British Journal of Radiology (2006) 79, 165-166
© 2006 British Institute of Radiology
doi: 10.1259/bjr/53167057
Improvements in dose homogeneity for tangential breast fields from a selection of combinations of library compensators
R J Wilks, BSc, PhD1,
T Cammack, MPhys, MSc1 and
P Bliss, MRCP, FRCR2
Departments of 1 Medical Physics and 2 Clinical Oncology, Torbay Hospital, Newton Road, Torquay TQ2 7AA, UK
Correspondence: T Cammack
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Abstract
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Individually paired physical compensators are used in our centre to improve dose homogeneity for radiotherapy to the whole breast. This technical note describes the further improvements that may be achieved when all possible combinations of individual compensators within the library are considered. A retrospective study of 78 patients using a total of 16 (left-sided) and 14 (right-sided) sets of library compensators was evaluated, and the results expressed in terms of the standard deviation of the differential dose–volume histogram and the dose range within the breast volume. The mean of the standard deviations was 3.17% (uncompensated), 2.16% (paired compensators) and 1.97% (combinations) and the mean homogeneity was 15.3%, 11.8% and 11.1%, respectively.
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Introduction
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Wilks and Bliss [1] showed that it was possible to use a library of a relatively small number of reusable compensators as a routine procedure in reducing the dose variation to the whole breast from tangential field radiotherapy. This approach reduced the number of individual compensators requiring manufacture, thus reducing the workload on the staff involved. It was found that out of a 100 patients, approximately 50 required compensation and 45 could be treated with one of the library compensator pairs. The percentage of patients planned with a library compensator has subsequently increased to approximately 70% of all breast patients. In an attempt to both improve treatment dose variation and to reduce the number of individual compensators requiring manufacture, it was decided to evaluate the likely improvements to be gained from using the library compensator plates in different combinations. Accordingly, a retrospective study was undertaken of 78 patients of whom 4 patients were treated with their own individually made compensators, 15 were treated without compensators and 59 were treated with paired library compensators. When these patient treatments were originally planned, a nominal threshold for the standard deviation of the dose–volume histogram of 2.5% was used to decide whether the uncompensated dose distribution was acceptable for treatment. Increased computer processing speeds have since made it practicable to investigate compensation for all patients.
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Method
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Each patient treatment was re-planned using additional software on the Osiris planning system [1], which allowed the individual library compensator plates to be used in combination with all (or none) of the other possible plates. Only those compensators whose dimensions were the same or greater than the tangential fields used to treat the patient were chosen for analysis. This means that the smaller field sizes had more possible combinations of plates than the larger field sizes. Of N possible pairs of compensator plates, the number of combinations was 2N (single plates) + N2.
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Results
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It was found possible to select a mixed compensator combination for all 78 patients, whereas only 73 patients' plans were improved with respect to no compensation when using the standard paired compensators. Of the remaining five patients, four were treated using individual compensators. The fifth was treated uncompensated as the standard deviation of the dose–volume histogram was below the nominal threshold (2.5%) for production of individual compensators. For the other three patients treated with individual compensators, in two of them the standard deviations of the differential dose–volume histograms were improved by 0.1%, using mixed compensator combinations. The third resulted in no change. Therefore, using mixed compensator combinations would have meant that no additional individual compensators would have needed to be made.
Overall, for the 78 mixed compensator combinations chosen, the dose variation was unchanged for 18 (23%) patients and improved (i.e. reduced) for 59 (76%) patients. One patient showed a small increase in dose variation for a mixed pair over an individually manufactured compensator pair. The compensator combination selected improved on the standard library pairs method and would probably have been judged to be acceptable by the consultant, even though the standard deviation was greater than the nominal threshold (2.8% rather than 2.5%).
Some improvements were more marked than others, since many original plans were already close to their optimum. Several measures of the dose uniformity were used to assess the differences in the dose distributions: (1) the standard deviation (SD) of the differential dose–volume histogram; (2) the dose range; (3) the breast volume which had a dose greater than 5% above the reference dose. The improvements of the SD ranged from 0.0 to 0.6%. Table 1
shows a summary of the improvements obtained. As Table 1 shows, there is a systematic but not highly significant improvement in dose uniformity when using mixed combinations of compensators. However, mixed compensators have proved to be applicable to more patients than the standard library pairs, as expected.
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Table 1. Measures of the improvements achieved by the use of compensator combinations. Each figure is followed by the standard deviation(SD) of that parameter
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Conclusion
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The technique of employing combinations of reusable compensators has a definite advantage in both increasing the likelihood of achieving a more uniform distribution and in reducing the number of additional individual compensators, which would otherwise require manufacture.
Received for publication June 9, 2005.
Revision received August 16, 2005.
Accepted for publication August 31, 2005.
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References
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- Wilks RJ, Bliss P. The use of a compensator library to reduce dose inhomogeneity in tangential radiotherapy of the breast. Radiother Oncol 2002. 62:147–57.
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Abstract
Introduction
