PencilBeamScanning

Pencil beam scanning (PBS) is an active beam delivery technique used in proton therapy and other charged-particle radiotherapies, in which a narrow beam is steered to illuminate the tumor volume point by point. By painting the target with a high-precision dose distribution, PBS can create complex, highly conformal dose shapes while limiting exposure to surrounding healthy tissue. PBS is commonly implemented with protons but can also be used with heavier ions such as carbon.

The method operates by directing a beam from a cyclotron or synchrotron through a gantry to the patient. Scanning magnets rapidly deflect the beam in two dimensions to place discrete dose spots on a planned grid within the tumor. The beam energy is changed layer by layer to position the Bragg peak at different depths, so a three-dimensional dose distribution is built up as the spot grid is swept through the target. Dose for each spot is adjusted in real time according to the treatment plan, and modern systems integrate dosimetry and imaging to verify delivery during treatment.

Advantages of PBS include high conformality to irregular tumor shapes, reduced integral dose to healthy tissue, and the ability to perform dose painting by modulating spot intensities. This technique can lower unnecessary exposure and is especially beneficial for pediatric patients and tumors near sensitive structures. PBS also enables advanced planning approaches, such as robust optimization over uncertainties and multi-field arrangements.

Challenges include sensitivity to patient motion and setup errors, which can cause dose degradation unless mitigated. Motion management techniques such as rescanning, gating, and 4D optimization are often employed. PBS systems are technologically complex and costly, requiring sophisticated quality assurance and precise planning to realize their benefits.