Example of 3 Field Technique Telecobalt Isodose Charts used

1. Manual Isodose Plotting
Example of 3 Field Technique
Telecobalt Isodose Charts used
Dr.V.Lokesh
Professor & HOD
Department of Radiation Oncology
KMIO
Compiled By Dr.Hemant Vasan
Post Graduate Resident

2. Place the template in the correct orientation

3. Choose the appropriate isodose chart and place beneath
the transparency

4. Align the central axis of the beam to the central axis of
the Tumor at the surface of the body contour and mark
the entry point

5. Mark the entry point at the central axis in the direction of
the beam

6. Start drawing the corresponding isodose lines and write
the dose percent each with a unique Color , here the first
line is traced with green Color and labelled 90

7. Trace out each curve with a unique colour noting the
correspond dose or the edge

8. The AP Beam Isodose curve would look like this as it
exits

9. Now , Flip the chart and repeat the same process for the
PA beam using the same colours corresponding to the
AP beam

10. The AP and PA beams together will look like this

11. For a 3Field , as in this case , an ipsi- lateral beam has
been placed , Align with the central axis and repeat the
same process using corresponding colours

13. This will be the result after tracing all the 3 beams ,Note
the central axis direction of each beam and the
corresponding isodose lines

14. Now, Systematically mark all the points of intersection
and add up the doses at the point of intersection Eg here
the the point would add up to as 80 (red) + 35(blue) =
115

15. Systematically find points of intersection and summate
corresponding doses

16. In case it gets very crowded, you can overlay a second transparency to
plot the points of intersection and their respective summated doses

17. Join the points of same doses with unique colours and
smooth curves to finally get an isodose line distribution
like this

18. 90% -95% isodose selected for prescription