements have been applied to the defined leaf positions with deliveries without displacements applied. The equations are then applied to two clinical intensity modulations: an inverse planned prostate field and a breast compensating field. It is shown that leaf position accuracy is more critical for a highly modulated low-dose intensity profile than a moderately modulated high-dose intensity profile. Suggestions are given regarding the implications for quality control of dynamic MLC treatments.

Algorithms↗

PubMed1999

Customised compensation using intensity modulated beams delivered by dynamic multileaf collimation.

BACKGROUND AND PURPOSE: This paper describes the development of customised compensation by intensity modulated radiation therapy (IMRT), delivered by dynamic application of a multileaf collimator (MLC), in order to improve dose homogeneity in treatments of the pelvic region. The introduction of this simple IMRT procedure will help facilitate the clinical implementation of more complex 3D conformal therapy techniques. MATERIALS AND METHOD: Computer software is used to generate profiles of the intensity modulated beams which are required to deliver a uniform dose in a plane, passing through the isocentre and normal to the beam axis, under an irregular surface contour. These profiles are then operated on by interpreter software which determines the leaf trajectories that are necessary to deliver these beam profiles using a single, unidirectional sweep of the MLC leaves. A full dose calculation based on the calculated leaf positions is subsequently performed, allowing further fine adjustments to the modulation where required. RESULTS AND CONCLUSION: The compensation procedure has been successfully tested using films placed under a test phantom. The effect of the compensation procedure on dose distributions in the transverse plane has been investigated using an anthropomorphic phantom. Overall dose homogeneity has been improved through the use of customised compensation delivered by dynamic multileaf collimation.

Humans↗

PubMed1998

Quality assurance of the dose delivered by small radiation segments.

The use of intensity modulation with multiple static fields has been suggested by many authors as a way to achieve highly conformal fields in radiotherapy. However, quality assurance of linear accelerators is generally done only for beam segments of 100 MU or higher, and by measuring beam profiles once the beam has stabilized. We propose a set of measurements to check the stability of dose delivery in small segments, and present measured data from three radiotherapy centres. The dose delivered per monitor unit, MU, was measured for various numbers of MU segments. The field flatness and symmetry were measured using either photographic films that are subsequently scanned by a densitometer, or by using a diode array. We performed the set of measurements at the three radiotherapy centres on a set of five different Philips SL accelerators with energies of 6 MV, 8 MV, 10 MV and 18 MV. The dose per monitor unit over the range of 1 to 100 MU was found to be accurate to within +/-5% of the nominal dose per monitor unit as defined for the delivery of 100 MU for all the energies. For four out of the five accelerators the dose per monitor unit over the same range was even found to be accurate to within +/-2%. The flatness and symmetry were in some cases found to be larger for small segments by a maximum of 9% of the flatness/symmetry for large segments. The result of this study provides the dosimetric evidence that the delivery of small segment doses as top-up fields for beam intensity modulation is feasible. However, it should be stressed that linear accelerators have different characteristics for the delivery of small segments, hence this type of measurement should be performed for each machine before the delivery of small dose segments is approved. In some cases it may be advisable to use a low pulse repetition frequency (PRF) to obtain more accurate dose delivery of small segments.

Biophysical Phenomena↗