Three-dimensional dose verification using BANG gel: a clinical example

Object The authors sought to demonstrate the possible value of three-dimensional dose verification by using gel dosimetry. Methods. In this study, commercially available BANG-25 Gy gel was used. This polymer gel is tissue equivalent and the relaxation rate (R2) measured using magnetic resonance (MR) imaging is proportional to the absorbed dose in the gel. A cylindrical container filled with BANG was mounted within an anthropomorphic head phantom and was handled using the same process as would be used for a patient undergoing gamma knife radiosurgery (GKS). An irregular target outline was constructed and a dose plan was created consisting of seven shots, three using the 8-mm and four using the 4-mm collimator helmet. The maximum dose specified was 25 Gy. A combination of several single spin-echo MR imaging sequences with different echo times was used to calculate the R2. The geometric resolution of the MR images was approximately I mm(3). To compare the measured dose distribution with the calculated one, isodoses were overlaid in three orthogonal planes by using specially designed analysis software. Conclusions. Comparisons of the measured and calculated relative dose distributions showed good overall agreement, with differences of less than 3 mm between measured and calculated isodoses. High resolution BANG gel dosimetry for GKS can be useful for the verification of clinical treatment plans, especially when multiple shots are involved. Further verifications will be done using additional imaging parameters and absolute dose calibrations to improve the method.