A system of dosimetric calculations 2005/02/21

Percentage depth dose (PDD)

PDD

PDD vs. SSD

Mayneord factor  unsuitable for isocentric techiniques The dependence of PDD on SSD  unsuitable for isocentric techiniques

Tissue Air Ratios (TAR)

The limitations of TAR It’s hard to measure in free space when the beam energy higher than Co-60. As the beam energy increases, the size of the chamber build-up for in-air measurements has to be increased. Karzmark et al.  tissue-phantom ratio (TPR) Holt et al.  tissue-maximum ratio (TMR)

Effective primary dose The effective primary in a phantom may be thought of as the dose at depth minus the phantom scatter. The effective primary dose may be defined as the depth dose excepted in the field when scattering volume is reduced to zero while keeping the collimator opening constant.

Collimator scatter factor (Sc) output factor It may be defined as the ratio of the output in air for a given field to that for a reference field. Sc may be measured with an ion chamber with a build-up cap of a size large enough to provide maximum dose build-up for the given energy beam.

Phantom scatter factor (Sp) It takes into account the change in scatter radiation originating in the phantom at a reference depth as the field size is changed. At the depth of maximum dose may be defined as BSF

Total scatter factor It’s defined as the dose rate a reference depth for a given field size r divided by the dose rate at the same point and depth for the reference field size(10*10 cm). It contains both the collimator and phantom scatter.

Arrangement for measuring Sc and Sc,p

Tissue-Phantom Ratio (TPR) It’s defined as the ratio of the dose at a given point in phantom to the dose at the same point at a fixed reference depth, usually 5 cm. TPR(r)=TPR(0)+SPR(r) The corresponding quantity for the scattered Dose is called the scattered-phantom ratio

The definition of TPR/TMR

Tissue-Maximum Ratio (TMR) TMR is a special case of TPR. It may be defined as the ration of the dose at a given point in phantom to the dose at the same point at the reference depth of maximum dose.

Properties of TMR TMR is independent of SSD FS ↑, TMR ↑ depth ↑, TMR ↓

The area factor It’s the ratio of the dose rate for a given field size to that of a reference field size, usually 10 * 10 cm. The dose rate for a reference field multiplied by the area factor for the treated field gives the dose rate for the treated field. The dose rates may have been measured and tabulated for a large number of field sizes, which eliminates the need for an area factor. The cGy/MU may have been set so that 1 cGy is the same as 1 MU for a reference field, is usually 10 * 10 cm.

Dose-Calculation’s Parameters Prescribed dose SSD: PDD SAD: TAR, TMP, TPR Field size factor (Khan: Sc,p) Dose per Monitor Unit (Dose/MU) Accessory factors ( tray, wedge…etc.)  MU

Techniques in treating patients Fixed source-surface distance technique Fixed source-axis distance technique

Fixed source-surface distance technique

Dose calculation at depth of maximum dose Consider the treatment of a 12 * 12 cm field using a 4-MV photon beam at 100-cm SSD. The Dose/MU for a 10 * 10 cm field is 1. The field size factor for 12 * 12 cm is 1.03. To find the necessary to deliver prescribed Dmax dose:

To deliver 200 cGy at Dmax which for 4 MV is at 1-cm depth, the number of MU would be:

To deliver 200 cGy at 5 cm

Fixed source-axis distance technique

Parallel opposed fields using an isocentric technique A treatment is delivered via equally weighted parallel opposed beams using 4-MV photon beams. The number of MU is found from

To deliver 100 cGy at 12-cm depth The patient is 24 cm in diameter, the SAD is 100 cm, and the fields are 12 * 12 cm. The Dose/MU at Dmax is 1.000 The FSF is 1.030 for 12 * 12 cm The TMR from A-18 at 12-cm depth is 0.716.

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