The evaluation of the agreement between measured and calculated dose plays an essential role in the quality assurance (QA) procedures for intensity modulated radiation therapy (IMRT). Film dosimetry ...has been widely adopted for this purpose due to excellent film characteristics in terms of spatial resolution; unfortunately, it is a time-consuming procedure and requires great care if film has to be used as an absolute dosimeter. If this is not the case, then an independent ionimetric measurement is mandatory to assess the absolute dose agreement. Arrays of detectors are now replacing films for routine IMRT QA, since they permit very simple verification procedures. They show excellent characteristics in terms of linearity, repeatability, and independence of the response from the dose rate, but at the same time present a poor spatial resolution, due to the limited number of detectors available. In our institution, a diode matrix (MapCHECK™, provided by Sun Nuclear) is adopted for routine QA. The aim of this work is to compare the performances of absolute film dosimetry with this matrix in QA procedures and to investigate the origin of possible discrepancies between the two methods. The results we present show a very good agreement between the two detectors when used to assess the mean dose deviation between calculated and measured doses (in both cases 0.2%). If the
γ
matrix method is adopted, MapCHECK™ response shows a slightly better agreement with computed dose distribution than film dosimetry (mean percentage of points satisfying the constraint
γ
⩽
1
: 96% versus 94%). This difference is shown not to depend on the different field sampling, but on the detectors’ capabilities. Moreover, we show that the diode matrix is able to identify eventual delivery errors as well as film. Our conclusion is that the diode matrix may effectively replace both film dosimetry and ionimetric measurements in routine IMRT QA.
Proton Computed Tomography (pCT) is a medical imaging technique based on the use of proton beams with energies above 200MeV to directly measure stopping power distributions inside the tissue volume. ...Prima (PRoton IMAging) is an Italian collaboration working on the development of a pCT scanner based on a tracker and a calorimeter to measure single protons trajectory and residual energy. The tracker is composed of four planes of silicon microstrip detectors to measure proton entry and exit positions and angles. Residual energy is measured by a calorimeter composed of YAG:Ce scintillating crystals. A first prototype of pCT scanner, with an active area of about 5×5cm2, has been constructed and characterized with 60MeV protons at the INFN Laboratori Nazionali del Sud, Catania (Italy) and with 180MeV protons at Svedberg Laboratory, Uppsala (Sweden). A new pre-clinical prototype with an extended active area up to 20×5cm2, real time data acquisition and a data rate up to 1MHz is under development. A description of the two prototypes will be presented together with first results concerning tomographic image reconstruction.
This paper describes the development of a proton Computed Tomography (pCT) apparatus able to reconstruct a map of stopping power useful for accurate proton therapy treatment planning and patient ...positioning. This system is based on two main components: a silicon microstrip tracker and a YAG:Ce crystal calorimeter. Each proton trajectory is sampled by the tracker in four points: two upstream and two downstream the object under test; the particle residual energy is measured by the calorimeter. The apparatus is described in details together with a discussion on the characterization of the hardware under proton beams with energies up to 175MeV.
The aim of this work was to test the suitability of a PTW diamond detector for nonreference condition dosimetry in photon beams of different energy (6 and 25 MV) and field size (from
2.6
cm
×2.6
cm
...to
10
cm
×10
cm
).
Diamond behavior was compared to that of a Scanditronix p-type silicon diode and a Scanditronix RK ionization chamber. Measurements included output factors (OF), percentage depth doses (PDD) and dose profiles. OFs measured with diamond detector agreed within 1% with those measured with diode and RK chamber. Only at 25 MV, for the smallest field size, RK chamber underestimated OFs due to averaging effects in a pointed shaped beam profile. Agreement was found between PDDs measured with diamond detector and RK chamber for both 6 MV and 25 MV photons and down to
5
cm
×5
cm
field size. For the
2.6
cm
×2.6
cm
field size, at 25 MV, RK chamber underestimated doses at shallow depth and the difference progressively went to zero in the distal region. PDD curves measured with silicon diode and diamond detector agreed well for the 25 MV beam at all the field sizes. Conversely, the nontissue equivalence of silicon led, for the 6 MV beam, to a slight overestimation of the diode doses in the distal region, at all the field sizes. Penumbra and field width measurements gave values in agreement for all the detectors but with a systematic overestimate by RK measurements. The results obtained confirm that ion chamber is not a suitable detector when high spatial resolution is required. On the other hand, the small differences in the studied parameters, between diamond and silicon systems, do not lead to a significant advantage in the use of diamond detector for routine clinical dosimetry.
A detailed investigation of the performance as a dosimeter of state-of-art polycrystalline CVD (pCVD) diamond detectors operated in photovoltaic regime for applications in clinical radiotherapy has ...been carried out with conventional 6–10–18
MV X-photons, as well as with a 10
MV photon Intensity Modulated Radiation Therapy (IMRT) beam from a linear accelerator. Our results show that the performances of a pCVD diamond dosimeter improves dramatically when operated in null-bias conditions. Main improvements with respect to operation with an external voltage applied are: a reduced pre-irradiation dose; an excellent time stability, characterised by standard deviations less than 0.5%; a rise-time comparable to that of commercial reference dosimeters; a linearity with dose proven over three decades; a reduced deviation from linearity of the current vs. dose–rate curve, output factors comparable to that of commercial reference dosimeters. These results represent a significant step towards clinical applications as IMRT with synthetic polycrystalline CVD diamond films.
► Cr/Au metallization process on pCVD provides Schottky barrier at electrodes. ► Null-bias operation of pCVD Diamond viable solution for dosimetry. ► Null-bias operation of pCVD in radiotherapy improves their performance as dosimeters. ► Clinical applications as Intensity Modulated Radiation Therapy with pCVD diamond films.
A commercial amorphous silicon electronic portal imaging device (EPID) has been studied to investigate its potential in the field of pretreatment verifications of step and shoot, intensity modulated ...radiation therapy (IMRT),
6
MV
photon beams. The EPID was calibrated to measure absolute exit dose in a water-equivalent phantom at patient level, following an experimental approach, which does not require sophisticated calculation algorithms. The procedure presented was specifically intended to replace the time-consuming in-phantom film dosimetry. The dosimetric response was characterized on the central axis in terms of stability, linearity, and pulse repetition frequency dependence. The
a
-
Si
EPID demonstrated a good linearity with dose (within 2% from 1 monitor unit), which represent a prerequisite for the application in IMRT. A series of measurements, in which phantom thickness, air gap between the phantom and the EPID, field size and position of measurement of dose in the phantom (entrance or exit) varied, was performed to find the optimal calibration conditions, for which the field size dependence is minimized. In these conditions (
20
cm
phantom thickness,
56
cm
air gap, exit dose measured at the isocenter), the introduction of a filter for the low-energy scattered radiation allowed us to define a universal calibration factor, independent of field size. The off-axis extension of the dose calibration was performed by applying a radial correction for the beam profile, distorted due to the standard flood field calibration of the device. For the acquisition of IMRT fields, it was necessary to employ home-made software and a specific procedure. This method was applied for the measurement of the dose distributions for 15 clinical IMRT fields. The agreement between the dose distributions, quantified by the gamma index, was found, on average, in 97.6% and 98.3% of the analyzed points for EPID versus TPS and for EPID versus FILM, respectively, thus suggesting a great potential of this EPID for IMRT dosimetric applications.
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