After a mastectomy, a woman often gets an implant, and then the chest is irradiated to kill any remaining cancer cells. Proton therapy is great because it hits the target precisely and spares the heart and lungs. But in some patients, a dense scar capsule forms around the implant: it squeezes the implant, making the breast hard and painful. This is called capsular contracture.
Researchers looked at the records of 24 women who underwent this treatment between 2015 and 2021. Eight developed contracture, sixteen did not. For each, they calculated how much radiation and at what energy the thin tissue layer around the implant (5 mm thick) received. It turned out that risk increases when, in some spots of this layer, dose and energy are both high, while in others they are too low. It's like an unevenly heated pan: some parts burn, others stay raw.
The researchers found three threshold values. If, in the 5-mm layer around the implant, a dose above 55.8 gray (adjusted for biological effect) combines with proton energy above 2.2 keV per micrometer, then the volume of such tissue must be less than 0.0033%. The second constraint is similar but for energy 5.4 keV/μm. The third, conversely, requires that almost the entire layer (more than 97%) receive at least a small dose with low energy. It's like a recipe: don't overheat, and don't leave cold spots.
These rules were tested on a computer model that predicts contracture risk. The model correctly identified 91.7% of cases. Now doctors can plan proton therapy in advance to stay within safe limits. For women who have had breast cancer, this is a chance to preserve not only health but also quality of life after reconstruction.
Capsular contracture is the most common reason for reoperation after breast implant placement, even without any radiation.