At a depth in the patient, well beyond the buildup layer, 1.5 cm for
6 MV and 3.5 cm for 18 MV photon beams, the transmission factor for one
cm of tissue can be found from a Tissue-Maximum Ratio table by dividing
a TMR value by the corresponding entry one cm less deep, for
representative field sizes and depths beyond the buildup region. Here
are some examples:
|energy||field size||depth, d||TMR(d)||TMR(d-1)||transmission,
|6 MV||10x10 cm2||10 cm||.792||.821||.9647|
|18 MV||10x10 cm2||10 cm||.904||.922||.9805|
So we can see that the change in transmission factor for 6 MV
photons is about 3.6% per cm and for 18 MV about 2.3% per cm of depth,
regardless of field size and depth, to within the precision of beam
data measurements and roundoff errors in the table.
|An error in distance alone results in a change in intensity of the radiation beam which depends inversely on the square of the distance from a point source such as an x-ray target, since the emitted energy spreads over an area proportional to the square of the distance. The relative change in absorbed dose depends on the ratio of distances squared. Examples for 1 cm difference:|
|Thus the difference in absorbed dose is about 2% per cm of displacement for points of clinical interest.|
What if we have parallel-opposed fields? A 1 cm error in SSD setting is exactly balanced by an opposite error in the opposing field, provided we do not reposition the patient between fields but rely on the precision of gantry motion for the second setup.
The depth to a point in the patient is unchanged, so the 3.6% difference in transmission for 6 MV does not apply. The 2% per cm difference due to distance error would apply. Errors in lateral positioning would be most critical, since the dose gradient at beam margins is so high that dose differences are near 100% for points inside or outside a beam.
What do we mean by ISOCENTER?
A point in the therapy beam which stays at the same placeThus it is the coincidence of the gantry rotation axis (a horizontal line in space), the table rotation axis (a vertical line in space), and the collimator rotation axis (which traces a plane perpendicular to the gantry axis).
during gantry, collimator, and table rotations.
Why is it useful?
Because a radiation beam defined by symmetric collimator jaws is centered there.But it is hiding inside the patient except at 100 cm SSD!! To make the isocenter useful for patient positioning we use ORTHOGONAL pointer lights that project perpendicular planes, which cross in perpendicular lines that are the axes of gantry rotation (horizontal), table rotation (vertical), and collimator rotation (when set for a horizontal beam). These coordinate axes point toward the isocenter and can be used with skin marks to reproduce the patient position once it is established.
So we see that "ISOCENTRIC" treatment planning has two meanings:
Treatment planning computer systems use this data format regardless of how depth-dose data was acquired and entered to the program system; accuracy of calculated dose distributions is better near the isocenter. Any plan that we enter to calculate isodose curves is handled this way internally by the program system, regardless of the conceptual system we use to specify treatment ports.The system of depth-dose data relative to the dose at isocenter in a standard field geometry, as a function of depth of overlying tissue and field size in surrounding irradiated volume.
2.We rely on the precision of mechanical rotations to achieve the geometry of combined fields that is planned, and errors in patient positioning result in compensating errors in dose distribution relative to the isocenter.The system of patient positioning so that multiple ports are treated without repositioning the patient because the center of gantry and table rotation, the isocenter, is the same point in the patient for every field.
However we must rely on positioning aids to make the planned treatment volume coincide reliably with the anatomical target volume, and avoid geometrical misses which are the most frequent cause of failure in treatments which should have good prognosis.
When extended distances are required to achieve larger field sizes or because of access restrictions, a multiple port plan with axes crossing at a common point has been called TELECENTRIC.
Let us borrow from the vocabulary of earthquake geology the word EPICENTER to refer to a surface point above the phenomenon of interest. An EPICENTRIC TREATMENT PLAN is a multiple-port plan where various fields are positioned so that orthogonal skin marks are at the gantry isocenter. This was the standard system for cobalt treatments at 80 cm SSD. The target volume is centered at a point on the beam axis that is a different distance from the isocenter for each field, so this is NOT an isocentric plan, and it does not automatically compensate for distance errors because the patient is repositioned for each field.
Let us borrow also the term HYPOCENTER to refer to a focus below the surface. A HYPOCENTRIC therapy field is one where the gantry isocenter is at a depth in the patient but not at the same point for multiple ports in the treatment plan. For example the combination of AP and PA fields positioned for isocenter at the midplane, with lateral or oblique fields using a different location of the gantry isocenter in the patient, can be called a HYPOCENTRIC plan.
There may be good reasons for using such a plan, for example when the target volumes are different for an initial course of therapy and a planned boost, or when blocking or asymmetric collimator settings make the isocenter unusable as a representative point for the nominal dose prescription in some or all of the fields.
In such cases we must recognize the necessity to establish and
locate appropriate calculation points so that absorbed doses from
different fields to the same anatomical location can be added. Also we
must establish different orthogonal skin marks or prescribe table
motions to reposition the patient for each HYPOCENTER location.
Why do we use isocentric treatment plans?
Because the therapy machine calibration is measured at isocenter?
No. In a recent survey by the Quality Assurance Review Center, twice as many institutions used fixed-SSD geometry for machine calibration as used isocentric setups.
No. We have relative depth-dose data to calculate the dose anywhere.Because our depth-dose data is formatted for calculating dose at any point relative to isocenter?
No, because computer programs are so fast and reliable that the time required to reformat the data is not a problem.
Yes, but we must get the right position to begin with.
Yes, but lateral errors do not.
Yes, but SSD and depth errors do not compensate except for parallel-opposed ports.
Yes, but this is also a good reason for fixed-SSD plans.