Multileaf collimation versus conventional shielding blocks: a time and motion study of beam shaping in radiotherapy.

The work processes for the planning and delivery of shaped beams using a Philips multileaf collimator were studied for two treatment techniques and compared to those for conventional shielding blocks. The MLC proved faster in all cases of treatment delivery providing time reductions of 19-48% for parallel opposed beams and 6-44% for conformal isocentric beams. The workload in the mould room and workshop would be reduced if multileaf collimation is used. Time spent manufacturing and mounting blocks (average 2 h 30 min and 37 min, respectively) is eliminated for the techniques studied. The physics process for generating conformal MLC beams proved faster (average 1 h 36 min) than for blocks (average 2 h 30 min); this was not so for parallel opposed beams. Overall the results suggest that using the MLC is a time- and resource-saving alternative to blocks.

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.

A study testing the routine use of ultrasound measurements when selecting the electron energy for breast boost radiotherapy.

The determination of the depth of the tumour bed within the breast requiring an electron therapy boost dose is generally judged clinically and can be inconsistent between individual radiotherapists. High frequency ultrasound provides a reproducible, safe and quick method of measuring this depth. In order to improve current working practice at the Royal Marsden NHS Trust the routine use of ultrasound when planning breast boost radiotherapy was established. Fifty-three early stage postoperative breast cancer patients had both clinical and ultrasound assessments of boost depth performed. These measurements were converted into electron energy and compared. Measurements ranged from 0.8 cm to 4.9 cm and electron energy from 4 MeV to 15 MeV. As a direct result of the ultrasound measurements taken, 60% of patients had their electron energy changed from that chosen by the clinically assessed measurement. Overall, the energy was as likely to be increased as decreased. Breast size did not influence the need for change but patients with small breasts never required an increase in the energy from that chosen clinically. It was concluded that the use of ultrasound, once integrated into the planning process, can improve accuracy when selecting electron energy for patients receiving breast boost irradiation.