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Photon Therapy II:
Intermediate Photon Therapy
Measuring kVp for a Superficial/Orthovoltage Photon Beam:
- Method 1 - Fluorescence Method:
- This method uses two chambers: one to measure transmission through an oblique absorbing material and one to measure scatter off of this material.
- The voltage is adjusted until a maximum value of the ratio of the two chambers is seen.
- This value corresponds to the K-edge.
- Method 2 - Penetrameter Method:
- This method uses an experimental cassette containing a series of filters and a film that is exposed to the photon beam.
- The exposed cassette is compared to a standard cassette to derive kVp information.
- (This is mentioned for historical interest.)
- Method 3 - kVp Meter:
- The simplest way to measure kVp is to acquire a calibrated meter that you place in the beam and take an image.
- These meters can instantly tell you the kVp and mAs of the beam for QA purposes.
- These effects refer to dose rate dependencies for both Co-60 and LINACs when turning a beam on or off.
- The effect itself is caused by a ramping up of the dose rate as a Co-60 source is translated into a beam-on position or a general ramping up of the dose rate for the linear accelerator. This effect should be negligible for normally operating linear accelerators, but corrections are needed for Co-60 based external beam therapy.
- Historically for Co-60, this has also been termed the shutter effect. This is because, for some Co-60 irradiation units, the Co-60 source was housed in a shielded beam-off position. The source would be translated into an “exposed” position via a remotely operated hydraulic or motorized mechanism. As this source would move into an open position, it would result in a short period of time wherein the radiation source was partially exposed. This, in turn, would result in a decreased dose rate. Hence, a correction factor for irradiation times was needed to account for the shutter effect based on when your timer would begin.
- In order to solve for a correction time that accounts for the end/shutter effect, we may consider the following process:
- Take two readings, R1 and Rn at the same setup conditions. R1 is taken with a sufficiently long exposure time, t, while Rn is taken with a series of exposures (of equal times) that sum to equal the time, t.
- Logically, then, R1 can be equated to: , where is the correction time.
- Then, if you take reading, Rn, with the same total time, t, then this reading should be:
- Since the reading rate did not change between the two measurement sets, we can equate the rates as follows:
- Solving for the correction time:
- Note that the value for the correction time should be positive and should be subtracted from the treatment time for the Co-60 exposure.
The Clarkson/Cunningham Scatter Technique:
- The three components of the external photon therapy beam are:
- Non-scatter, non-attenuated primary beam,
- Scatter off of the collimators,
- Scatter in the medium.
- Note that depth dose data for radiation therapy beams is usually tabulated for square fields. However, the majority of applications in radiotherapy require rectangular or irregularly shaped fields. Therefore, semiempirical methods have been developed to relate central axis beam data for square and rectangular fields. These involve the use of equivalent squares and area/perimeter values. But, these methods do not apply for the cases of off-axis points and for irregular (blocked) fields.
- Thus, in order to relate tabulated data for off-axis and irregular fields, we turn, most commonly to the Clarkson/Cunningham method. This method is based on the principle that the scattered component of the depth dose (which depends on field size and shape) can be calculated separately from the primary component which is independent of the field size and shape. We, therefore, may split TAR into two components: one representing the primary component of the beam, independent of field size and shape, and the other representing the scattered component of the beam, dependent on field size and shape (as represented by the following equation):
- The idea behind this technique is that we cannot refer to our tabulated data for TAR values directly (including both primary and scatter air ratios) for these irregularly blocked fields.
- It may be worth noting here that we can return to depth dose data then by recalling the following relationship:
- SSD - is equal to the SSD in the %DD setup
- Sp - is the phantom scatter factor for the field size depth d or dmax in the %DD setup
- The primary component of the beam, TAR0, can be determined by noting that it is the TAR for a theoretical 0 x 0 cm2 field size. We can then deduce values for TAR0 by extrapolating back to zero field size using data points of other TAR curves of varying field size.
- Once TAR0 has been found you may calculate SAR tables using your measured TAR tables and TAR0.
- For an irregularly shaped field, the SAR value can be obtained semi-empirically by measuring the radii of several lines emanating from a point in the treatment depth plane. For blocked fields, these radii combine via addition and subtraction to provide an SAR value which can then be compared with other radii SAR values to find an average.
- Then, by combining the primary and scatter air ratios, we can then derive an average TAR value that can be used in traditional hand calculations.