RTH-21

TG-51 absorbed dose to water

Dw = M kQ ND,w PTP. Chamber reading to water dose.

Reading speed

CalibrationAAPM TG-51

Governing equation

D_w=M\,k_Q N_{D,w}P_{TP}

$M \in \left[0.5, 40\right]$

where

M: Corrected reading (nC)
N_{D,w}: Absorbed-dose cal. coeff. (Gy/nC)
k_Q: Quality conversion kQ (—)
T: Temperature (°C)
P: Pressure (kPa)
D_w: Dose to water (Gy)

The figure, part by part

Each numbered marker wears the color of its card. Hover a card to light the marker. Tap a marker to jump to that card. The slider walks a red point along the curve; Animate drawing traces it like a pen.

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①

Governing law — red

D_w=M\,k_Q N_{D,w}P_{TP}

$M \in \left[0.5, 40\right]$

$M = 0.5$ $D_w = 0.024$

Along the curve

②

Domain of the variable — blue

M_{\min} = 0.5,\quad M_{\max} = 40

$M \in \left[0.5, 40\right]$

$M = 0.5$ $M = 0.5$

Along the curve

③

Response curve — green

D_w=M\,k_Q N_{D,w}P_{TP}

$M \in \left[0.5, 40\right]$

$M = 0.5$ $D_w = 0.024$

Along the curve

④

Rate of change — amber

\frac{\mathrm{d}D_w}{\mathrm{d}M}

$M \in \left[0.5, 40\right]$

$M = 0.748$ $D_w^{\prime} = 0.048$

Along the curve

Lecture brief

Historical brief

The linear-quadratic model, BED, tissue-maximum ratio and Clarkson scatter turned radiotherapy into fraction arithmetic. MU calculation on these sheets is that clinical closed form. This sheet (RTH-21 — TG-51 absorbed dose to water) is the form associated with AAPM TG-51. Working symbols: $M$ , $N_{D,w}$ , $k_Q$ , $T$ , $P$ $\rightarrow$ $D_w$ . PTP = ((273.2+T)/295.2)(101.325/P). kQ accounts for beam quality versus 60Co.

Purpose

Purpose: compute

D_w

from

M

N_{D,w}

k_Q

T

P

in Radiotherapy via

D_w=M\,k_Q N_{D,w}P_{TP}

Dw = M kQ ND,w PTP. Chamber reading to water dose. Use it when a real radiotherapy question must be answered in SI before a code check.

Live realistic example

In symbols

Live case. Given

M = 12.000\,\mathrm{nC}

N_{D,w} = 0.048\,\mathrm{Gy/nC}

k_Q = 0.995\,\mathrm{—}

T = 22.000\,\mathrm{^{\circ}C}

P = 101.300\,\mathrm{kPa}

, the governing relation

D_w=M\,k_Q N_{D,w}P_{TP}

yields

D_w = 0.5733\,\mathrm{Gy}

. A Farmer chamber in a water tank, a calibrated Dw. Move a slider: the numbers are this situation, not a canned story.

Calculator

Inputs

Corrected reading M12 nCAbsorbed-dose cal. coeff. N_{D,w}0.048 Gy/nCQuality conversion kQ k_Q0.995 —Temperature T22 °CPressure P101.3 kPa

Outputs

Dose to water D_w0.5733 Gy

Reading speed

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Narration of this film

A Farmer chamber in a water tank, a calibrated Dw.

PTP = ((273.2+T)/295.2)(101.325/P). kQ accounts for beam quality versus 60Co.

Reading speed

Watch on YouTube