AAPM Task Group:

TG-35, TG-45

SAMIR LAOUI, Ph.D.

TG-35

Accelerator Safety Hazards

Most common hazards

Incorrect radiation dose

Dose delivered to wrong region

Collision between patient and machine

Incorrect beam energy or modality

Electrical/mechanical problems

Accelerator Safety Hazards

AAPM Task Group Report #35 (TG-35) covers safety

issues that the medical physicist should be aware of.

Two FDA classifications of hazards

Class I – causes serious injury or death

Type A hazard are directly responsible for life-threatening

complications

Type B hazard increases probability of unacceptable outcome

(complication or lack of tumor control)

Class II – hazards where the risk of serious injury are small

TG-35: Two levels of class I hazards (type A

and type B) are defined

Type of Hazards

Type A hazard: Involves improper delivery of

25% of prescribed dose

The total dose error threshold is on the order of 10-

15 Gy

Radiation overdose caused by a faulty machine

Dose outside the intended radiation field (Critical

Organs)

Patient-machine collision

Incorrect beam energy or modality

Type of Hazards

Type B Hazards:

The total dose error threshold is on the order of 4 Gy

Radiation overdose caused by a faulty machine

Dose outside the intended radiation field

Incorrect energy or mode

Underdose

Accelerator hazards occurrence rate

TG-35: Unable to find published accident

frequencies for radiation therapy devices

Estimation: 40 per day per machine with 15 year

machine lifetime. If 1 type A incident occurs

5 X 10-6 per patient treatment.

Error rate for faults leading to type B problems

should be set at about 10-4

Procedures in response to

potential hazard It is highly desirable to prepare and adopt a set of procedures for

medical physicists to follow if faced with safety hazards

It is essential to establish a written reporting and documenting

mechanism

In the occurrence of significant un-prescribed radiation exposures

to the patient, the radiation oncologist must take an active primary

role in

The immediate evaluation of the affected patient

Informing the patient of the occurrence and potential for acute and late effects

Informing the other physicians in respective specialties responsible for the

patient’s care

The termination or alteration of the fraction size from the intended regimen of

radiation therapy

Implementing a rigorous timeline for followup patient care to assess the

occurrence of acute and late effects

Reporting of incidents, malfunctions or

machine breakdowns

The most frequent problems can be classified

into three categories 1. Clear breakdowns-no beam

2. Machine suffers frequent interlock interrupts-beam available if

radiation therapy technologist continually resets interlock

3. Machine gives occasional problems that can easily be overridden by

radiation therapy technologist

Seemingly benign faults that are easily reset can

be misleading. They must be reported to physics

Therapist should be trained to identify unusual

situations and change in machine performance

Documentation and reporting

It is essential to establish a written reporting and

documenting mechanism

It is important that the technologist leave the

machine in its “fault” state to help service

personnel diagnose the problem

Bodies to report problems to

Manufacturer

U.S. Pharmacopeia (USP)

The Problem Reporting Program (PRP)

TG-45

PART 1: ADMINISTRATION

Radiation Oncology needs Housing and support equipment

Equipment Selection Acceptance Testing

Commissioning Initial and continuing staff training

Maintenance Quality Assurance (Periodic)

Facilities be staffed at levels that closely follow the guidelines

given in the “Blue Book”

Recommends the radiation oncology physicist be certified in

radiation oncology physics by either the American Board of

Radiology, American Board of Medical Physics, or the Canadian

College of Medical Physics

Medical Physicist

The Physicist is responsible for Acceptance testing

Commissioning

Calibration

Periodic QA

Dosimetry

Treatment planning

Research

the qualified radiation oncology physicist is the

sole individual who can make a decision on the

working conditions of a medical accelerator for

patient treatments

Implementation of Linac involves

Equipment Selection Post-Acquisition

FACILITY PLANNING AND

RADIATION PROTECTION The radiation oncology physicist should be involved in the

design of the facility

Shielding design

Neutron concern at above about 10 MV

Maze (Leads to lighter door)

Door weight and manual access

Room location and orientation

Radiation safety survey

Neutron leakage

Radiation levels outside the room

Radiation Protection Regulation

Regulatory bodies

Linear Accelerators

National Council on Radiation Protection and

Measurements (NCRP)

Individual states (Suggested State Regulations for Control

of Radiation, SSRCR)

Cobalt-60

Nuclear Regulatory Commission (NRC)

Exposure Limits

NCRP Report #116 replaces Report #91

Occupation Limits (controlled areas)

Whole body – 50 mSv / yr (1 mSv / wk)

Lens of Eye – 150 mSv / yr

Pregnant Worker – 5 mSv / term (0.5 mSv / mo)

Lifetime – 10 mSv x Age (years)

Public Limits (noncontrolled areas) Whole body – 1 mSv / yr (0.02 mSv / wk)

Extremities, Skin, Lens of Eye – 50 mSv / yr

ACCEPTANCE TESTING

Purpose:

Machines meets or exceeds the contractual specifications

Assures the safety of patients and machine operators

Provides critical baseline data for future QA

Checking the treatment area:

During Installation: Warning signs, training, warning lights, audio and

video, door interlocks, emergency power failure

At first delivery of beam: validation of the proper operation of the

door interlock system, Emergency off switches proper operation,

Calibration of the machine output in all modes, Radiation levels outside

the room

ACCEPTANCE TESTING

Mechanical Checks

Alignment of collimator axis and collimator jaws

Collimator axis, light localizer axis, and cross hairs

Light field and radiation field congruence and coincidence

Light field and radiation field symmetry: Collimator jaws symmetry

Light field, radiation field and field readout agreement, and accuracy:

Over full range of collimator and gantry rotations.

Mechanical isocenter location: Sphere containing the

intersection point for all machine orientations.

Radiation isocenter location

With respect to the collimator axis

With respect to couch

With respect to gantry angle

ACCEPTANCE TESTING

Other mechanical system tests Patient support system: Couch verification with and without load

Anti-collision system(s)

Beam Modifiers: electron collimators, Beam stoppers

Console system tests Mode selection

Computer-controlled accelerator software validation: Software updates

must be subjected to an ATP

Readout linearity: Can be done during initial tests cited earlier

Record and verify system: Maintain and record patient’s TX parameters

Radiation beam parameters

Beam output

Calibration TG-21; TG 51

Adjustability and range: cG/MU range

Stability: cG/MU adjustment should be stable

Timer must be accurate and stable

Monitor characteristics

Linearity and end effects: MU linearity

Dose rate accuracy (MU per unit-time)

Dose rate dependence (MU per unit-time settings)

Constancy of output with gantry position

Radiation beam parameters

Flatness

Maximum variation of dose in central 80% of the FWHM of the open field.

X-ray off-axis ratios (“horns”)

Symmetry

Maximum percent deviation of the “left-side” dose from the “right-side” dose at the 80% of the FWHM.

Penumbra

Film is choice because of spatial resolution

Radiation beam parameters

X-ray beam energy

Specified as depth of dmax and/or %dd at 10-cm depth for a

10x10-cm2 field. Example: 18 MV at 3.3 cm. PDD 80% at 10 cm

Electron beam energy

Usually specified at depth of 80% and 50% dose for a 10x10-cm2

field. 12 MeV 80% at 3.8 cm 50% at 5.2 cm

COMMISSIONING

Definition: refers to the process whereby the needed

machine-specific beam data are acquired and

operational procedures are defined

Includes but not limited to:

Beam data acquisition

Entry of beam data into an RTP system and testing of its accuracy

Development of operational procedures

Training of all concerned with the operation of the new accelerator.

Commissioning pre-requisites

Enough time to perform the tests

Using water phantoms and precise equipment

Commissioning of photon beam

One of the most important tasks in commissioning

photon beams is selecting a method for dose calculations

and then collecting the necessary beam data

The accuracy of computerized radiation treatment

planning (RTP) systems for photon beams can be verified

using the data provided by AAPM Task Group 23

The treatment planning system data should be modified

such that the resultant isodose curves match the measured

data

Commissioning of photon beam

Square and rectangular photon beams MU

calculation To calculate the number of MU required to deliver a prescribed absorbed

dose, the following are needed:

Commissioning of photon beam

Wedged photon beams Wedge factor: Ratio of the absorbed dose at specified depth, measured

in the standard geometry with the wedge in place , to the absorbed dose

in the same geometry without the wedge.

Beam-shaping blocks for photons Purpose: conform the basic rectangular field shape to the beam’s eye view

of the target volume

Block transmission factor BTF

Commissioning stationary

electron beams Dose distributions in an electron field depend strongly on the

design and construction of the collimating system- applicators

Dosimetry data for electron

beams The following characteristics must be measured

Output factors

Central axis depth dose curves

Isodose charts

Cross beam profiles

Dosimetry parameters depend strongly upon electron energy that must be

specified for each clinical electron beam

Dose: TG-25

Because of the complexity of the spectrum, there is no single energy parameter

that will fully characterize the electron beam

Commissioning of special procedures

Total and half body photon irradiation Inverse square law may not accurately predict the decrease in beam

intensity at large distances

Change in photon spectrum for large fields (Change in mass abs. coef.)

Large phantoms

Total Skin Electron Irradiation

Additional load placed on linacs by increased dose rate

Intraoperative Radiotherapy

Multidisciplinary procedure: Radiation therapy +

Surgery

Dose delivered at the time on the surgery

Requires special cones

PPD change associated with cones has to be

measured

Stereotactic radiosurgery

Delivers a large dose in a single fraction for the

treatment of intracranial targets

SRS is distinguished from routine TX:

Routine SIM is not done for target volume

determination

3D planning is necessary

More stringent isocenter criteria

Smaller beams are utilized

One fraction

SRS Commissioning

Imaging: Target localization

Treatment planning:

3D dose computation along fined grids (1mm)

Dosimetry: Small fields dose measurement

challenges

QUALITY ASSURANCE

PROGRAMS

Radiation protection of personnel and patients

Safe maintenance and operation of machine

Safety: Emergency situations

Security

Maintenance: the physicist must be aware of what

was done and how it might affect the accelerator’s

operation

Training: Accelerator operation and emergency

procedures

Logistics

QUALITY ASSURANCE

PROGRAMS

Accuracy of dose delivery

An effective quality control program to prevent

treatment errors due to machine malfunction

Maintenance of QA records

Critical parameters should be checked frequently

Refer to AAPM TG-40 for more details