Accuracy of Dose Delivery in Gynecological Brachytherapy

Abstract

Mailed thermoluminescence dosimetry (TLD) has been used in many dosimetry intercomparisons and quality assurance procedures, however mainly in external beam therapy (1 6). For brachytherapy, 19% source calibration intercomparison with mailed TLD has also been performed (4). With careful in vivo TL dosimetry it is possible to determine the overall accuracy of the treatment procedure and find weak points in the chain of external radiotherapy (5). Since the similar principle should also be valid for brachytherapy, we have performed in vivo rectal measurements with TLD during gynecological intracavitary treatments and the results have been compared with the corresponding dose calculations, source strength measurements and specifications. Material and Methods. With mailed TLD it is possible to omit the calibration intercomparison of TLD chains of each centre, once the calibration of the TLD for the reference centre has been checked. Although easily performed, rectal in vivo TLD has not been found very useful in routine patient dosimetry with single measurements, as in the estimation of rectal maximum dose, due to the problems with probe localization (7). Instead, the technique has been used for testing the agreement between actual and calculated dose in intracavitary radiotherapy (8) where the results of multiple measurements have been used in the statistical analysis. A flexible probe has been used widely for in vivo rectal dosimetry (7-9). We constructed a flexible silicon tube which was filled with 4 TLD pellets (lithium borate), lead markers and water equivalent spacers (8). The calibration of the TL probe was checked in water for Iq7Cs and 'Wo and the intrinsic accuracy was better than *2'% Six gynecological afterloading units in Finland were included in the study: three Selectron (Nucletron) high dose rate (HDR) units (labelled afterwards with Nos. I , 2 and 3) with V o sources, two Selectron medium dose rate (MDR) units (Nos. 4 and 5 ) with 7Cs sources and an old non-commercial HDR unit with one '37Cs source (No. 6) . In MDR treatments the probe was irradiated for one hour (8) and in HDR treatments for the whole treatment time. Ten successive patients were investigated. Thus, the maximum number of the measurement points was 40 in each institute. The accurate localization of the flexible probe close to any anatomic structure or reference point, like the ICRU reference points (10) or the point of rectal maximum dose, is difficult and it was not objected. Instead, the aim of the present study was to investigate the general accuracy of the dose calculation and delivery in gynecological intracavitary radiotherapy. The stated dose for each site of the TL dosimeters was calculated by the treatment planning system routinely used in each institute. The source configuration data were based on the anterior-posterior and lateral radiographs. Due to the lead markers inside the probe the sites of the TLD pellets could easily be determined on the radiographs (8). The dose delivered by the radiographs was considered negligible. Because of a few poor quality radiographs some distant calculation points had to be omitted. In addition, a comparison between dose calculation programs was performed. The Theraplan V05 (Theratronics) system was selected as a reference (units Nos. 5 and 6 ) and the stated doses of the unit No. 4 (dose calculation with non-commercial Finnish software called SisDos) and No. 2 (dose calculation with Cadplan by Varian-Dosetek) were recalculated from the original radiographs by the reference system using the same input data. Because the stated doses of the units Nos. 1, 2 and 3 were calculated with the Cadplan system, only the data of unit No. 2 was selected for recalculation with Theraplan. The reference treatment planning system had been checked against measurements both in reference conditions and in vivo (8). In addition to the spatial information of the sources essential physical parameters affecting the dose delivery are the source strength specified either by activity or air-kerma rate and the treatment time. Since errors in source strength and timer contribute directly to the delivered dose, both parameters were determined independently. In Finland the source specification data of the manufacturer is used in treatment planning if the difference between the commissioning measurements and the manufacturer's data is below 5% We extracted the measured source strength from the commissioning documents for the units Nos. 1, 2, 3, and 4, but a new measurement was made in air with the calibrated ionization chamber for the units Nos. 5 and 6. The results were presented as a ratio of the measured to stated source strength as follows