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Due to their proximity to lymph nodes, cardiac implantable electronic device leads can present a challenge to target coverage in proton therapy of breast cancer patients. Moreover, shortcomings in treatment planning dose calculations when modelling the interaction of proton beams with metal components can result in greater uncertainty in the delivered dose. The aim of this study was therefore to dosimetrically investigate the dose degradation caused by two types of leads with 3D radiochromic dosimeters and 2D gafchromic films.

Radiochromic dosimeters (5×5×7 cm³) were fabricated from silicone, curing agent, chloroform and leucomalachite green (LMG). Before curing, a cylindrical insert with a 3 mm diameter was placed in the centre of each dosimeter, to fit the lead during irradiation. Leads of two different widths (Ø1.6 mm and Ø2.2 mm) were used. EBT3 gafchromic films (5 × 7 cm²) were placed 1 cm above and 1 cm below the leads (see Fig 1).  The entire setup was CT-scanned and imported to Eclipse (Varian Medical Systems) where two spot-scanning proton therapy plans were prepared using 2.5 or 7.5 cm thickness solid water (SW) build-ups, such that the leads were positioned at the spread-out Bragg peak. A 5 cm range shifter (water equivalent thickness of 5.7 cm) was used in both plans that also delivered 6 fractions of 2 Gy (RBE dose) to the region containing the lead. Following our previously established dosimetry protocols, gafchromic films were scanned 24  hours before and after irradiation with an Epson Expression Pro scanner while the radiochromic dosimeters were scanned 2 hours before and after irradiation using an optical CT scanner (Modus Medical) with 1000 projections over a 360° rotation. Subsequent data reconstruction for the 3D dosimeters was performed with 0.5 mm³ voxel size.

With respect to dose degradations, the largest under dosage (12%) was observed for the thick lead (Ø2.2mm) and 7.5 cm SW build-up, while the smallest under dosage (6%) was observed for the Ø1.6mm lead and 2.5 cm SW build up. Under dosage was localised behind the leads in the beam direction and had approximately the same width and shape as the leads themselves. No overdosage due to backscatter radiation could be observed with neither films nor radiochromic dosimeters.

Both lead thicknesses caused dose degradations, with the smallest shadowing effect measured for the thin lead (Ø1.6 mm). Since underdosage was localised in the beam direction downstream of the leads, its effect could be minimized in patient plans by using more than one field at different incidence angles.