Hypofractionation – Physics Todd Pawlicki, Ph.D.Professor & Vice-Chair of Medical Physics

Objectives

• Understand special technical issues for hypofractionated treatments

• Be able to highlight relevant technical reports

• Discuss safety issues and mitigation strategies with hypofractionated treatments

Acknowledgements:

• UCSD physics team• William Song, PhD

The Issue

Dose deviation from the prescription for an error in a single fraction.

Two Categories

• Cases with stationary/rigid targets– Brain– Spine– etc

• Cases with moving/deforming targets– Lung– Liver– etc

Simulation

Planning

Treatment

Prescription

Guidance documents

Solberg et al PRO 2011

Potters et al IJROBP 2010

AAPM TG-101: Table of Contents

• Introduction and Scope• History and Rationale for SBRT• Current Status of SBRT-Patient Selection Criteria• Simulation Imaging and Treatment Planning• Patient Positioning, Immobilization, Target Localization,

and Delivery• Special Dosimetry Considerations• Clinical Implementation of SBRT• Future Directions

Safety and Quality

• Multidisciplinary team – May be everyone for smaller centers

• Choose a treatment site– Identify special issues for that site

• Team members express concerns, find solutions

• Everyone on the same page– Move forward only when preparation is done

Patient Selection

• Cooperative and understands instructions

• Can tolerate prolonged setup and treatments

• No resting tremors, uncontrolled pain, etc.

• Non-urgent cases only

Hamilton et al. Neurosurgery 1995Immobilization (body)

Framed Frame-less

AAPM TG-101 (2010)

SBRT Immobilization Accuracy

Accuracy summary:2 – 3 mm

Immobilization (head)

Framed Frame-less

Head immobilization accuracy ~ 1-2 mm

Immobilization – Other issues

• Patient comfort– Treatment delivery may also requires longer sessions– Simulation can take longer than usual (e.g., 4DCT)

• Treatment beams– Multiple beam angles– Arms over head even for lower spine targets

CT Simulation

Static targets• Can be similar as for non-

hypofractionated cases – Head vs. body

• MR can be helpful for target and normal tissue contouring (e.g. cord)

Moving targets• Should use a motion

management strategy– Breathing control,

compression, 4DCT

• Know the limitations of your strategy

Motion-Induced Artifacts

Example

Rietzel et al., Med Phys 2005;32(4):874-889

Motion-Induced Artifacts

Real Life

AAPM TG-76 2006 The Management of Respiratory Motion in Radiation Oncology

No general pattern of respiratory behavior can be assumed for each patient.

Respiratory Signal: TermsPeriod

Amplitude 0%

50%

100%

30% 70%

100% duty cycle = Beam ON between 0%-100% window50% duty cycle = Beam ON between 30%-70% window

Time

Am

plitu

de

Duty CycleGate 100 Gate 3070 Static Case

PTV PTV PTV

100% duty cycle0-100% window

50% duty cycle30-70% Window

4DCT Simulation

4DCTRespiratory signal

from system

X-ray ON

First couch position Second couch position Third couch position

Problems With Phase Sorting

Period Amplitude

Baseline

*Phase-to-amplitude relationship changes…

ITV ContouringMaxIP100(MIP100)

MaxIP3070(MIP3070)

AvgIP MinIP

30%40%50%60%70%80%90%

20%10%

0%Range:

30%40%50%60%70%

Range:

30%40%50%60%70%80%90%

20%10%

0%Range:

30%40%50%60%70%80%90%

20%10%

0%Range:

Contouring/Planning

• MaxIPs useful in lung (tumor density is higher than surrounding tissue)– If GATE100 treatment, then use MIP100 CT for ITV– If GATE3070 treatment, then use MIP3070 CT for ITV

• MinIPs useful in liver (tumor density is lower than surrounding tissue)

• Fuse the ITV onto AvgIP

• Plan on AvgIP

Treatment Planning

• Understand basic planning guidelines– Include entire dose calculation region

• Always use 3D planning– Conventional or intensity modulation– Achieve high dose conformality

• Multiple and/or non coplanar beams/arcs– Collision avoidance procedures in place

Multiple Beams

B Hoppe et al, IJROBP, 2008

5 – 11 static beam OR 1 – 3 arcs

To Gate or Not to Gate?

• Relevant questions– Motion > 0.5 cm?– Breathing regular? Period > 4 sec?– Patient compliance? – Patient tolerate longer treatment

time with gating?

• Decision at planning stage– GATE100 or GATE3070

Figure 9: AAPM TG-76 (Report 91), 2006

Planning • Decide on criteria for gating

– Note, most cases are not going to be gated

• ITV-PTV margin– At least 5 to 10 mm– Motion management does not mean zero margin

• Generally, prescription isodose covers 100% ITV– Adequate PTV coverage is also of concern

• Hotspot inside/outside PTV?– If inside, typically ≤ 110%– If outside, re-plan necessary

Isodose Coverage

• Isodose covers tightly around the ITV/PTV • No hotspots should be outside the PTV

Planning

• Normal tissues and constraints– Lungs, aorta, spinal cord, brachial plexus, esophagus, heart, rib cage– Tolerances are still evolving

• Generally, 5-11 fields (spacing ~ 20°, non-opposed)– Typically more beams are used with complex shaped targets

• 2.0 - 2.5 mm dose grid is adequate

• Use heterogeneity corrections and convolution/superposition algorithm

T10 (18 Gy x 1): IMRT Plan

T10 (18 Gy x 1): Dose Distribution

VMAT SRS18Gy x 1

6F

Multiple PTVs

1

CBCTGate100

1 1 1

1

3

2

Gate100 Setup Strategy

Step 1: Acquire kV/kV – register to boneStep 2: Acquire CBCT – register to superior border

Geometric center

Gate100 Match

Gate3070 Setup Strategy

1

CBCTGate3070

1 1 1

End-of-Expiration = Target at Superior Border

1

3

23

2

Step 1: Acquire kV/kV – register to boneStep 2: Acquire CBCT – register to superior border

Matched at the superior edge

CBCTReference CT

Gate3070 Match

Patient Number2019181716151413121110987654321

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

vector by Patient Number

GYN – bony anatomy

Prostate – Fiducials

Image Interpretation

Efficient Treatment is Important

• Mean intra-fractional tumor position– Measured as a function of the interval between localization and repeated CBCTs

• 5.3 mm if the time > 34 min• 2.2 mm if the time < 34 min

Quality Assurance Overview

• Acceptance

• Commissioning

• Clinical implementation

Commissioning & Implementation• SBRT equipment

– Ensure geometric precision/accuracy for immobilization devices

• Check the inter-operation of all equipment– It should not be assumed one component that works in one clinical

situation will work for SBRT treatments

• Treatment planning system (TPS)– TPS accuracy for small fields– End-to-end testing

• Establish independent checks– e.g. RPC dosimetry– Audits for process checks

Thoughts on Commissioning

• Allow enough time for commissioning – do not rush to treat a patient

• Regular QA should verify equipment/process changes

• Ensure overall accuracy using end-to-end tests for each new treatment site

• Phone a friend, e.g., on-line collaborations

• Mechanical and radiation isocenter verification– E.g., Winston-Lutz test

• MV and kV isocentercoincidence verification

• AAPM TG-142, 2009– for other routine linac tests

On-going QA

TG-142 Linac QA Example

• Daily, Monthly, Annual• EDWs, MLCs, Imaging systems

Other Relevant AAPM Reports

• TG-42 (1995)– Stereotactic cranial radiosurgery

• TG-66 (2003)– CT and the CT simulation process

• TG-135 (2011)– Robotic radiosurgery

Patient-Specific Physics QA

• Planning requirements are different– Is the treatment plan acceptable?

• Appropriate imaging sequence

• Patient-specific measurements

• Directly supervise each treatment fraction– Appropriate setup– Appropriate beam-delivery parameters– Motion management input

Patient-Specific Physics QA cont.

• Independent review of setup and treatment parameters should be completed

• Validate initial setup instructions and check plan against prescription

• Review the plan with your physician– Contours Ok– Dose distribution makes sense

Summary and Final Caveats

• Correct images from simulation to planning• Contour on correct images• Multiple fields • Correct procedures for treatment setup• Don’t miss the big picture

– TPS commissioning, Linac output, end-to-end tests

• Use quality management program– Practice patterns and technology evolve