IMRT Dosimetry Tools
Point dosimetery
1) Ionization chambers:
Cylindrical ionization chambers are used for point-dose measurements in
megavoltage photon radiation therapy because of their excellent stability,
linear response to absorbed dose, small directional dependence, beam-quality
response independence, and traceability to a primary calibration
standard.High-spatial resolution is important for IMRT measurements.
a)Ionization
chambers should be used:
(i) In homogeneous dose regions (ii) To verify
monitor unit outputs (iii) To verify critical structure doses.
b)Ionization
chambers should not be used:
(i)
To measure beam profiles that will be employed to model the IMRT beam penumbra.
(ii)
When the leakage current will yield an integrated charge of >5% of the
expected radiation-induced charge. Leakage current corrections should be
applied if the expected uncorrected error is >2%.
c)Chamber
selection
(i)
A cylindrical ionization chamber should be used.(ii) The size of the ionization
chamber should be appropriate for the task (1) For measuring output factors:
The radiation field should be 1.5 cm wider than the effective length of the
ionization chamber.(2) For IMRT dose measurements: The ionization chamber size
should be small enough to limit
the dose heterogeneity across the chamber active volume to 10% and 5% if the measurements are being compared against
volume-averaged and point doses, respectively.(iii)The ionization chamber
electrode should be fabricated out of low-Z materials(e.g.,aluminum).
When high-Z electrodes are used, the chamber should be cross-calibrated
in conditions that minimize photon spectral differences(e.g., the same depth
and minimizing field size differences).
d)
Measurement protocols
(i)
When ionization chambers are used to validate single-point IMRT absolute doses:
(1) The ionization chamber should be placed in a region of the dose
distribution where the expected dose
heterogeneity is less than 10% or 5% across the ionization chamber and the
expected dose heterogeneity is less than 10% within 2 mm from the intended
ionization chamber position, if the measurements are being compared against
volume-averaged or point doses, respectively
(2) While point-dose comparisons
are possible with all treatment planning systems,the measured dose should be
compared to the calculated doses that have been
averaged throughout the active ionization chamber active
volume. This is typically done by computing the dose using
a CT scan with the ionization chamber in place, contouring
the ionization chamber
volume, and querying the treatment planning system dose statistics for the mean dose. This
system can also be used to identify the expected dose heterogeneity.
2)solid
state detectors: P-type semiconductor
diode detectors have some attractive characteristics for megavoltage photon
beam dosimetry, especially for small-field measurements. not
only is the active volume of diodes much smaller than the smallest ionization
chamber but also the sensitivity of these diodes can be 20–100 times greater. In
contrast to silicon diode detectors, diamond detectors are almost soft-tissue
equivalent in terms of atomic composition (although they have a physical
density much greater than water at 3.5 g cm−3). Recently,
chemical vapor deposition (CVD) diodes have been investigated for radiation
therapy dosimetry. Another type of small-field dosimeter that has been used in IMRT
is the thermoluminescence dosimeter (TLD).
Diode detectors( i.e. single-point
detectors)
a)
Diode detectors should be used:
(i)
For measuring relative dose distributions, particularly for measuring MLC
penumbras used in beam modeling in a treatment planning system (ii) For
providing dose measurement points supplemental to ionization chamber measurements
b)
Diode detectors should not be used:
(i)
To measure absolute doses (ii) As the sole measurement device for measuring beam
profiles that will be employed to model the IMRT beam penumbra. This is due to
the potential for over-response in the low-dose regions outside the field
boundaries. Other detectors with little energy response variations should be
compared to diode-measured profiles to assure that the diode profile provides
accurate out-of-field results.
(c)
Diode selection
(i)
Use unshielded diode detectors (ii) Diode detectors designed for in vivo dosimetry
should not be used for in-phantom measurements.
(d)
Measurement protocols
(i)
Diode response varies with orientation, so the relative orientation of the
diode to the radiation beam should be carefully considered.
Thermoluminescent dosimeters
(chips)
a)
TLD detectors should be used:
(i) When the phantom geometry will not allow
ionization chamber measurements.(ii) When multiple simultaneous point measurements
are desired.
b)TLD
detectors should not be used:
(i)To
measure absolute doses if the overall measurement precision needs to be better
than 3%.
c) TLD
selection
(i)Low-atomic
number TLDs (e.g., LiF) should be used.
(d)
Measurement protocols
(i)
A strict annealing and calibration protocol should be adopted that provides
relative response factors for individual chips. (ii) Care should be taken to
assure that the user accurately knows the TLD positions with respect to the
linear accelerator.
Electrometer
The
basic requirements for electrometers are (1) accuracy,(2) linearity, (3)
stability, (4) sensitivity, (5)high impedance,and (6) low leakage. These
electrometers have much lower leakage currents than most of the older models.
Physicists should also assess the performance of electrometers with automatic
leakage correction.
2D Dosimetry
The current commercial
options for two-dimensional (2D) dosimetry are radiographic film, radiochromic
film, computed radiography, diode arrays,and ionization chamber arrays.
Film:
Film is an excellent tool for 2D dose mapping due to its extremely high spatial
resolution with grain sizes typically having dimensions on the order of microns
with appropriate densitometer.
a)Radiographic film
Two
commercial radiographic films are most commonly used Kodak EDR2 and XV2. XV2
should no bet used to measure doses greater than 100 cGy and EDR2 should not be
used to measure doses greater than 500cGy.
i Radiographic film should be used:
1)For
relative IMRT dose distribution measurements. 2) To measure beam profiles that
will be employed to model the IMRT beam penumbra. 3) For measuring relative
output factors of small fields
ii
Radiographic film should not be used:
1)
For absolute dose measurements 2 )To verify monitor unit outputs
(iii)
Measurement protocols
1) A
sensitometric curve should be measured for each radiographic film experiment.2)
The sensitometric curve films should be selected from the same batch as the measurement
films.3) The sensitometric curve films should be processed at the same time as
the measurement films.
4 )The
recommendations of AAPM Task Group 69 should be observed for radiographic film
and densitometry.5) Handle film carefully with clean hands or light cotton
gloves.6) Bending, stretching, or scratching films should be avoided.7) For
EDR-2, wait at least 1 h after irradiation before processing.
b) Radiochromic film
i Radiochromic film should be used:
1)
For measuring relative dose distributions.2) For measuring dose distributions
that will be used to model the IMRT beam penumbra.3) For measuring relative
output factors of small fields.
4)
When a radiographic film processor is not available
ii_Radiochromic
film should not be used:
1)
For absolute dose measurements.2) To verify monitor unit outputs
iii
Film selection
1
EBT-2 is the only film commercially available with appropriate sensitivity
(iv)
Measurement protocols
1)A
sensitometric curve should be measured for each radiochromic film experiment.2)
The sensitometric curve films should be selected from the same batch as the
measurement films.3) The optical density distribution should be measured no
sooner than 1 h after irradiation.4)The sensitometric curve and measurement
films should be irradiated on the same day.5)The sensitometric curve and
measurement films should be scanned on the same day.6) The orientation of the
sensitometric and measurement films during scanning needs to be consistent with
respect to the original orientation.7) Handle film carefully with clean hands
or light cotton gloves.8) Bending, stretching, or scratching films should be
avoided.
Array
detectors
Array
detectors calibrated to yield multiple cumulative readings of absorbed dose
across a 2D plane represent a recent and popular new addition to the tools
available for routine clinical IMRT QA. Commercially available two-dimensional diode array detector
utilized n-type diode technology. This device is called the Mapcheck .
The Mapcheck contains an array of 445 variably spaced diodes over an area of
22x22 cm2. The diode spacings are 7.07and 14.14 mm in the central 10x10 cm2 and
the outer regions, respectively. The diode plane has an effective build-up
depth of 2 cm and a backscattering thickness of 2.3 cm. The physical cross
section of each diode is 0.8 mm2. The 2D ionization chamber arrays have
detector spacingof 1x1 cm2.
(1)Useful
for efficient routine QA of a precommissioned IMRT technique. Initial commissioning
should be performed with a system with higher spatial resolution(e.g.,
film).(2) For calibration and all measurements with the device, the linear
accelerator dose repetition rate should be the
same
as for the clinical treatment.(3) The device calibration should be checked
monthly, or as specified by the manufacturer or published literature.(4)
Careful consideration should be given to the development of pass/fail
acceptance criteria for the evaluation of the results from an array detector.
For example, AAPM Task Group 119
demonstrated pass rates of >90% of the evaluated points when using 3
mm/3% distance-to-agreement (DTA) and dose-difference criteria,respectively,
when reporting institution’s planar diode detector measurement QA results. Each
physicist should determine acceptance criteria that are appropriate for the
treatment site, the treatment objectives, and the clinic’s policies.
Computed
radiography
Computed
radiography (CR) has been available for more than 20 years, but is gaining
interest in radiation oncology as a dosimeter because of the removal of film
processors from radiation oncology departments.CR techniques have been
successfully used for megavoltage beam relative dosimetry by employing
low-energy filters, but care should be taken if CR is used for IMRT QA because
the photon spectrum varies widely across IMRT fields.
3D dosimetry/ gel-dosimetry
Truly
comprehensive dosimetric verification of IMRT requires a dosimetry system with
full capability in 3D. A goal of 3D dosimetry has been formulated in the
resolution-time-accuracy-precision (RTAP) criteria. The RTAP ideal
functionality incorporates a spatial resolution of 1 mm3, short read-out time
of <<1 h, accuracy within 3% of true value, and a noise level within 1%.
The three principal categories of 3D dosimeters are polyacrylamide (PAG) gels,
Fricke gels, and radiochromic plastics.
The
original PAG gel consisted of bis (3%), acrylamide (3%), nitrogen, and gelatin
(5%) by weight. PAGs utilize the mechanism of radiation-induced polymerization
of monomers, where small monomer molecules join together under the influence of
ionizing radiation. The resultant polymer microparticles are fixed in the gelatin
lattice, yielding a stable impression of the dose. High resolution 3D maps of dose
have been achieved both by MR scanning and optical-CT scanning.
Fricke and
radiochromic gels and plastics: A limitation of gelatin Fricke
dosimeters is that the radiation-induced ferric ions diffuse through the gel
matrix, leading to degradation and eventual loss of the recorded dose
distribution with time. The practical implication is that Fricke gel dosimeters
must be scanned within 2 h postirradiation, presenting logistical difficulties
in a clinical setting. Recently, a promising new
radiochromic material, PRESAGE, has been introduced and detailed studies of the basic
dosimetric properties have confirmed its promise for 3D dosimetry.
PHANTOMS
Verification
processes for IMRT vary significantly in their phantom requirements. Phantoms
are typically constructed using either water or water-equivalent plastic. Open
water phantoms can be used when the beam is perpendicular to the phantom
surface, and where great flexibility in detector positioning is desired. With the proper procedures and design, water-equivalent
plastic phantoms can support multiple detectors, radiographic film, and rapid
and efficient setup reproducibility. Such phantoms can also include the
substitution or addition of heterogeneous materials. To conduct an overall
evaluation of an IMRT delivery system, anthropomorphic phantoms are useful in
conjunction with other phantoms.
Geometric
phantoms
Simple
geometric phantoms that can accommodate ionization chambers and film are used
for measuring single point and planar doses. Cubic
phantoms, comprised of slabs, are easy and accurate to set up and allow for
measurements at multiple depths. A NOMOS phantom was modified to
accommodate TLDs and multiple ionization chamber positions by using different
spacers of water-equivalent material. Rectangular
phantoms are useful for measuring single field or composite dose distributions.
Cylindrical phantoms have a convenient geometry for coplanar composite IMRT
delivery verification, while allowing for multiple ionization chamber
positions. A novel cylindrical phantom that places radiographic film in
a spiral slot and an ionization chamber at the phantom center was developed for
tomotherapy verification.
Anthropomorphic
phantoms
Anthropomorphic
phantoms are fabricated in the shape of a human. anthropomorphic phantoms have
been effectively used for limited measurements to evaluate the process of
patient treatment planning and delivery and to identify treatment planning or
dose delivery problems that are not evident in simple homogeneous geometric
phantoms.While anthropomorphic phantoms are good for assessing the overall IMRT
planning and delivery process, many commercial phantoms are composed of thick
transverse slices, which limit the flexibility in film and point-dosimeter placement.
The sources of this article is AAMP reports.
Ram Narayan Yadav
Medical Physicist
Department of radiation oncology
BP Koirala Memorial Cancer Hospital
Bharatpur,Chitwan.
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