Assessment of radiotherapy set-up error for limb sarcoma using electronic portal imaging (EPI)

1. 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 13th Annual Meeting, Seattle, USA

2. 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

3. Radiotherapy process Mould Room & planning CT scan Volume definition Plan generation Treatment Treatment verification by EPI

4. 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

5. Immobilisation: Lower limb

6. Immobilisation: Upper limb

7. Radiotherapy target definition

8. Gross tumour volume (GTV) Volume occupied by the gross visible tumour GTV

9. Clinical target volume (CTV) Volume surrounding macroscopic tumour to include local sub-clinical microscopic spread CTV

10. 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

11. Treatment verification: EPI • Verification of accuracy of treatment • Take images of treatment portals regularly during treatment • Compare with planning images

12. 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)

14. ENSURE THAT ALL IMAGES INCLUDE A JOINT FOR REFERENCE PURPOSES ISO PVs DAY 1 WEEK 1 REVIEW BEFORE 2nd FRACTION * If unable to resolve consult doctor ? delay or continue ≤ 5mm iso deviation is acceptable ≥ 8mm > 5mm, < 8mm ≤ 5mm PRETREAT PV 2# RADIOGRAPHER APPROVAL DR TO SIGN OFF ON DAY 2 REPEAT IMAGE AND REVIEW BEFORENEXT # Error > 5mm < 8mm If ≤ 5mm If ≤ 5mm > 8mm WEEKLY ISO CHECKS Random Error > 5mm < 8mm PRETREAT PV 3RD # If < 5mm If > 5mm < 8mm systematic variation MOVE ON SET RECORD AND GET DR TO SIGN OFF If < 5mm PV # 4 * CONSULT WITH SIMULATOR, REVIEW SET UP If >5mm < 8mm

15. 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

16. Tumour location

17. 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

18. 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%)

19. Absolute values of set-up variation for all patients

20. Cumulative percentage for set up variation 67% of all images were within 3mm

21. Cumulative percentage for set up variation 90.5% of images within 5mm

22. Cumulative percentage for set up variation 95.9% of images within 7mm

23. Distribution of R-L variation of anterior isocentre EPIs

24. Distribution of A-P variation of lateral isocentre EPIs

25. Distribution of S-I variation of anterior isocentre EPIs

26. What margin should be added to CTV to create a PTV?

27. 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

28. Quantification of error Mean of means = group systematic error RMS = group random error

29. 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

30. Systematic/Random errors Margin = (2.5 X SD of group systematic error) + (0.7 X SD of random error) - Van Herk, 2000, 2004

31. 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