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

Abstract

In stereotactic radiosurgery the choice of appropriate detectors, whether for 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 relative dose distributions. In addition a small diode was used to estimate the relative output factors. The data obtained using radiochromic and diode detectors were compared with measurements performed with other conventional methods of dosimetry, with calculated values by treatment planning systems and with data prestored in the treatment planning system supplied by the Leksell Gamma Knife (LGK) vendor. Two stereotactic radiosurgery techniques were considered: Leksell Gamma Knife (using gamma-rays from 60Co) and linac-based radiosurgery (LR) (6 MV x-rays). Different detectors were used for both relative and absolute dosimetry: relative output factors (OFs) were estimated by using radiochromic and radiographic films and a small diode; relative dose distributions in the axial and coronal planes of a spherical polystyrene phantom were measured using radiochromic film and calculated by two different treatment planning systems (TPSs). The absolute dose at the sphere centre was measured by radiochromic film and a small ionization chamber. An accurate selection of radiochromic film was made: samples of unexposed film showing a percentage standard deviation of less than 3% were used for relative dose profiles, and for absolute 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 chamber are in good correlation with radiochromic film-generated data, for both 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) except the 4 mm helmet used in the LGK unit. In this case, differences exist between diode and radiochromic film measurements and both detectors show data values larger than the prestored OF value of 0.80. Dose profiles measured by radiochromic film and calculated are in excellent agreement for both LGK 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 radiochromic and diode detectors, show that the 4 mm helmet relative output factor could be underestimated.

Dosimetry tools and techniques for IMRT

“…95,105,[131][132][133][134] For IMRT commissioning, radiochromic film can be used to acquire beam profiles that will be used for modeling beam penumbra. 99,133 It can also be used to measure relative output factors for very small fields 100,133,[135][136][137] and can be used to measure phantombased IMRT dose distributions, for example, dose distribution measurements that will be used to verify the treatment planning system's programming of the linear accelerator delivery parameters ͑"monitor unit outputs"͒.…”

NCS Report 25: Process management and quality assurance for intracranial stereotactic treatment

“…Recently, some authors have investigated both radiographic and Gafchromic film in the measurement of relative outputs, with promising results. Somigliana et al [144] used both types of film and a diode to measure relative outputs on both a Gamma Knife unit and a Linac. All 3 detectors produced equivalent results on the Linac for collimator diameters between 24 and 14 mm.…”

Quality Assurance in Stereotactic Radiosurgery/Radiotherapy according to DIN 6875-1

“…The output factor has to be determined on the central axis at the reference depth relative to the calibration condition for the dose monitor. Output factors have to be determined for all fi xed tertiary collimators and for the minimal and maximal, as well as for a medium, rectangular fi eld size for micro-multileaf collimators [5] .…”

“…The detectors were placed with an uncertainty of approximately 0.1 mm in the centre of a homo- The fi gures in parentheses for the pin point chamber were not taken into account for calculating the mean values due to systematic errors [5]. 1 Standard deviation of mean value.…”