Module 1

Unit 2: Learning tasks


Tasks

Open patient case “8 Phantom for comparison”, Case 2 from the archive. Enter your name in the textbox, click on "Accept cmts" and save the case to the checkpoint database (select "My own storage").

This case provides an opportunity to investigate how different beam parameters influence the dose distribution of a single beam in a water phantom. The virtual phantom used here has the dimensions 40 x 40 x 40 cm³. The density of the phantom is set to 1.0 g/cm³ corresponding to water at room temperature.


Preliminary steps

In order to compare the effects of different parameters, it is sometimes useful to have a reference plan.

Plan 1 is used as the reference plan with standard settings (set plan colour to green)
Plan 2 is the plan that you can modify (set plan colour to red)

Open both plans (corresponding to the same phantom). Add a transverse view to both plans and compute the dose distribution.

To normalize the plans to the maximum dose on the central axis, compute the point doses in the Point dose panel and set the dose at depth of the dose maximum (for 6 MV: zmax=1.6 cm) to 100 cGy.

NOTE Only absolute doses are possible to be displayed in Prism. In order to normalize a plan we pretend that 100 cGy corresponds to 100%.


Plan 2 is the plan that you can modify. Henceforth you'll only work on Plan 2! Keep Plan 1 open as reference.

Documentation

Use the to document your work. Save the answer template as Module1_YourName.doc or ".pdf" and submit after you have completed all units of Learning Module 1.


Task 1: PDDs

a) Based on the isodose lines and the corresponding point doses, plot the PDD for the standard settings. Use the to plot the PDDs.


Change the parameters listed below one by one. Record the PDD for each setting in the same graph. Note, do not forget to normalize the plan to zmax after each calculation.

Note: don't forget to set back to standard settings before changing the next parameter.
Isodose linesValues and colours of the predefined isodose lines:
Value in cGy
 Colour
107
 red
100
 white
95
 blue
90
 green
80
 yellow
70
 magenta
50
 cyan
30
 gray
Parameters to be changed:

  • Change of field size to 20 x 20 cm².

  • Change of SSD (change CouchHT).

    • Try to find out to which SSD a couch height of 0.0 cm corresponds to!
      Hint: Open a transverse view and measure the SSD with the ruler.

    • If you increase the couch height, does the SSD increase or decrease?

    • Change SSD to 50 cm and 150 cm.

  • Change of energy (use the SL20A-18MV-VJC, hint: what is zmax for 18 MV?)

(This is what it should look like: .)



b) For a field size of 10 x 10 cm², 100 MU, 6MV photon beam, what SSD results in an absolute dose of 1 Gy in the depth of the dose maximum?



c) Switch back to standard settings and

  1. Write down the dose at zmax for SSD = 100 cm.

  2. Now change the SSD to 105 cm (CouchHT = - 5 cm).

  3. Use the inverse square law to calculate how many MUs are now required to achieve the same dose at zmax as in I. Check your result with Prism.



Task 2: Wedges

Switch back to standard settings. In the Beam panel, click the button labelled "No wedge" to add a wedge to your beam. Explore what happens to the dose distribution for different wedges.

Determine the wedge factor for a 45° wedge at zmax.



Task 3: Parallel opposed beams

Switch back to standard settings. Use the Copy 180-function in the Beam panel to add an additional parallel opposed beam. Compute the dose and normalize the beams to 100 cGy at the centre of the phantom (Point 14, 20 cm depth).
Why is the isodose distribution not symmetrical?
How would you change the beam weights to obtain a more symmetrical dose distribution?

(Here are .)



Task 4: Effect of inhomogeneity corrections

Open patient case “8 Phantom for comparison”, Case 3 from the archive. For this case an organ is embedded in the phantom.

The organ has the same density as the water phantom (1.0 g/cm³). Compute the isodose distribution.
 
Compare this isodose distribution to the isodose distributions of the two following cases:

  1. The density of the organ is greater than the density of the phantom (compact bone ~1.8 g/cm³).

  2. The density of the organ is less than the density of the phantom (lung tissue ~0.3 g/cm³).


To change the density settings for the organ, open the Volume editor by clicking the "Anatomy"-button in the Patient panel. Select the organ “ORGAN” in the upper right corner. Change the value of the organ’s density in the textbox labelled “Den.” in the lower left corner!

The remaining functions of the Volume editor will be described in Learning module 2!

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