Dosimetry of Gamma Knife and linac-based radiosurgery using radiochromic and diode detectors

In stereotactic radiosurgery the choice of appropriate detectors, whether f or absolute or relative. dosimetry, is very important due to the steep dose gradient and the incomplete lateral electronic equilibrium. For both linac-based and Leksell Gamma Knife radiosurgery units, we tested the use of calibrated radiochromic film to measure absolute doses and relat ive dose distributions. In addition a small diode was used to estimate the relative output factors. The data obtained using radiochromic and diode det ectors were compared with measurements performed with other conventional me thods of dosimetry, with calculated values by treatment planning systems an d with data prestored in the treatment planning system supplied by the Leks ell Gamma Knife (LGK) vendor. Two stereotactic radiosurgery techniques were considered: Leksell Gamma Kni fe (using gamma-rays from Co-60) and linac-based radiosurgery (LR) (6 MV x- rays). Different detectors were used for both relative and absolute dosimet ry: relative output factors (OFs) were estimated by using radiochromic and radiographic films and a small diode; relative dose distributions in the ax ial and coronal planes of a spherical polystyrene phantom were measured usi ng radiochromic film and calculated by two different treatment planning sys tems (TPSs). The absolute dose at the sphere centre was measured by radioch romic film and a small ionization chamber. An accurate selection of radioch romic film was made: samples of unexposed film showing a percentage standar d deviation of less than 3% were used for relative dose profiles, and for a bsolute dose and OF evaluations this value was reduced to 1.5%. Moreover a proper calibration curve was made for each set of measurements. With regard to absolute doses, the results obtained with the ionization cha mber are in good correlation with radiochromic film-generated data, for bot h LGK and LR, showing a dose difference of less than 1%. The output factor evaluations, performed using different methods, are in good agreement with a maximum difference of 1.5% for all field sizes considered (LGK and LR) ex cept the 4 mm helmet used in the LGK unit. In this case, differences exist between diode and radiochromic film measurements and both detectors show da ta values larger than the prestored OF value of 0.80. Dose profiles measure d by radiochromic film and calculated are in excellent agreement for both L GK and LR with a maximum deviation of less than 1.0 mm, when full widths of the dose profiles at 20%, 50%, 80% levels are considered. When external photon beams are used in stereotactic radiosurgery, the 'well selected' radiochromic films are very accurate detectors both for relative and absolute dosimetry. The experimental results, obtained using both radi ochromic and diode detectors, show that the 4 mm helmet relative output fac tor could be underestimated.