Effect of ion chamber sensitive volume on absolute dose verification in CyberKnife plan / 中华放射肿瘤学杂志

ABSTRACT

Objective To evaluate the effect of ion chamber sensitive volume on absolute dose verification in CyberKnife plan.Methods Solid water phantoms were scanned by a CT scanner, single-field plan, multi-field isocentric plan and sequential optimized plan were designed by the treatment planning system.Absolute doses were measured at the specified point in each plan using the ion chambers with sensitive volumes of 0.007 cm3(A16), 0.24 cm3(A12 s), and 0.6 cm3(PTW30013) and compared with calculated values.Results For the single-field plan, the relative error increased as the aperture size of collimator decreased;with relative errors within ±2%, the smallest aperture sizes of collimator were 12.5 mm (A16), 25 mm (A12 s), and 30 mm (PTW30013).For the multi-filed isocentric plan, the relative errors were 0.26%±3.90%(A16),-6.28%±14.33%(A12 s), and-9.41%±14.10%(PTW30013).For the sequential plan optimized with 15 mm cone, the relative error was 0.79%±1.43%;for the sequential plan optimized with 7.5 mm cone, the relative error was 2.01%±8.39%.In absolute dose verification for clinical plans, there was no significant difference between the results measured by these ion chambers (P=0.985).Conclusions There is no significant effect of ion chamber sensitive volume on absolute dose verification in CyberKnife plan under the following two situations(1) the collimator with a relatively large aperture is used;(2) the sensitive volume of ion chamber is totally covered by the prescription isodose line.