Plotting of Mannual Issodose line in different Cancer cases in Radiotherapy treatment plan and it is compared with Varian Eclipse Treatment Planning System. Comparision done in 2019 at Tata Memorial Hospital, Parel, Mumbai.

1. PIJUSH PAUL
RESIDENT MEDICAL PHYSISCIST
TATA MEMORIAL HOSPITAL

2. Content

3. Lines connecting the points of equal
percentage depth dose(PDD)
Drawn at regular interval.
Normalised at reference point.
Isodose line

4. Purpose
• Representation plannar variation of abosorbed dose.
• To get idea about the isodose line covering the PTV and
OAR.

5. Correction for contour
irregularities

6. Isodose shift method
 Used for Manually correction
of Isodose lines.
 Along each grid lines(1 cm),
Isodose line is shifted by an
amount:
k x h

7.  h (tissue deficit or tissue excess) & “k” a factor
less than 1 and is given in the following Table:
Isodose position along the grid are joined after
correction.

9. Energy

10. shift factor formula
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and
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t
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f
t
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if
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f
t
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t
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meanings
usual
have
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and
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D
y
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where
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D
t
f
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D
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t
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p













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


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

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



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








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,where G is relative
dose gradient

11. Gf
t
t
where
Gf
Gf
t
t
t
F
factor
Shift
2
..
..
2
)
(
_ '
'





FS=10x10cm2 ,SSD(f)=100cm
and for depth=7cm G=0.068
then
Shift factor:
71
.
0
100
068
.
0
2
100
068
.
0
2






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Gf
Gf
F
 Simillarly, corresponding values for
5x5cm2 and 20x20cm2 is 0.74 and
0.68.
(Giessen PH.A method of calculating the isodose shift in correcting for oblique incidence in radiotherapy.Br J
Radiol. 1973;46:978.)

12. • K = f(gap,radiation energy,SSD,field area and depth in absorbing medium)
• When gap(t) is small compared to SSD , then k is independent of gap.
• Example:If SSD is 50cm then k is not independent of gap
• But it is dependent on other factors.
• For depths below about 7cm, the factor is sensibly constant.
• It is shown that a single value of the factor may be used for all fields and
depths(for given radiation energy and SSD) with an error up to 1% to 2%.
(Giessen PH.A method of calculating the isodose shift in correcting for oblique incidence in radiotherapy.Br
J Radiol. 1973;46:978.)

13. Effective SSD method
It include correction of PDD with the help of ISF.
Here isodose lines are shift by an amount h (tissue deficit or
tissue excess)

14.  Here h is tissue deficit
 As we are interested to find PDD at
point A as shown in the figure.

15. Tissue-air ratio method
Correction of PDD done with help of TAR/TMR
actual
corrected PDD
CF
PDD *

)
(
)
(
h
d
TMR
d
TMR
CF

 ,Here d and h are in cm.

16. Corrections for tissue
inhomogeneities

17. Isodose shift method
Used for Manually correction of Isodose lines.
Proposed by Greene, Steward and Sundblom.
Shift are done by amount equal to n(emperically obtained
value) times the thickness of inhomogeniety.
It is measured along the line parallel to the central axis and
passing to the point of interest.
The shift is away from skin for lung and air cavity and
opposite in case of bone.

19. Small bulges are due to inoization chamber of
0.6 mm diameter fixed at position R.
Measured with air filled hole where the isodose
curve moved distally by 0.6 times the thickness
of air gap.
(Greene D, Stewart Jr. Isodose curves in non_uniform Phantoms.Br J Radiol.1965;38:378.)

20. Measured with cork
(simulating Lung density )
where the isodose curve
moved distally by 0.4 times the
thickness of cork/ Lung.
(Greene D, Stewart Jr. Isodose curves in non_uniform Phantoms.Br J Radiol.1965;38:378.)

21. • The value of n is given by the following table:
• It is seen that the factor depends on the properties of material of different
densities and independent of field size.
• Error percentage is about 2% when the above emperical rules is compared
with measured Isodose lines .

22. Other correction factors
• Tissue_Air Ratio Method.
• Power Law Tissue_Air Ratio Method.
• Equivalent Tissue_Air Ratio Method.

24. Standard Isodose Chart
Butterpaper
Drawing Tools.

25. • First take a contour of a patient CT
image with possible accuracy on a
Butterpaper.
• Take Standard Isodose Chart
• Place Contoured Butterpaper over
isodose chart

26. • Adjust the isodose line chart with the surface of body
contour.
• Draw isodose line coressponding to chart for
number of fields required.
• Apply correction factor whenever required.

28. Meningioma brain
tumour

29. Beam Data
• Standard Isodose Chart:A112111_807
• Field Size:8W x 10 cm2
• Wedge Angle: 30°
• Technique: SSD(80 cm)
• SWD: 45 cm
• Machine:THERATRON 780_C
• Prescriptions:2000cGY/5#

30. RA
RL
Wedge Angle: 30°

31. 100%
94%
88%
PTV

32. View in grid
method

33. Eye

34. Summing up=182%
Along RA=97%
Along RL=85%
RA
RL

35.  Summing of dose at different grid points

36. 100％
99％
98％
95％
90％
 100％=184%=2000cGy
 99％=183%=1980cGy
 98=180%=1960cGy
 95％=175%=1900cGy
 90％=166%=1800Cgy
Eye

37. 100％
99％
98％
95％
90％
 100％=184%=2000cGy
 99％=183%=1980cGy
 98=180%=1960cGy
 95％=175%=1900cGy
 90％=166%=1800Cgy

38.  Treatment time calculation
• Fraction size=400cGY
• Machine: Theratron_780C
• Field size=8 X 10 cm2
FIELD EFS
(cm2)
DEPTH
(cm)
PDD (%) OUTPUT
(cGy/min)
WEDGE
FACTOR
TT(min)
RA 9 2.5 91.2 34.53 0.74 8.58
RL 9 2.5 91.2 34.53 0.74 8.58

39. Comparison
180％ 180％
with grid method
184％
183％

40.  Comparison
100％
98％
99％
95％
90％
PTV
 100％=184%=2000cGy
 99％=183%=1980cGy
 98=180%=1960cGy
 95％=175%=1900cGy
 90％=166%=1800Cgy

41. 1cm away (isocenter) 2cm away(isocentre)
1977cGy(TPS)
1980cGy
1985cGy(TPS)
1976cGy(TPS)
1978cGy
1984cGy(TPS)
1980cGy
1977cGy(TPS)
2000cGy
1977cGy(TPS)
1971cGy(TPS)
1980cGy
1978cGy
1978cGy
Variation(%)=0.15
Variation(%)=0.10
Variation(%)=1.16
Variation(%)=0.20
Variation(%)=0.35
Variation(%)=0.05
Variation(%)=0.45

42. Time required for physicist
Procedure Time(Approx)
Contouring 20min
RA(Wedge ) Field 20min
RL(Wedge ) Field 20min
Grid
+
summing up dose
+
joining isodose points
3Hr

43. BREAST

44. Beam Data
• Standard Isodose Chart:A112111_812
• Field Size:10W x 15 cm2
• Wedge Angle: 30°
• Technique: SSD(80 cm)
• SWD: 45 cm
• Machine:THERATRON 780_C
• Prescriptions:4005cGY/15#

45. MT
LT
Wedge Angle: 30°
Heart
LUNG

46. Without correction factor
100%
94%
89%
83%
Heart
LUNG

47. Correction factor applied
FORMULA: k X h
k=0.66(Co-60)-SSD_80cm
Correction for contour irregularities
Heart
LUNG

48. 3.2cm
 Shift towards the skin(tissue excess) =0.66 X 3.2cm=2.1cm
2.1cm
central axis central axis
Heart
LUNG
Heart
LUNG

49.  With correction factor(irregularity correction factor)
Heart
LUNG
100％
94％

51. Heart
LUNG
104％
103％
101％
96%
90％

52.  Treatment time calculation
• FRACTION SIZE=267cGY
• FIELD SIZE=10W x 15 cm2
FIELD EFS
(cm2)
DEPTH
(cm)
PDD
(%)
OUTPUT
(cGy/min)
WEDGE
FACTOR
TT(min)
MT 12 5 79.5 35.76 0.71 6.61
LT 12 5 79.5 35.76 0.71 6.61

53.  Comparison
104％
103％
101％
96%
100％
104％
103％
101％
96%
90％
90％
Normalization at Isocentre
 104％=163%=4165cGy
 103％=160%=4125cGy
 101％=158%=4045cGy
 100%=156%(Normalization )
 96%=150%=3845cGY
 90％=149%=3805cGy

54. Time required for physicist
Procedure Time(Approx)
Previous procedures 3.50Hr
Irregularities Corrections 1Hr

55. LUNG

56. • Standard Isodose Chart:A102411-080
• Field Size:10 X 10 cm2
• Technique: SSD(80 cm)
• SWD: 53 cm
• Machine:THERATRON 780
• Prescriptions:6000cGy/30#

57. Table used for inhomogeneties
correction

58.  Thickness of lung=6cm
 Factor for Lung(Co-60)=0.4
 Shift in isodose lines along anterior
towards centrak axis=0.4 X 6cm=2.4cm
6cm
2.4cm
PTV

59. INHOMOGENITIES CORRECTION APLLIED
 Comparison
 90％=110%=5400cGy
 80％=90%=4800cGy
PTV
90%
85%
80%
100%
90%
85%
80%

60. Treatment time calculation
• Fraction size=200cGy
• Machine: Theratron_780C
• Field size=10 X 10 cm2
FIELD EFS
(cm2)
DEPTH
(cm)
PDD (%) OUTPUT
(cGy/min)
TT(min)
AP 10 10 56.4 56.24 3.15
PA 10 7 69.3 56.24 2.56

61. Time required for physicist
Procedure Time(Approx)
procedures
+
Inhomogenities Corrections
2Hr

62. PELVIS CASE

63. • Standard Isodose Chart:A102411-080
• Field Size:8 X 8 cm2
• Technique: SSD(80 cm)
• SWD: 53 cm
• Machine:THERATRON 780
• Prescriptions:3625cGY/5#

67. Comparison with grid method
Grid method

68.  98%=3553cGy=261%
 95％=3444cGy=253%
 90％=3263cGy=240%
Comparision with TPS
100%
90%
98%
100%
98%
90%

69. Treatment time calculation
• Fraction size=725cGy
• Field size=8 X 8cm2
FIELD EFS
(cm2)
DEPTH
(cm)
PDD (%) OUTPUT
(cGy/min)
TT(min)
AP 8 10 54.8 55.18 5.99
PA 8 12 47.2 55.18 6.96
RL 8 18 30.2 55.18 10.87
LL 8 18 30.2 55.18 10.87

70. Time required for physicist
Procedure Time(Approx)
Four Fields(Grid) 4Hr

71. Conclusion
• Time consuming.
• Grid method shifts towards more accuracy.
• Grid method gives max dose recieving by OAR along
with PTV coverage.

72. References
• KHAN'S(The physics of radiation therapy) by Faiz M. Khan and John
P.Gibbons-5th Edition published by Wolters Kluwer(2014).
• Giessen PH.A method of calculating the isodose shift in correcting for
oblique incidence in radiotherapy.Br J Radiol. 1973;46:978.
• Greene D, Stewart Jr. Isodose curves in non_uniform Phantoms.Br J
Radiol.1965;38:378.