Optimized low dose rate pellet configurations for intravaginal brachytherapy.
OBJECTIVE
The objective was to identify configurations of low dose rate pellet sources that optimize short treatment length brachytherapy dose distributions for a set of four intravaginal applicators.


METHODS
The method of simulated annealing was used. Dose rates at calculation points on the surface of 2.0, 2.5, 3.0, and 3.5 cm diameter applicators along a fixed 3.0 cm treatment length were optimized for Cs-137 sources of strengths 0.74 and 0.63 GBq in trains having maximum lengths of 3.0, 3.5 and 4.0 cm. Variations in the optimization algorithm involving two different objective functions and different combinations of selectable parameters were investigated in an effort to standardize the approach.


RESULTS
An objective function based on the maximum dose rate difference at the calculation points in conjunction with a single parameter set proved suitable for all applicators. Optimized solutions involving both a single configuration of sources and a combination of two such configurations were successfully identified. The latter consistently afforded superior dose rate uniformity, particularly for the smaller diameter applicators. A maximum source train length of 3.5 cm was found to provide a good compromise between attaining dose rate uniformity along the 3.0 cm treatment length and minimizing irradiation of adjacent normal tissues. For each applicator, an optimized 3.5 cm pellet train yielded better surface dose rate uniformity than a corresponding optimum-length linear source.


CONCLUSIONS
Pellet configurations that optimize dose distributions for intravaginal brachytherapy applicators can be reliably identified with modest computational effort using the method of simulated annealing. The method is therefore suitable for use in routine clinical treatment planning for this site.
Sloboda
R S
RS
Cross Cancer Institute, Edmonton, Alberta, Canada.
Pearcey
R G
RG
Gillan
S J
SJ
D000465:Algorithms; D001918:Brachytherapy / Q000295:instrumentation*; D005260:Female; D006801:Humans; D011874:Radiometry / Q000379:methods*; D011879:Radiotherapy Dosage; D014625:Vaginal Neoplasms / Q000532:radiotherapy*