Stereotactic Body Radiation Therapy: The Report of AAPM Task Group 101

1. Stereotactic Body Radiation Therapy: The Report of AAPM Task Group 101 JOURNAL CLUB Slides prepared By Dr Wang Fuqiang, Registrar, Radiation Oncology, NCCS Daniel Tan Course Director Associate Consultant Department of Radiation Oncology National Cancer Centre Singapore

2. Stereotactic Body Radiation Therapy: The Report of AAPM Task Group 101 Aims: • Know the existence of this resource • Know the contents of this resource • Briefly run through this resource to make a mental note

3. Stereotactic Body Radiation Therapy: The Report of AAPM Task Group 101

4. I. Introduction and Scope

5. I. Introduction and Scope Contents

6. I. Introduction and Scope • SBRT • emerging RT procedure for the treatment of early stage primary and oligometastatic cancer • delivery of large doses in few fractions resulting in high biological effective dose • to minimise normal tissue toxicity, need to ensure high conformity and rapid fall-off doses away from the target • therefore high level confidence in accuracy of treatment is required for SBRT • this is achievable by integrating modern imaging, simulation and treatment planning and delivery technologies

7. I. Introduction and Scope • ‘In SBRT, confidence in this accuracy is accomplished by the integration of modern imaging, simulation, treatment planning, and delivery technologies into all phases of the treatment process; from treatment simulation and planning, and continuing throughout beam delivery. ‘

9. II. History and Rationale of SBRT • outcomes of SBRT for both primary and metastatic disease compare favourably to surgery • many conceptual theories: • sites of gross disease containing highest number of clonogenic cells not eliminated by chemotherapy • oligometastatic disease which can be eradicated if numbers are limited • Norton-Simon hypothesis whereby cancer increases from low undetectable level to a phase of exponential growth and a lethal plateau, therefore SBRT may aid in the reduction of systemic burden to delay lethal tumour burden

10. II. History and Rationale of SBRT • immunomodulation • palliative treatment • clinical patient outcomes first published in 1995 • initially focused on liver and lung lesions • subsequently other studies included spinal lesions

11. III. Patient Selection Criteria • mostly lung/ liver/ spinal lesions • well circumscribed tumours up to 5cm • SBRT has been used as a boost in addition to regional nodal irradiation • careful evaluation of normal tissue function and dose distribution (typically pulmonary function and volume of liver irradiated) • important structures should be avoided

12. III. Patient Selection Criteria • Recommendations • formal group trials with appropriate protocols • or an institution treatment protocol/ guidelines as developed by radiation oncologists and physicists

13. IV. Simulation Imaging and Planning • A. Simulation Imaging • CT/ 4D CT/ MRI/ PET • Recommendation: • simulation done in treatment position • cover target and all OARs • 5-10cm superior and inferior of normal treatment borders (~15cm if non-coplanar treatment techniques) • tomographic slice thickness of 1-3mm

14. IV. Simulation Imaging and Planning B • B. Data Acquisition • Multiple sources for organ/ tumour motion during simulation • Population based margins may be incorrectly applied • refer to AAPM Task Group 76 report on various tumour motion strategies

15. IV. Simulation Imaging and Planning • C. Imaging Artifacts • If target and radiosensitive critical structures cannot be localised on section imaging modality with sufficient accuracy because of motion and/ or metal artifacts, SBRT should not be pursued as a treatment option

16. IV. Simulation Imaging and Planning • D. Treatment Planning • Limited volume of tissues containing the gross tumour and close vicinity are targeted for high dose per fraction treatment, hot spots within the target are deemed acceptable • Volume of normal tissue receiving high doses should be minimised by a sharp dose fall-off outside of the target

17. IV. Simulation Imaging and Planning • D. Treatment Planning • ICRU 50/ 62 • GTV/CTV considered identical • Variation in CTV due to motion/ organ filling accounted for by ITV • PTV

18. IV. Simulation Imaging and Planning • D. Treatment Planning • 1. Dose Heterogeneity, gradient and fall-off and beam geometry • dose prescription specified at lower isodose with small or no margins for penumbra • hotspots within target deemed acceptable and clinically desirable • use of multiple nonoverlapping beams to achieve sharp dose fall-off • beam energy (6MV smaller penumbra) • resolution of beam shaping (as determined by MLC leaf width-> 5mm adequate)

19. IV. Simulation Imaging and Planning • D. Treatment Planning • 2. Beam selection and beam geometry • restricting entrance dose to <30% of cumulative dose and avoiding beam overlaps to prevent acute skin reactions • increased number of beams yield better conformity but not practical (VMAT may overcome this issue)

20. IV. Simulation Imaging and Planning • D. Treatment Planning • 3. Calculation grid size • 2mm grids required for IMRT • Recommendation: 2mm of finer for SBRT, >3mm not acceptable • a 2.5 mm isotropic grid produces an accuracy of about 1% in the high-dose region of an IMRT plan consisting of multiple fields • Another report indicated an accuracy of +/- 5% for an isotropic grid resolution of 4 mm. • Chung et al. found a dose difference of 2.3% of the prescribed dose for 2 mm calculation grids as compared to 1.5 mm grids, rising to 5.6% for 4 mm grids. • conclusion is that 2 mm grids are required for IMRT procedures, especially in high-dose gradient areas.

21. IV. Simulation Imaging and Planning • D. Treatment Planning • 4. Bioeffect-based treatment planning • NTD derived from conventional RT unlikely to be applicable to SBRT • Bioeffect measures (BED/ NTD/ EUD) required to rank and compare SBRT plans with conventional plans

23. IV. Simulation Imaging and Planning • D. Treatment Planning • 5. Normal Tissue Dose Tolerance • Recommendation: Normal tissue dose tolerance in the context of SBRT still evolving , limited experiences to draw recommendations

24. IV. Simulation Imaging and Planning

25. IV. Simulation Imaging and Planning

26. IV. Simulation Imaging and Planning E • E. Treatment plan reporting • prescription dose/ ICRU reference point / number of fractions/ total treatment delivery period/ target coverage • plan conformity • heterogeneity index • dose fall-off outside of target • notable areas of high/ low dose outside of PTV • dose to OARs

27. V. Patient Positioning, Immobilisation, Target Localisation and Delivery

28. V. Patient Positioning, Immobilisation, Target Localisation and Delivery • B. Image-guided localisation • For SBRT, image guided localisation techniques should be used to guarantee the spatial accuracy of delivered dose distribution • gantry mounted kV units capable of fluoroscopy, radiographic localisation and cone beam imaging • implantation of fiducials • ultrasound imaging • radiofrequency tracking

29. V. Patient Positioning, Immobilisation, Target Localisation and Delivery • C. localisation, tumour tracking and gating techniques for respiratory motion management • 1. Image-guided techniques • Cone beam imaging with acquisition time >60s • fast CT less ideal because position of tumour may be captured at random

30. V. Patient Positioning, Immobilisation, Target Localisation and Delivery • C. localisation, tumour tracking and gating techniques for respiratory motion management • 2. Optical tracking techniques • stereoscopic infrared cameras and video photogrammetry used to track 3D coordinates of points on patient’s skin

31. V. Patient Positioning, Immobilisation, Target Localisation and Delivery • C. localisation, tumour tracking and gating techniques for respiratory motion management • 3. Respiratory gating techniques • delivery of dose at certain phases of breathing • issue of reproducibility • recommend patient-specific tumour motion assessment for thoracic/ abdominal targets

32. V. Patient Positioning, Immobilisation, Target Localisation and Delivery • D. Delivery data reporting • report that QA process is in use and proper documentation for accurate treatment delivery

33. VI. Special Dosimetry Considerations • A. Problems associated with dosimetry of small/ narrow field geometry • an appropriate dosimeter with a spatial resolution of ~1mm or better • maximum inner diameter of a detector should be <half the FWHM of smallest beam measure

34. VI. Special Dosimetry Considerations • B. Problems associated with small-field heterogeneity calculations • when target is surrounded by low-density tissue • Monte Carlo precalculated dose-spread kernels and employing convolution/ superposition techniques • AAPM Task Group 65 recommend inhomogeneity corrections be used for patient dose calculation

35. VII. Clinical Implementation of SBRT • Critical steps involved • establish scope of program • determine treatment modality • equipment requirements • personnel needed • acceptance/ commissioning • establish work flow guidelines/ reporting/ QA • conduct personnel training

36. VII. Clinical Implementation of SBRT • A. Establishing the scope and clinical Goals • 1. Equipment considerations • integration of treatment machines with pre-existing planning system and imaging localisation

37. VII. Clinical Implementation of SBRT • A. Establishing the scope and clinical Goals • 2. Time and personnel considerations • additional physicist involvement

38. VII. Clinical Implementation of SBRT • B. Acceptance, commissioning and QA • acceptance test procedures by vendors • commissioning tests developed by physicists • QA procedures for both treatment and patient

39. VII. Clinical Implementation of SBRT • C. Patient safety and the medical physicist • recommend one medical physicist to be present throughout first treatment fraction and available for subsequent fractions

40. VII. Clinical Implementation of SBRT • D. Quality process improvement: Vigilance in the error reduction process in the treatment planning and delivery process • regular review of existing QA procedures with the objective of assessing and critiquing the current QA practice

41. VIII. Future Directions • incorporation of strategies for the adaptive conformation of treatment fields • incorporation of bioeffect knowledge into treatment process • incorporation of improvements in small field dosimetry performance in clinical treatment planning system • incorporation of chemotherapy • incorporation of molecular imaging • incorporation of tumour-motion effects into the treatment planning and the methods of evaluation for the delivered SBRT dose to a dynamic target • volumetric modulated arc therapy • proton and heavy ion therapies

42. Goldmine