Background and purpose: The clinical benefit of irradiating the intact breast after lumpectomy must be weighted against the risk of severe toxicity. We present a study on cardiac and pulmonary dose-volume data and the related complication probabilities of tangential breast irradiation having the following objectives: (1) to quantify the sparing of the organs at risk (ORs), the heart and the lung, achieved by three-dimensional (3-D) conformal tangential irradiation (CTI) as compared to standard tangential irradiation (STI); (2) to elucidate the uncertainty in radiation tolerance data; and (3) to analyse the relation between the amount of OR irradiated and the resulting morbidity risk.
Material and methods: Computed tomography (CT)-based 3-D treatment plans of 26 patients prescribed to CTI of the intact breast were applied. Contour-based STI has been our routine treatment, and was reconstructed for all patients. Dose-volume data and normal tissue complication probability (NTCP) predictions from the probit and relative seriality models with several cardiac and pulmonary tolerance parameterizations were analysed and compared.
Results and conclusions: A significant amount of normal tissues can be spared from radiation by using CT-based CTI, resulting in a 50% reduction of the average excess cardiac mortality risk in the left-sided cases. The risks for pericarditis and pneumonitis were too low to reveal any clinically significant difference between the treatments. For the STI set-up, a regression analysis showed that the excess cardiac mortality risk increased when larger parts of the heart were inside the fields. However, the different excess cardiac mortality and pneumonitis tolerance parameters resulted in statistically significant different NTCPs, which precluded the ability to accurately predict absolute NTCPs after tangential breast irradiation. Despite this uncertainty the different series of cardiac and pulmonary risk predictions were in relatively good agreement when small volumes of the ORs were irradiated. From the present data and without consideration of patient or organ motion, it therefore appears that tangential breast irradiation with less than 1 cm of the heart and 2-2.5 cm of the lung included inside the treatment fields will cause at most 1 per thousand risk for cardiac mortality and pulmonary morbidity. CT-based CTI should be considered, in particular for the left-sided cases, if these requirements cannot be met.