ACCURACY IN MANTLE FIELD IRRADIATIONS - IRRADIATED VOLUME AND DAILY DOSE

Accuracy of patient positioning and dose delivery in mantle field irra diations was investigated on 29 patients treated between August 1990 a nd December 1991. Patients were treated in two different centers, Univ ersity Hospital St. Rafael in Leuven and Institut Gustave Roussy in Vi llejuif, where different techniques and procedures were used. Measurem ents were performed on 341 portal films and entrance doses were measur ed in 518 treatment set-ups. The impact of systematic errors occuring during treatment preparation and day-to-day variations on the accuracy of treatment execution were separately analysed. Daily reproducibilit y, defined as the deviation from the respective mean measured value fo r a treatment was demonstrated to be good for both the treated volume and the delivered dose and no difference between the two techniques wa s shown. Comparing the successive portal films of individual patients (reproducibility of a treatment, once it has started), only small day- to-day variations are found: the SD is 3.4 mm for craniocaudal movemen ts and 2.6 mm for lateral movements. For dose delivery very narrow dis tributions are obtained with SDs of, respectively, 1.5% and 1.85% for the Leuven and the Villejuif group. This suggests that the position of the patient, which is often thought as the critical point in this com plex set-up can be done in a very accurate way, regardless of the posi tion used. To assess the global accuracy of the treatment, the actuall y treated volume and delivered dose were compared with the planned val ues. Apart from reproducibility this also takes into account the whole preparatory procedure between planning and the start of the therapy ( first session). Comparing the simulated treatment volume with the actu al daily irradiated volume, the mean value of the deviations is less t han 1 mm. The SD is 5.2 mm for craniocaudal and 3.8 mm for lateral mov ements. Errors (> 3 mm at patient midline level) in block manufacturin g and fixation (34% of the checked blocks) proved to be the major caus e of deviations. It was demonstrated that most of the patient movement s were not, as often assumed, simple translations in craniocaudal or l ateral direction, but frequently complex movements such as rotations i n a sagittal and/or transverse plane. No systematic deviation in dose delivery was seen in the first center: mean value +0.1% with a SD of 2 .5%. In the second center, however, a deviation of +1.4% from the mean was found, the SD was 3.7% and in 22% of the set-ups an error of more than 5% was detected. The large deviations were caused by a systemati c error linked to an erroneous set-up procedure: wrong source-skin dis tance (SSD) adjustments for all four patients treated with an isocentr ic technique resulted from the impossibility of direct reading on the SSD meter.