Comparison of Radiotherapy Treatment Plans for Left-sided Breast Cancer Patients based on Three- and Four-dimensional Computed Tomography Imaging

Abstract

Aims The target volume for breast radiotherapy after conservative surgery for breast cancer may be affected by breathing motion. We investigated differences between conventional and four-dimensional computed tomography-based treatment planning and whether gating could improve dose volume parameters. Materials and methods Ten patients with left-sided breast cancer and surgical clips at the excision site had conventional treatment planning computed tomography and four-dimensional computed tomography. Treatment plans using two tangential beams (6 MV X-rays) were optimised for target coverage and homogeneity using a field in field technique for the three-dimensional scan. This plan was applied directly to four-dimensional datasets representing individual phases of the breathing cycle and combinations thereof (average and maximum intensity projection). Optimised plans were generated for the maximum inhalation scan to study what could potentially be achieved in gated radiotherapy. Results Four-dimensional computed tomography with effective doses of around 10 mSv proved to be adequate for treatment planning in all patients. The average motion of the surgical clips was 3.7 mm (range 1.7–6.5 mm), which was similar to the movement of the chest wall. With a margin of 7 mm for the whole breast to planning target volume, conventional three-dimensional computed tomography-based planning was found to adequately cover the target as seen on four-dimensional computed tomography without significant differences in normal tissue sparing. Improved sparing of the heart and lung could only be achieved by reducing the posterior margin of the target volume, which may be justified if four-dimensional computed tomography is used to determine the target and its motion. Conclusion No significant benefit has been shown for the use of four-dimensional computed tomography-based planning if motion management is not implemented concurrently with a reduced posterior margin between clinical and planning target volumes.