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 13 th 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 PVs DAY 1 WEEK 1 REVIEW BEFORE 2nd FRACTION RADIOGRAPHER APPROVAL DR TO SIGN OFF ON DAY 2 REPEAT IMAGE AND REVIEW BEFORENEXT # PRETREAT PV 2# ≤ 5mm iso deviation is acceptable ≥ 8mm ≤ 5mm WEEKLY ISO CHECKS PRETREAT PV 3RD # MOVE ON SET RECORD 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 – 14/11/06over 12 months All EPIs for all patients analysed in 3 dimensions Each image match recorded greatest variation

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: (86%)

Absolute values of set-up variation for all patients

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

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

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

Distribution of R-L variation of anterior isocentre EPIs

Distribution of A-P variation of lateral isocentre EPIs

Distribution of S-I variation of anterior isocentre EPIs

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 DayPt 1Pt 2Pt 3Pt Mean SD 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: , 2000 Van Herk et al, Seminars in Radiation Oncology, 14: 52-64, 2004

Systematic/Random errors Group systematic error Group SD of random error Calculated CTV→PTV margin L-R on AP 0.2 mm +/- 1.4 SD +/- 1.1mm4.3 mm S-I on AP -0.2 mm +/- 1.7 SD +/- 1.7mm5.3 mm A–P on Lat 0.3 mm +/- 1.5 SD +/- 0.8mm4.4 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