Purpose: To compare reproducibility of set-up for two different treatment techniques for external irradiation of the breast.
Methods and materials: In total, the analysis comprised 56 pairs of portal and simulator films for 14 consecutive patients treated following breast conserving therapy and 98 pairs of portal and simulator films for 20 consecutive patients treated after mastectomy. For the first group the tangential field technique (TF technique) was used, for the second the inverse hockey stick technique (IHS technique). Evaluation of the treatment reproducibility was performed in terms of systematic and random error calculated for the whole groups, comparison of set-up accuracy by means of comparison of cumulative distribution of the length of the displacement vector.
Results: In the IHS and TF techniques for medial and lateral fields, displacement larger than 5 mm occurred in 28.3, 15.8 and 25.4%, respectively. For the IHS technique, the systematic errors for lateral and cranial-caudal direction were 1.9 and 1.7 mm, respectively (1 SD), the random errors for lateral and cranial-caudal direction were 2.0 and 2.5 mm. For the TF technique, the systematic errors for ventral-dorsal and cranial-caudal direction were 2.6 and 1.3 mm for medial field and 3.7 and 0.7 mm for lateral fields, respectively, the random errors for lateral and cranial-caudal direction were 2.2 and 1.0 mm for medial field and 2.9 and 1.1 for lateral field, respectively. Rotations were negligible in the IHS technique. For the TF technique the systematic and random components amounted to about 2.0 degrees (1 SD).
Conclusions: Both the inverse hockey stick and standard tangential techniques showed good reproducibility of patients' set-up with respect to cranial-caudal direction. For the TF technique, the accuracy should be improved for the medial field with respect to the ventral-dorsal direction.