IMRT Dosimetry Concerns:

  1. Small Field Sizes.
  2. Large Numbers of Monitor Units.
  3. Leaf Transmission Leakage (Intraleaf Leakage) - typically around 2%.
  4. Interleaf Leakage - typically around 3-4%.
  5. Leaf-End Leakage - typically around 15-20%.
  6. Neutron Head Leakage - as photon beam energy increases, there is an increase of the magnitude of neutron contamination being emitted from the linac head (at 6X, the leakage negligible, but at 18X, this value has increased to contribute 0.15% of the head leakage).  It is for this reason that 6X and 10X photon energies have been employed for IMRT applications versus 18X beams.

 

The Dosimetric Leaf Gap:

  1. The dosimetric leaf gap (DLG) is a term that must be calculated for all MLCs and directly impacts (is used in models) dose calculations in treatment planning systems.
  2. Physically, it is the leaf-end leakage and arises for each of the major MLC designs currently used in radiation oncology.
  3. For Varian MLCs, the dosimetric (not physical) gap arises because we have MLCs with rounded leaf ends meeting from each bank.  Even when the leaves are completely closed against each other, there is a substantial dose that can be measured as a thin strip where the leaves meet.  The width of this strip is the DLG.
  4. There are several ways to measure this factor which is then inputted into the treatment planning system as a dosimetric term.  Two will be described as follows:
  1. Film:
  1. With your calibrated film, make a series of exposures with varying physical leaf gaps between the leaves of the two banks (e.g. 2, 4, 6, 8, 10 mm, etc.).
  2. Each gap should then have its own film.
  3. Analyze these films to discover the full-width at half-maximum (FWHM) for each data point.
  4. Plot FWHM versus leaf gap.  This plot should be largely linear.  Extrapolate your linear fit back to an FWHM of zero (i.e. the x-intercept).  The absolute value of this x-intercept in mm will be your DLG.
  1. Chamber:
  1. The basic idea for this procedure is that you expose an open field and then you expose uniform fields that are made with sliding MLCs with varying leaf widths between banks A and B.  As long as the MLCs move with constant speeds, there should be a uniform exposure made.
  2. We repeat the uniform exposures for a series of leaf widths, and again, plot a detector reading (it could be film or a chamber).
  3. By tracing the linear fit back to the x-intercept of chamber reading versus leaf gap width, the DLG can be measured as the absolute value of the x-intercept.

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  1. A typical value for Varian MLC’s is about 1.5-2 mm.

 

IMRT and Dose Rate:

  1. An important concern when using IMRT delivery methods using a sliding window technique is the dose rate used to deliver the treatment.
  2. MLCs have a maximum speed at which they can move (usually about 2 cm/s) and if the dose rate is too high they end up delivering no primary radiation while getting into position.
  3. This results in increased leakage which can reduce the degree to which you can shield healthy structures.
  4. It has been found that about 300-400 MU/min is optimal for delivery time versus tissue sparing.

 

Large Field Radiotherapy:

  1. The following are various diseases that may be treated with hemibody or total body photon irradiation:
  1. Multiple Myelomas.
  2. Limited Immunity Deficiencies.
  3. Limited Genetic Disorders (e.g. Diamond Blackfan Anemia).
  4. Bone Palliation.
  5. Leukemia.
  1. The purpose of total body irradiation (TBI) is to suppress a patient’s immune system and decrease the likelihood of rejecting the graft and at the same time helps eliminate existing bone marrow.
  2. Patients are treated to about 12-14 Gy given twice per day separated by 6 hours, the dose rate must be kept around 10 cGy/min to reduce lung toxicities.

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