Quality control in interstitial brachytherapy of the breast using pulsed dose rate: treatment planning and dose delivery with an Ir-192 afterloading system

Background and purpose: In the Radiotherapy Department of Leuven, about 20% of all breast cancer patients treated with breast conserving surgery and e xternal radiotherapy receive an additional boost with pulsed dose rate (PDR ) Ir-192 brachytherapy. An investigation was performed to assess the accura cy of the delivered PDR brachytherapy treatment. Secondly, the feasibility of in vivo measurements during PDR dose delivery was investigated. Materials and methods: Two phantoms are manufactured to mimic a breast, one for thermoluminescent dosimetry (TLD) measurements, and one for dosimetry using radiochromic films. The TLD phantom allows measurements at 34 dose po ints in three planes including the basal dose points. The firm phantom is d esigned in such a way that films can be positioned in a plane parallel and orthogonal to the needles. Results: The dose distributions calculated with the TPS are in good agreeme nt with both TLD and radiochromic film measurements (average deviations of point doses < +/-5%). However, close to the interface tissue-air the dose i s overestimated by the TPS since it neglects the finite size of a breast an d the associated lack of backscatter (average deviations of point doses -14 %). Conclusion: Most deviations between measured and calculated doses: are in t he order of magnitude of the uncertainty associated with the source strengt h specification, except for the point doses measured close to the skin. In vivo dosimetry during PDR brachytherapy treatment was found to be a valuabl e procedure to detect large errors, e.g. errors caused by an incorrect data transfer. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.