Proton modulated arc therapy for the treatment of left-sided breast cancer

Current proton techniques for left-sided breast cancer use an anterior or lateral anterior oblique (LAO) proton beam which places the distal edge of the beam immediately against the ipsilateral lung and the cardiac structures. Tangential proton modulated arc therapy (tPMAT) would place the distal edge within the target, reducing the dose and linear energy transfer (LET) to the lung, the heart or the left anterior descending (LAD) artery.

Six patients treated with a single LAO proton beam (sLAO), were re-planned using a tPMAT beam extending from the left lateral to the mediastinal side of the patient. The plans consisted of fifty mono-energetic beams, separated by 2o each. A custom wedge was devised for each patient to enable full target coverage. Dose and volume constraints for PTV, heart, lung, LAD in both treatment modalities were reported, and the statistical significance of their difference was determined using the t-test. LET distributions were calculated and reported for all patients. The robustness of both plan modalities were tested by calculating perturbed doses with isocenter shiftsof 3mm in all 6 principal directions and considering ±3.5% range uncertainty.

tPMAT showed equal or better capacity to cover the target with 95% of the dose while maintaining the dose below prescription for the organs at risk (OARs). The use of tPMAT reduces the LET in the OARs compared to the sLAO. tPMAT is also more robust to isocenter shifts and range uncertainties. A universal wedge can be used in tPMAT without changing the dosimetric or biophysical properties of the plan.

tPMAT provides robust dosimetric and biophysical benefits over the current sLAO technique. The possibility of using universal wedges indicates the possibility of doing tPMAT without altering the current fast turnaround of PBS planning and delivery.