3. MARCO TEÓRICO
3.4. TECNICAS Y ESTRATEGIAS DIDACTICO PEDAGOGICAS INNOVADORAS
3.4.4. ESTRATEGIAS, ACTIVIDADES DE APRENDIZAJE COOPERATIVO
as the same number of mu (or time period) was set for both water and plastic measurements, the doses cancel out, but the uncorrected readings M’w and M’pl have to be corrected for the influence quantities. the resulting formula is given by:
' 273
' 273
w pl w
pl
pl w pl
M P T
k M P T
= × × +
+ (5)
ionization chamber has to be placed at the appropriate reference depth for each beam quality. the energy correction factor is the ratio of the calibration coefficient at the relevant energy to that for a 60co beam. however, it was recommended that the measured Ncal for each energy should be used. the measurement of the energy correction factor was to ensure the consistency of the in vivo dosimetry system. typical values of the energy correction factor for lif would be about 1.02–1.03 for 6 mv and 1.03–1.04 for 15–18 mv beams.
4.4.1.3. Fading correction
The fading correction factor is the ratio of the dosimeter reading taken with the reference delay Δtref, for which no fading is assumed, to the dosimeter readout taken with the delay Δt,
tref fad
t
M k D
M D
∆
∆
=
(8)
for tlD, it usually is a ratio of the response of a dosimeter irradiated and read on the day t1 to the response of the dosimeter irradiated on the day t1 and read on the day t2. however, fading will not be relevant for tlD unless there is a significant variation in time between the irradiation and reading for different tlDs. the time delay should always be kept approximately constant. also, tlDs should not be read out within 30 minutes of irradiation to avoid short-time fading. tlDs should be kept in the dark for most of their life.
Diodes are read out immediately, and fading is not therefore a consideration.
for mosfets, fading is only relevant if they are to be used to store measurements for an individual patient.
It was not intended that this would be done in this project. It was recommended that readouts of mosfets should be performed 2 minutes after exposure.
for oslD, fading is rapid in the first few minutes after irradiation, and therefore it was recommended that the dosimeters should be read after a constant time delay (e.g. 10 minutes after irradiation).
4.4.2. Beam dependent correction factors
the beam dependent correction factors described below were measured for each photon beam quality to be used for in vivo dosimetry, both for 60co and linear accelerators. for each beam, the appropriate buildup caps were used.
4.4.2.1. Angle of incidence correction
the dosimeter was placed on the surface of the solid phantom with the sensitive volume at the isocentre, and readings were taken for 100 mu (or equivalent for 60co) with the gantry at –60º, –45º, –30º, –15º, 0º, 15º, 30º, 45º, 60º. If Mang is the reading at each angle, then the correction factor kang is given by:
0 0
ang ang
ang
M D M
k M M
D
° °
= =
(9)
kang is not normally needed except when measuring areas such as the breast and larynx. the primary measurement of this factor was carried out at the middle energy to be used clinically (e.g. 6 mv). It was checked at 45º for each energy being used. If the factor differed by more than 1.0% from the primary measurement, then it would be necessary to carry out a full set of measurements for each energy.
4.4.2.2. SSD correction
In general, the ssD correction originates from three effects. firstly, there is the geometric effect of the different distances from source to ionization chamber and from source to in vivo dosimeter. secondly, there is the dose rate dependence pertaining to diodes and mosfets (see section 4.1.5). the third effect is relevant for dosimeters with insufficient buildup and relates to the contribution to the dosimeter signal from contaminant electrons.
for dosimeters with appropriate buildup caps exhibiting low dose-rate dependence (tested as described in section 4.1.5), the need for an ssD correction should be obviated by the specific inverse square correction between the depth of dose maximum and the position of the active volume of the dosimeter. however, it is necessary to verify that, in practice, no correction is required. the procedure was to place the dosimeter on the surface of the phantom as for the dosimeter calibration (section 4.3.5) and perform measurements at 70, 80, 90, 100 and 110 cm with a fixed collimator opening (the same as for the reference conditions). the dose D0 = 100 cgy at dmax, (corresponding to 100 mu or the equivalent time for 60co) was set for each distance. for this purpose, it was necessary to calculate the predicted dose at the extended ssD. this was calculated using the Burns formula as already described in section 4.1.5. the dosimeter reading must be corrected for ssD to the depth dose maximum using the formula in section 4.3.1.
the resulting formula is given by:
(10)
where SSD0 corresponds to the standard treatment distance (as used for the dosimeter calibration), and SSD is the distance of interest.
the corrected dosimeter readings were then plotted against the ssD. If the graph implied that the correction needed for the range of ssDs to be used was greater than 1%, it would be necessary to include a correction.
4.4.2.3. Field size correction
the field size correction factor may be needed if the dosimeter is used for a wide range of beam sizes and the buildup thickness is insufficient as the changes in dose response originate from contaminant electrons, and the dosimeter is exposed to varying scatter conditions depending on the field size. the field size correction factor is defined as
0 10 10 field
X X
M k D
M D
×
×
=
(11)
D0refers to the absorbed dose to water at dmax, and ion chamber measurements should be converted from the reference depth in water to dmax. If both the dosimeter and ion chamber measurements were made in plastic, an appropriate conversion to derive absorbed dose to water at dmax should be used. alternatively, ion chamber measurements could be made in water, while the measurements for the in vivo dosimeter were made on a solid phantom. for convenience, the formula describing kfield can be converted to group in vivo dosimeter measurements in the numerator and ion chamber measurements in the denominator.
( )
( )
( )
( )
2 0 0
0 0
2 s max SSD
SSD s
SSD max
SSD d M
D SSD d k
SSD d M
D SSD d
−
×
+
= × +−
4.4.2.4. Wedge correction
the wedge correction factor is sometimes needed because of the change in the beam energy spectrum associated with introducing a wedge into the beam. for virtual wedges, obtained by moving the linac jaws, this is unlikely to be necessary.
the wedge correction factor is defined as the ratio between the wedge transmission factor (WF) for a 10 cm × 10 cm open field, measured with an ionization chamber at the reference depth and converted to dmax, and the wedge transmission factor for the same field size, measured with the in vivo dosimeter placed on the surface of the solid phantom.
(13)
where the wedge transmission factor is defined by:
(14)
the measurement was carried out as follows. the solid phantom was set up as for calibration, but with the dosimeter accurately placed on the beam’s central axis. Dopen = 100 cgy, i.e. 100 mu (or 100cgy in a cobalt-60 beam) was delivered without a wedge. the mus were then divided by the wedge factor, Wf, in order to give the same dose at dmax as for the open field, muwedge = 100/Wf and the relevant number of monitor units delivered.
under the measurement conditions (Dopen = Dwedge), the wedge correction formula is simplified to:
open wedge
wedge
k M
=M (15)
If Mopen and Mwedge differ by more than 1%, a correction will be needed. however, if the correction does appear to be significant, the value of Wf as defined here should be carefully checked. If kwedge is large for a 10 cm × 10 cm field size, it may be necessary to repeat the measurements for different field sizes, depending on the clinical use.
4.4.2.5. Block and tray correction
If in vivo dosimetry is used for blocked fields, the block and tray correction may be needed. measurements of the correction factors are performed similarly as for wedge factors (see section 4.4.2.4).
( ) ( )
( )
( )10 10
0 10 10 X X field
X X
M k M
D D
×
×
×
×
= (12)
wedge
open open ic
wedge
wedge det
wedge open
M D
D
D WF
k M M WF
D M
= = =
100 100 max max
Dose at d for MU withwedge Dose at d for MU without wedge WF=