Comparison of radiography- and computed tomography-based treatment planning in cervix cancer in brachytherapy with specific attention to some quality assurance aspects.

INTRODUCTION A modern approach in treatment planning for cervix carcinoma is based on a series of computed tomography (CT) sections and 3D dose computation. When these techniques were not yet available, dose evaluation was based on orthogonal radiographs. The CT based planning provides information on target and organ volumes and dose-volume histograms. The radiography based planning provides only dimensions and doses at selected points. The aim of the presented study is to correlate the information obtained with the two approaches for high dose-rate (HDR) brachytherapy of cervix carcinoma. METHODS For the study 28 patients with 35 applications receiving HDR treatment with Ir-192 were investigated. The planning system PLATO (Nucletron) was used. The different aspects of available data, results and inaccuracies regarding quality assurance were looked at. RESULTS From the CT based planning, the volume, location and dose-volume histograms were calculated for the CTV, rectum and bladder. From the radiography-based planning, the dose to point A (prescription), point B, rectum and bladder ICRU reference points [14], points related to the bony structures could be evaluated as well as volumes receiving different dose levels. These two sets of information were compared and following mean values derived. For a dose prescription of 7 Gy at point A, as an average, 83% (44 cm(3)) of the clinical target volume (CTV) receives at least 7 Gy. The mean dose at the rectum ICRU reference point is 4.3 Gy, and 12% (9 cm(3)) of the rectum is encompassed by the 4.3 Gy isodose. The mean dose at the bladder ICRU reference point is 5.8 Gy, and 8% (16 cm(3)) of the bladder is encompassed by the 5.8 Gy isodose. The maximum dose to the rectum is 1.5 times higher than the dose at the ICRU reference point, and for the bladder 1.4 times higher. Uncertainties caused by the reconstruction of the applicator and merging of isodoses could be evaluated. DISCUSSION The subdivision of different approaches and the transfer from point doses to volumes in treatment planning is possible and practical for the treatment of cervix carcinoma in brachytherapy.