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

JOURNAL CLUB

Slides prepared By Dr Wang Fuqiang, Registrar, Radiation Oncology, NCCS

Daniel TanCourse DirectorAssociate ConsultantDepartment of Radiation OncologyNational Cancer Centre Singapore

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

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1)Know the existence of this resource

2)Know the contents of this resource

3)Briefly run through this resource to make a mental note

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

I. Introduction and Scope

I. Introduction and Scope

Contents

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

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. ‘

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

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

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

III. Patient Selection Criteria

Recommendations formal group trials with appropriate protocols or an institution treatment protocol/ guidelines as

developed by radiation oncologists and physicists

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

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

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

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

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

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)

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)

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.

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

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

IV. Simulation Imaging and Planning

IV. Simulation Imaging and Planning

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

V. Patient Positioning, Immobilisation, Target Localisation and Delivery

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

V. Patient Positioning, Immobilisation, Target Localisation and Delivery

C. localisation, tumour tracking and gating techniques for respiratory motion management

1. Image-guided techniquesCone beam imaging with acquisition

time >60s fast CT less ideal because position of

tumour may be captured at random

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

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

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

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

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

VII. Clinical Implementation of SBRT

Critical steps involved

1. establish scope of program

2. determine treatment modality

3. equipment requirements

4. personnel needed

5. acceptance/ commissioning

6. establish work flow guidelines/ reporting/ QA

7. conduct personnel training

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

VII. Clinical Implementation of SBRT

A. Establishing the scope and clinical Goals 2. Time and personnel considerations

additional physicist involvement

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

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

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

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

Goldmine