QA of Breast Radiotherapy: Variability of Tangent Beam Techniques

Purpose/Objective: Problem: Observation over time reveals that tangent beam geometry varies between clinics and individual radiation oncologists in USA. In addition to outcome, this variance may influence the evolution of IMRT, part-breast radiotherapy and quality assurance (QA) procedures. Purpose: To review published tangent beam geometry recommendations in order to seek consensual simulation and treatment portal QA procedures, in accord with American College of Radiology (ACR) radiation oncology practice guidelines. Materials/Methods: Methods: Contemporary (1980-2003) English language radiotherapy and radiotherapist textbooks (n=14), protocol instructions (n=2) and American College of Radiology standards were reviewed for tangent beam geometry recommendations and tabulated to elucidate conformity and variance. In particular, recommendations were compared for body treatment position, arm position, beam margin on skin to breast border, set-up for postero-medial beam border, anterior flash margin, inferior beam margin from breast, superior transverse beam border level, superior corner in simulation/portal radiograph illustrations, lung-to-rib and abdomen-to-rib distances, medial and lateral beam entries/exits as well as inclusion of drain incision. Results: Results: The recommendation for supine body position was almost unanimous, ipsilateral arm abduction was more common than the bilateral, beam margin options to palpable and anterior breast borders varied from 1-2 cm. A NSABP protocol contained the only recommendation for inclusion of the tail of Spence. There were many ways to design the postero-medial beam border; three of thirteen authors advocated inclusion of internal mammary nodes in the tangent beams. The inferior beam margin to the breast varied from 1-3 cm, and the superior borders transverse level in the patient varied from the supra-sternal notch to the manubrium. The superior beam corner of the simulation/portal radiograph illustrations varied from lung to rib to outside the ribcage while the inferior corner commonly was shown to expose the abdominal organs. Lung exposure, as seen as the distance from beam border to chest wall, was allowed to vary between 1-3.5 cm , with over half of the authors not giving a measure of exposure. Most authors preferred the midline for medial and the mid-axillary line for lateral beam entry-exit. A few allowed both ipsi- and contralateral beam borders at the anterior skin and either the posterior axillary line or the breast border laterally. One textbook recommended inclusion of the drain incision. Conclusions: Discussion: Tangent beam geometry recommendations from textbooks and literature are variable, especially for lymphatic efflux , chest wall, beam margins, abdominal organ and drainage site coverage. Trial protocols often contained few beam restrictions, maybe reflecting the broad variance in respected textbooks. Many recommendations could be questioned simply because they do not satisfy expected clinical coverage of pertinent tissues (e.g.lymphatic efflux, chest wall) , especially when considering the fact that the penumbra of parallel opposed beams is, at a minimum, close to 1 cm. The collective impression from this study was that there were few comprehensive accounts among those reviewed and that the observed collective variance is fairly well represented around the country. This creates problems for peer review and quality assurance procedures, especially since this beam geometry, for the majority of breasts, also invariably results in violations of ICRU guidelines on dose homogeneity. We have consolidated the recommendations in order to reach a clinically acceptable baseline that incorporates consensus on what may be considered important to treat in each individual patient. A composite 2 cm margin concept for individual patients is therefore proposed, based on CT reconstruction and 3-D planning as an absolute requirement. Conclusion: This study has examined the non-conformity in published guidelines. It is an attempt to achieve conformity by enabling physicians to select collective criteria toward a common consensus and carry out recommended QA on administered radiation.

SST18-04

QA of Breast Radiotherapy: Variability of Tangent Beam Techniques