Abstract
Purpose: Adjuvant accelerated partial breast irradiation (APBI) results in low local recurrence risks. However, the survival benefit of adjuvant radiotherapy APBI for low-risk breast cancer might partially be offset by the risk of radiation-induced lung cancer. Reducing the lung dose mitigates this risk, but this could result in higher doses to the ipsilateral breast. Different external beam APBI techniques are equally conformal and homogenous, but the intermediate to low dose distribution differs. Thus, the risk of toxicity is different. The purpose of this study is to quantify the trade-off between secondary lung cancer risk and breast dose in treatment planning and to compare an optimal coplanar and non-coplanar technique.Methods: A total of 440 APBI treatment plans were generated using automated treatment planning for a coplanar VMAT beam-setup and a non-coplanar robotic stereotactic radiotherapy beam-setup. This enabled an unbiased comparison of two times 11 Pareto-optimal plans for 20 patients, gradually shifting priority from maximum lung sparing to maximum ipsilateral breast sparing. The excess absolute risks of developing lung cancer and breast fibrosis were calculated using the Schneider model for lung cancer and the Avanzo model for breast fibrosis.Results: Prioritizing lung sparing reduced the mean lung dose from 2.2 Gy to as low as 0.3 Gy for the non-coplanar technique and from 1.9 Gy to 0.4 Gy for the coplanar technique, corresponding to a 7- and 4-fold median reduction of secondary lung cancer risk, respectively, compared to prioritizing breast sparing. The increase in breast dose resulted in a negligible 0.4% increase in fibrosis risk. The use of non-coplanar beams resulted in lower secondary cancer and fibrosis risks (p < 0.001). Lung sparing also reduced the mean heart dose for both techniques.Conclusions: The risk of secondary lung cancer of external beam APBI can be dramatically reduced by prioritizing lung sparing during treatment planning. The associated increase in breast dose did not lead to a relevant increase in fibrosis risk. The use of non-coplanar beams systematically resulted in the lowest risks of secondary lung cancer and fibrosis. Prioritizing lung sparing during treatment planning could increase the overall survival of early-stage breast cancer patients by reducing mortality due to secondary lung cancer and cardiovascular toxicity.
Highlights
The prognosis of early-stage breast cancer patients is excellent, with a cancer-specific survival of almost 99% at 5 years [1]
One option is the use of accelerated partial breast irradiation (APBI) instead of whole breast irradiation (WBI) for early stage breast cancer patients that are eligible according to international guidelines [6,7,8,9,10,11]
Dose comparison studies have shown that the dose to the lungs is significantly lower with APBI compared to WBI but varies greatly depending on the APBI technique used [3, 18,19,20,21,22,23]
Summary
The prognosis of early-stage breast cancer patients is excellent, with a cancer-specific survival of almost 99% at 5 years [1]. Reducing the amount of radiation to the lungs during treatment planning could reduce the long-term overall mortality of early stage breast cancer patients. The conformality and homogeneity of the different contemporary APBI techniques were similar [21,22,23] This means that the differences between the external beam APBI techniques are not in the high dose region but in the intermediate and low dose regions where radiation induced malignancies occur. In the case of APBI, reducing the dose to the lungs mainly results in a higher dose to the ipsilateral non-target breast tissue. This might result in more breast toxicity including fibrosis
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