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Assessment of radiotherapy set-up error for limb
sarcoma using electronic portal imaging (EPI)
Wendy Ella, Eleanor Gill, Anna Cassoni, Beatrice Seddon
London Bone and Soft Tissue Tumour Service, University College Hospital, London UK
CTOS 13CTOS 13thth Annual Meeting, Seattle, USA Annual Meeting, Seattle, USA
Purpose of study
• To measure the set up variation in patients with limb sarcoma with reference to the standard departmental EPI protocol
• To assess the reproducibility of current departmental limb immobilisation techniques
• Calculation of margin to be applied to CTV to produce PTV, on basis of set-up variation
Radiotherapy process
Mould Room & planning CT scan
Volume definition
Plan generation
Treatment
Treatment verification by EPI
Immobilisation
• Immobilises limb in same position every day• Reduces day to day variation in limb position
(potential source of error)• Impression of limb with patient in the optimum
treatment position: – Baseboard with customised foam moulded under
limb– sheet of thermoplastic (Orfit) moulded around
limb, fixed onto baseboard
Immobilisation: Lower limb
Immobilisation: Upper limb
Radiotherapy target definition
Gross tumour volume (GTV)
Volume occupied by the gross visible tumour
GTV
Clinical target volume (CTV) Volume surrounding macroscopic tumour to include local sub-
clinical microscopic spread
CTV
Planning target volume (PTV) Geometric concept of an additional margin to ensure
that CTV receives prescribed dose, accounting for variations in set-up that inevitably occur day to day
PTV
Treatment verification: EPI
• Verification of accuracy of treatment
• Take images of treatment portals regularly during treatment
• Compare with planning images
Electronic portal imaging
• Check isocentres on anterior and lateral portals
• Match structures• Asymmetric fields• Include a joint
• Match in 3 dimensions (left-right, superior-inferior & anterior-posterior)
• Assess variation of EPI from planning images, and whether it is within defined departmental tolerance (≤5 mm)
ISO PVsDAY 1 WEEK 1
REVIEW BEFORE 2nd FRACTION
RADIOGRAPHERAPPROVAL
DR TO SIGN OFF
ON DAY 2 REPEAT IMAGE AND REVIEW BEFORENEXT #
PRETREAT PV 2#
≤ 5mm isodeviation is acceptable
≥ 8mm
≤ 5mm
WEEKLY ISO
CHECKSPRETREAT PV 3RD #
MOVE ON SETRECORD AND GET DR
TO SIGN OFF
> 5mm, < 8mm
Error > 5mm < 8mm
If ≤ 5mm
If < 5mm
If > 5mm < 8mm systematic variation
* CONSULT WITH SIMULATOR, REVIEW SET UP
If < 5mm
PV # 4
If >5mm < 8mm
Random Error > 5mm < 8mm
* If unable to resolve consult
doctor ? delay or continue
If ≤ 5mm
ENSURE THAT ALL IMAGES INCLUDE
A JOINT FOR REFERENCE PURPOSES
> 8mm
Methods
• Patients receiving radical dose radiotherapy to a limb for a sarcoma under care of two consultants (AMC, BMS)
• 49 patients treated between 15/11/05 – 14/11/06 over 12 months
• All EPIs for all patients analysed in 3 dimensions
• Each image match recorded greatest variation
33
16
Upper limb
Lower limb
4
6
1
5Upper arm
Lower arm
Elbow
Shoulder/chestwall
18
9
2
4
Thigh
Lower leg
Knee
Foot
Tumour location
Results
• 614 EPIs were available for assessment
• 24 EPIs could not be matched (for technical reasons)
• 589 EPIs were analysed (mean 12 per patient)
• Mean additional dose of EPIs: 12mu
Results
• For 512 (90.5%) variation of the EPI from DRR was ≤ 5mm (i.e. within departmental tolerance)
• 63% of patients had all EPIs ≤ 5mm tolerance
– Mean number of EPIs within tolerance per patient: 10.39 (86%)
-0.80
-0.60
-0.40
-0.20
0.00
0.20
0.40
0.60
0.80
1.00
Dis
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in c
m A
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2.00
Dis
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Dis
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Absolute values of set-up variation for all patients
Cumulative percentage for set up variation
0
10
20
30
40
50
60
70
80
90
100
<0.14 <0.24 <0.34 <0.44 <0.54 <0.64 <0.74
Treated setup rate
67% of all images were within 3mm
Cumulative percentage for set up variation
0
10
20
30
40
50
60
70
80
90
100
<0.14 <0.24 <0.34 <0.44 <0.54 <0.64 <0.74
Treated setup rate
90.5% of images within 5mm
Cumulative percentage for set up variation
0
10
20
30
40
50
60
70
80
90
100
<0.14 <0.24 <0.34 <0.44 <0.54 <0.64 <0.74
Treated setup rate
95.9% of images within 7mm
Distribution of R-L variation of anterior isocentre EPIs
0
10
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80
-8m
m-7
mm
-6m
m-5
mm
-4m
m-3
mm
-2m
m-1
mm
0mm
1mm
2mm
3mm
4mm
5mm
6mm
7mm
8mm
Deviation to nearest mm
Nu
mb
er o
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ages
Distribution of A-P variation of lateral isocentre EPIs
0
10
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-8m
m-7
mm
-6m
m-5
mm
-4m
m-3
mm
-2m
m-1
mm
0mm
1mm
2mm
3mm
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5mm
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Deviation to nearest mm
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Distribution of S-I variation of anterior isocentre EPIs
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-8m
m-7
mm
-6m
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mm
-4m
m-3
mm
-2m
m-1
mm
0mm
1mm
2mm
3mm
4mm
5mm
6mm
7mm
8mm
Deviation to nearest mm
Nu
mb
er o
f im
ages
What margin should be added to CTV to create a PTV?
Error in radiotherapy planning and treatment
• Error – any deviation between planned and executed treatment:
– Systematic error – influences all fractions in identical way throughout treatment
– Random error – influences fractions randomly, e.g. due to patient or organ motion
Quantification of error
Day Pt 1 Pt 2 Pt 3 Pt 4
1 2 4 1 3
2 1 -2 -1 -3
3 1 2 2 -2
4 1 0 2 1
Mean 1.25 1 1 -0.25
SD 0.50 2.58 1.41 -0.25
Mean of means = group systematic error
RMS = group random error
CTV to PTV margin calculation
• Margin = (2.5 X SD of group systematic error) + (0.7 X SD of random error)
• Ensures that 90% of patients receive a minimum cumulative CTV dose of at least 95% of the prescribed dose
Van Herk et al, IJROBP, 47: 1121-35, 2000
Van Herk et al, Seminars in Radiation Oncology, 14: 52-64, 2004
Systematic/Random errorsGroup
systematic error
Group SD of random error
Calculated CTV→PTV
margin
L-R on AP
0.2 mm
+/- 1.4 SD
+/- 1.1mm 4.3 mm
S-I on AP
-0.2 mm
+/- 1.7 SD
+/- 1.7mm 5.3 mm
A–P on Lat
0.3 mm
+/- 1.5 SD
+/- 0.8mm 4.4 mm
Margin = (2.5 X SD of group systematic error) + (0.7 X SD of random error) - Van Herk, 2000, 2004
Conclusion
• Current immobilisation techniques result in good reproducibility of set-up
• >90% of EPIs were within departmental tolerance of ≤5mm
• Current departmental EPI protocol results in acceptably accurate treatment
• Calculated margin for CTV → PTV for our department = 5.3mm