Biomedical subjects

G S Shentall

Publications and source records attributed to G S Shentall.

6 recordsLinked to original sources

PubMed1998

A digital method for computing target margins in radiotherapy.

Computer methods for determining the planning target volume from the gross tumor volume for both conventional and conformal radiotherapy are presented. Production of a two-dimensional (2D) treatment plan is assisted by projecting outlines of the gross tumor onto a single transverse plane, so that the total extent of the tumor can be easily visualized. A 2D margin can then be added to the resulting outline, so as to account for microscopic tumor spread, organ motion, and setup uncertainty. The margin may be anisotropic to account for the known differences in setup accuracy in the anterior, posterior, left and right directions. For three-dimensional (3D) treatment planning, it is necessary to add a 3D margin to the gross tumor volume to define the planning target volume, the anisotropy of the margin now being allowed to extend to the superior and inferior directions also. Robust methods for automatically calculating these regions are described, and illustrated for the case of a prostate tumor. It is demonstrated that while a slicewise 2D margin is adequate for 2D planning, a fully 3D margin must be used for 3D conformal planning to avoid underdosing the superior and inferior extremities of the clinical target volume.

Biophysical Phenomena↗

PubMed1997

Dosimetric evaluation of compensation in radiotherapy of the breast: MLC intensity modulation and physical compensators.

BACKGROUND AND PURPOSE: Electronic portal images may be used to design the compensation required to maximise dose uniformity in the breast from opposed tangential beams. MATERIALS AND METHODS: Four methods of implementing the desired compensation have been studied: a simple wedge, a physical compensator in conjunction with a wedge; one open field plus four shaped multi-leaf-collimated (MLC) fields, and one wedged field in conjunction with three shaped MLC fields. Evaluation was performed using thermoluminescent dosimeters (TLDs) placed inside a phantom which was designed to mimic the human breast. The measured results are compared with both the prediction of the in-house compensation design software and with the dose predicted by the GE Target II planning system. The implications of each method for the time taken to plan and deliver treatment were analysed. RESULTS: The dose inhomogeneity, as measured at seven points in the central plane was greatest for the simple wedge (root mean square (rms) = 4.5%) compared to an open field plus four shaped MLC fields (rms = 2.2%), a wedged field plus three shaped MLC fields (rms = 3.3%), and the physical compensator (rms = 2.4%). The times required to plan and prepare these treatments varied considerably. The standard wedged treatment required under 15 min; both MLC-based and the physical compensator treatments required approximately 50 min. Differences of treatment delivery times were up to 8 min. CONCLUSIONS: These results indicate that the dose inhomogeneity can be reduced by beam intensity modulation designed using EPIDs.

Breast↗

PubMed1997

The potential for increased sparing of normal tissue in parallel opposed techniques with a multileaf collimator.

A multileaf collimator (MLC) can be used in parallel opposed techniques as a direct replacement for standard-shaped beam blocks. However, improved shielding is possible if the MLC field is designed to fit a target rather than to mimic a straight-edged block. This study has compared the treatment areas produced by the MLC and by conventionally blocked fields with the target area for 43 parallel opposed treatments. It was found in every case that the MLC treated less than 10% excess tissue, and, in over 70% of patients, the excess was less than 5%. The conventional fields, however, treated more than 10% excess tissue in 70% of patients. The effect of MLC orientation and the benefits of using an MLC are discussed.

Head and Neck Neoplasms↗

PubMed1997

Methods for transferring patient and plan data between radiotherapy treatment planning systems.

The effectiveness of conformal radiotherapy can ultimately only be assessed by the use of clinical trials. As large multicentre clinical trials become more widespread, methods of transferring patient and plan data between radiotherapy treatment planning systems become increasingly important. In this paper, the general strategy for the transfer of data is discussed, and also illustrated with reference to two specific systems: TARGET 2 (GE Medical Systems) and VOXELPLAN (DKFZ-Heidelberg). The transfer method involves using a computer program to translate the data formats used by each of the two systems for CT scans, patient outlines, plan information and block descriptions. This paper does not address the question of transferring beam data between systems: beam data must first be entered separately into both machines. The physical concepts encountered when transferring plans are described, with specific reference to the two planning systems used. Differences in the strategies used by the two planning systems for definition of irregular field shapes are compared. The dose calculations used by the two systems are also briefly evaluated. Isodoses produced by VOXELPLAN around a circular target volume are found to be up to 3 mm different in location to those produced by TARGET 2, owing to the use of a smooth field shape contour as opposed to a stepped field shape which closely models the leaves of a multileaf collimator. In general, dose distributions generated by both systems are comparable, but some differences are found in the presence of large tissue inhomogeneities. It is concluded that the transfer of patient and plan data between two different treatment planning systems is feasible, provided that any differences in field shape definition methods or dose calculation methods between the two systems are understood.

Humans↗

PubMed1995

The acceptability of a multileaf collimator as a replacement for conventional blocks.

A multileaf collimator (MLC) can be used as a replacement for conventional blocks as well as for conformal radiotherapy. This study has assessed the possibility of using a Philips MLC for 218 patients treated with conventionally blocked fields. It was found that MLC field shaping would have been appropriate for over 94% of such patients. The facility to treat large blocked fields has been found to be particularly useful. Use of the predefined shapes stored in the Regular Shape Library provided by Philips was evaluated and it was found that an appropriate shape was available in 52% of cases. The application of MLC fields to the treatment of different anatomical sites is discussed.

Equipment Design↗

PubMed1993

The Royal Marsden Hospital pelvic radiotherapy trial: technical aspects and quality assurance.

Planning and quality control procedures are described for a randomised trial designed to measure the effect on normal tissue toxicity of reducing the volume of normal tissue irradiated through the introduction of Beams-Eye-View designed customised blocks. Consideration is given to the accuracy with which blocks can be designed and to the potential application of multi-leaf collimator technology.

Adult↗