GAMOS tutorial

X-ray

Exercises

Pedro Arce DuboisCIEMAT

http://fismed.ciemat.es/GAMOS

Pedro Arce GAMOS radiotherapy tutorial 2

X-ray simulation Exercises

Ex. 1: X-ray tube

Ex. 2: Writing phase space

Ex. 3: Score dose in phantom

Ex. 4: Score lateral dose

The exercises are sequentialUse the commands of the previous exercise (only change what indicated)

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Exercise 1: X-ray tube geometry

Tube of inner diameter 60 mm, length 100cm. Wall of 2 mm of pyrex. Filled with vacuum (G4_Galactic material). The tube is placed rotated -90 degrees around the X axis

The tube has a window hole of 2.5 cm length and 2.5 cm width, placed at phi 90 degree (so that after rotation it will end at the positive Z side). In the middle of the window hole there is a foil of 125 microns of berilium at Z positive. The centre of the window foil is placed at position (0,0,0)

The tube is surrounded by a shield 2 cm thick of lead. In it there is also a hole at the position than the tube

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Exercise 1: X-ray tube geometry

The anode is built from an alloy of 90% tungsten and 10 % rhenium. It is a polycone of width 10 mm, radius 20 mm at one side and 5 mm at the other side. The surface has an inclination of 12 degrees. The anode axis coincides with the tube axis and it is placed at 80 mm from the tube wall, with the smaller radius towards the tube center

The source of electrons is a cylinder of air of 5 mm radius and 10 microns thick, placed so that its centre is in front of the centre of the anode inclined surface, at mid-distance of radius (=12.5 mm out of centre), and at 50 mm along the tube axis coordinate

The primary particles are electrons of 150 keV homogenously distributed in the source cylinder and with initial direction along the Y axis

Physics is low energy electromagnetic one

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Exercise 1: study gamma propagation

Plot the initial kinetic energy and direction theta angle of all gammas created

Plot the kinetic energy and direction theta angle of all gammas when they reach the inner side of the tube (or the window hole inner side)

Plot the kinetic energy and direction theta angle of all gammas when they exit the shield

Plot the total length that primary electrons travel inside the anode

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Exercise 2a: write phase space

Use setup of exercise 1 and write a phase space of the particles that reach a Z plane situated 10 cm below the tube window foil

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Exercise 2b: use bremsstrahlung splitting

Same as exercise 2a, but use Z-plane

bremsstrahlung splitting

Z plane defined at 10 cm from window foil and size 15 X

15 cm

Try splitting 100 times and see bias in phase space

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Exercise 3a: score dose in phantom

Use the phase space created in exercise 2Run 1000000 events, reusing phase space file

Score dose in phantom voxelsUse test_64x64x39.g4dcm

Print a report of the dose in each voxel, with errors

Make histograms of dose in voxels (PDD, profiles)

Write dose in a file, with dose squared, to allow summation of files

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Exercise 3b: place a protection before phantom

Repeat exercise 3a, but placing a protection of 2cm of Cerrobend, 10 X 10 cm wide, at the centre of the phantom, touching itCompare dose distributionsPlot length of tracks traversing protection

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Exercise 4a: Calculate lateral dose

Read phase space from exercise 2

Use point detector scorer to calculate dose in three points

At the Z of the window foil at Y = 25.1 cm (1 mm from phantom)At Z 1 meter below the window foil at Y = 25.1 cmAt Z 1 meter above the window foil at Y = 25.1 cm

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Exercise 4b: Calculate dispersion in patient

Same as exercise 4a, and place patient of exercise 3

Compare equivalent dose in each of the three points