Simple calculations based on SSD, PDD and TMR
SSD – the distance between the source and the surface of the patient
PDD – percentage depth dose. The percentage of dose at a given point compared to a reference of 100% dose at Dmax
TMR – Tissue maximum ratio. The ratio of dose at 2 different depths in a phantom, however the phantom is moved so that the source to measurement point distance is constant. Hence the ratio of dose is reflective of the differential attenuation and does not need to consider any differences due to SSD.
TMR is the ratio of dose at depth d to the dose at depth of d max, such that both points are placed at the isocentre or at a fixed distance (usually 100cm SSD).
If SSD moved closer: the PDD changes, but total dose changes as well due to inverse square law. Assuming no attenuation in tissue, the dose at two different distances is inversely proportional to the square of the distance from the source.
Dose at distance a / Dose at distance b = (distance b / distance a )2 (inverse square law)
FOR isocentric treatments use a new reference depth which is 10cm deep in water in a 10×10 field. The surface of the patient is closer to the source actually the reference point is at a depth in tissue, so with more attenuation that a classic FSD 100cm reference point on the surface of the patient.
Maynard F factor is used to calculate change in percentage depth dose at a point if the SSD is changed. (if using PDDs to calculate dose and SSD changes, it gets complicated, and hence for isocentric treatments TMR is usually used)
PPD 2 = PDD 1 x (SSD2 + d max/ SSD1 + d max )2 x (SSD1 +depth / SSD2 +depth )2
or PDD new / PDD old = (old total distance / old d max dist )2 x ( new d max distance/ new depth total distance )2
If for a given set up of fixed SSD, ask to calculate dose at two different points in a radiation field and only TMRs given:
Dose 1/Dose 2 = TMR1/TMR 2 x (distance 2)2/ (distance 1)2
REMEMBER – TMR only accounts for attenuation, NOT for inverse square law. PDD accounts for BOTH.
For head and neck and lung – usually 6MV beam used ( low energy will have smaller secondary build up regions at air/tissue interfaces in patient)
Pelvis – 8MV or higher usually (better PDD with higher dose, higher skin sparing and better penetration), more homogenous dose distribution