Purpose: In proton therapy, uncertainties in stopping power ratio (SPR) limit plan robustness and can compromise treatment quality. These uncertainties are exacerbated for high density and non-biological materials. We studied the dosimetric impact of 12 commonly used biliary and esophageal stents and provide recommendations for handling these objects during treatment planning.
Methods: Water equivalent thickness (WET) for five esophageal and seven biliary stents was measured using a multilayer ionization chamber (MLIC). The stents were subsequently placed in a water phantom and CT scans were acquired and imported into the treatment planning system (TPS). Dosimetric perturbations resulting from the presence of the stents in the field were evaluated with the TPS. Subsequently, 2D planar dose measurements were performed in a water phantom at two depths distal to the stents (0 and 1 cm) using calibrated radiochromic film. 2D gamma analysis was performed using various criteria to evaluate the dosimetric impact of the stents.
Results: Submillimeter WET values were measured for both esophageal (0.35-0.85mm) and biliary stents (0.15-0.45mm). Further analysis of film profiles showed dose differences of 0.5-1% conforming to the overall location of the stents. However, TPS indicated WET values as large as 2 mm stemming from the presence of CT artifacts and inaccuracies in SPR estimations.
Conclusion: While our measurements indicate that biliary and esophageal stents have minimal impact on proton dose, the inaccuracies associated with SPR estimation suggest their presence can have non-negligible impact on dose optimization and calculation dose. Appropriate material override can reduce the TPS overestimation of the stent’s effect.