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

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Absolute values of set-up variation for all patients

Cumulative percentage for set up variation

0

10

20

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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

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-8m

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Deviation to nearest mm

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Distribution of A-P variation of lateral isocentre EPIs

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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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Deviation to nearest mm

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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