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Adjacent Photon Fields:
- The reason that we gap in radiotherapy is to avoid exceeding the threshold dose for a series organ (e.g. spinal cord) because we may be joining multiple fields in a single treatment or we may have given prior radiotherapy to a nearby region.
- Gapping Methods:
- Simple Gapping - here, we introduce a geometric gap at the surface of the patient that is just large enough to ensure that there is no overlap at the Organ At Risk (OAR). By doing so, we avoid doubling the dose to the OAR due to overlap but may introduce cold spots.
- Moving Junction - here, we allow the divergent fields to overlap, but we only treat partially to the total tumor dose. After a certain fraction of the tumor dose is delivered, we slide the junction a certain distance away from the first junction to a second location to ensure that no single volume obtains excessive cold or hot spots.
- Beam Splitting - beam splitting refers to pushing one of the asymmetric primary jaws in front of the beam to the center to create a situation where we can line up the fields more easily without divergence. Relying on this requires exact and unforgiving knowledge of field alignment.
- Beam Spoilers - this device causes the edges of the fields that would be combined to have increased penumbra so that the match line is blurred resulting in smaller hot or cold spots due to poor beam alignment.
- Sometimes it can be necessary to compare two treatment plans with different delivery techniques. This may not boil down to simple tumor coverage and meeting dose constraints on normal tissues.
- The idea of integral dose is basically the total energy absorbed for the treatment and is the integrated weighted dose times the mass (so just energy).
- For instance, a VMAT plan may irradiate a lot of tissue to low doses whereas an IMRT plan may expose less tissue to higher doses. The concept of integral dose can be used to compare the plans.
- Note: this is seldom used clinically and is more of an academic pursuit.
- Integral decreases with increasing beam energy.
Radiation versus Mechanical Isocentricity:
- In order to measure the mechanical isocenter, the gantry, collimator, and table are usually rotated with a pen located at isocenter which is examined for movement (this should be within 1-2 mm).
- However, we are also interested in radiation isocenter which is tested using a Winston-Lutz test.
- This involves taking a series of films (or port films) of a small metal sphere located at the isocenter for a number of gantry, collimator and couch angle combinations.
- This should also be within 1-2 mm.