Beam Quality Measurement and Verification of a C-Series Linear Accelerator

Barrington Brevitt

Published on: 2018-12-28

Abstract

Objectives: To conduct beam quality measurements and verification for treatment plan quality assurance in radiation therapy.

Methodology: An IBA Pharma type ion chamber (FC65-G) and electrometer (IBA Dose 2) were used to obtain ionization charges for an 18MV and a 6MV beam from a C-Series linear accelerator. These measurements were taken using a 1D water phantom, dimensions 40cm* 35cm*34.5 cm at a source to detector distance (SDD) of 100cm with the chamber 10cm beneath the water surface. The “K” (quality conversion factor) was calculated using the TPR20,10 method. Two sets of measurements were taken at a depth of 20 cm and 10 cm beneath the water surface at a source to detector distance of 100cm. Three measurements were taken for both photon energies at polarities of +300, -300 and +100 on the electrometer.

Results: Ka values were calculated using the TPR20, 10 principles outlined in the TRS 135 protocol. The Ka value for the 6MV photon was determined to be 0.996 Gy while that for the 18MV was 0.973 Gy. These Ka values were then used to determine the tabulated percentage depth dose (PDD) for the photon energies. The tabulated PDD’s were 0.665 and 0.788 for the 6MV and 18MV beam respectively. From equation 2, Dw (10cm) for 6MV photon was calculated to be 0.68 Gy and that for the 18 MV photon was 0.81 Gy. The absorbed dose of the treatment unit at Dmax (Eq. 3) was calculated to be; (18MV) 1.03 Gy and (6MV) 1.03 Gy.

Conclusion: Due to the complexity of photon production and interactions and the need to precisely treat a tumour volume, beam quality verification should be conducted on a daily basis before treatment of patients.

Keywords

Beamlet; Photon Fluence; Quality Assurance; Isocentre; Collimator; IMRT; VMAT

Introduction

The aim of all radiotherapy treatment is to precisely deliver a prescribed dose of radiation to a tumour volume while simultaneously sparing the organs at risk (OARs) surrounding this tumour. Radiation therapy has evolved and the method of radiation generation varies, ranging from the controlled emission of gamma rays from a Cobalt 60 source to the production of particles such as; photons, electrons and protons produced in linear accelerators. The nature and location of the tumour volume dictate the approach with which radiation dose is delivered to ensure total tumour coverage and the equivalent beam fluency. These approaches evolved from 3-Dimensional conformal therapy to Intensity Modulated Radiation Therapy (IMRT) and Volumetric Modulated Arc Therapy (VMAT). It is also important to map the tumour volume before dose delivery due to involuntary physiological actions such as peristalsis, breathing or other muscular contractions. This mapping is done through a process known as Image Guided Radiation Therapy (IGRT) which ensures the patient reassumes the exact position for each treatment fraction. Portal images are taken of the area of interest and matched to set up fields taken during treatment planning and simulation. This matching process results in a delta couch shift that adjusts the treatment couch before the prescribed dose is delivered.

The high energy photons employed during tumour treatment can interact with components of the gantry such as lead jaws, and produce particles that may contaminate the treatment beam. These contaminants are produced through various interactions with the photons and the absorbing medium such as; Compton Interaction, Photoelectric Effect and Pair Production. Photons possess no mass or charge and travel at the speed of light. They have more penetrating power than charged particles of similar energy, and as they traverse a medium there is energy transformation to electron energy [1]. The prospect of an interaction per unit distance travelled is denoted by the principle , where µ is dependent on the energy of the photon and the materials through which it travels [2]. When photons have energies in excess of 1.022MeV or approximately 1 MV, it may interact with the medium in a process known as Pair Production. Here the photon is influenced by the strong field effects of the nucleus and may disappear producing a positive and negative electron pair. These electrons are not orbital but are created during an energy/mass conversion of the incident photon. The odds of pair production occurring within a medium increases as the photon energy and the atomic number (~Z2) of the medium increases [2]. The relationship between energy, voltage and charge is denoted by the following equation;

V (V) = E (eV) / Q (C)             (1)

The aim of this paper is to conduct beam quality measurements and verification for treatment plan quality assurance.

Methodology

An IBA Pharma type ion chamber (FC65-G) and electrometer (IBA Dose 2) were used to obtain ionization charges for an 18MV and a 6MV beam from a Varian C-Series linear accelerator. These measurements were taken using a 1D water phantom, dimensions 40cm* 35cm*34.5cm at a source to detector distance (SDD) of 100cm with the chamber 10cm beneath the water surface. The American Association of Physicist in Medicine (AAPM) Technical Report Series (TRS135) protocol was used to calculate the beam quality. The “Ka” (quality conversion factor) was then calculated using the TPR20,10 method outlined in the protocol. Here 2 sets of measurements were taken at a depth of 20 cm and 10 cm beneath the water surface at a source to detector distance of 100cm. Three measurements were taken for both photon energies at polarities of +300, -300 and +100 on the electrometer.

Beam Quality Measurement

The following equations were used to determine the beam quality as outlined by the AAPM TRS 135 protocol. The photon energies used were 6MV and 18MV, to deliver 100MU on a Varian C-Series linear accelerator.

Where: M1 is measured charge at reference depth (Dref); Ktp is the temperature and pressure correction factor; Kelec is the electrometer reading correction factor; Kpol is the polarization correction factor; Ka is the quality conversion factor; Ks is the ion chamber recombination factor; NDw is the chamber correction factor; PDD is the percentage depth dose and Dw is the absorbed dose rate to water at the reference depth (at Dmax and 10cm)

 

 

Where: P0 = 101.32 KPa (pressure at calibration); T0= 200C (temperature at calibration); T1 = 25.60C (temperature at measurement); P1 = 101.12 KPa (pressure at measurement); M+ = positive polarity reading; M- = negative polarity reading; M = routine polarity reading; V1 = measurement at voltage 1 and V2 = measurement at voltage 2.

 

 

Results And Discussion

Based on Eq.2 through 8 the measured charges were used to calculate the different parameters, which are shown in Table 1. It, therefore, follows that Mq (Eq. 4) for the 6MV beam equates to 14.22 x 10-9C and that for the 18MV beam was 17.14 x 10-9C. Ka values (Eq. 2) were calculated using the TPR20, 10 principles outlined in the TRS 135 protocol.

Table 1: Illustrating the variables calculated from Eq. 2 through 8.

 

6MV Beam

18MV Beam

Kelect

1

1

Kpol

1.001

1.001

Ks

1.001

1.003

Ktp

1.02

1.02

The Ka value for the 6MV photon was determined to be 0.996 Gy while that for the 18MV was 0.973 Gy. These Ka values were then used to determine the tabulated percentage depth dose (PDD) for the photon energies. The tabulated PDD’s were 0.665 and 0.788 for the 6MV and 18MV beam respectively. From Eq.2, Dw (10cm) for 6MV photon was calculated to be 0.68 Gy and that for the 18 MV photon was 0.81 Gy. The absorbed dose of the treatment unit at Dmax (Eq. 3), was calculated to be; (18MV) 1.03 Gy and (6MV) 1.03 Gy.

Table 2: Illustrating the charges obtained for both photon energies.

Photon Energy

Polarity of Ion Chamber

Charge Measured (10-9C)

6MV

300

13.94

-300

13.98

100

13.77

18MV

300

16.8

-300

16.84

100

16.58

According to (1) the work function of Lead is 4.08eV. Using the expression;

Where E = Energy of photon; EK = Kinetic Energy of electrons and ? = Work function

When EK = 0 then E = ?, and further the photon energy equivalent to the work function of lead is:

The photon beam under investigation for this scenario consists of 6MV beam lets. When 100MU is delivered and measured at a polarity of +300 the charge recorded was 13.9×10-9C. It follows that E = 0.1J (Eq. 9), since E (in J)=V (in V) ×Q (in C). It is therefore evident that the 6MV photons typically used for radiotherapy treatment possess enough energy to promote photoelectric interaction in the Lead components of the gantry. These interactions contaminate the treatment beam, highlighting the importance of beam quality verification on a regular basis, especially before and during treatment delivery. The photon fluency of the radiation beam is affected by particles produced at different points in the treatment unit.

  • Primary photons- produced from electron interaction with the target.
  • Extra-focal photons- produced due to the interaction of the primary photons and parts of the gantry such as the collimators and wedges.
  • Electron contamination- due to electrons produced from interactions with air and the collimator leaves.

Eq. 10 (3), represents the total absorbed energy of the treatment beam by superposition of the energies deposited from all three sources, namely; primary photons (a), extra-focal photons (b) and contaminating electrons (c):

The electrons contaminating the photon beam are also produced via interactions with the flattening filter. When beam modifiers are employed such as wedges, they absorb most of the electrons from the treatment beam; however, they become secondary sources of electrons as they interact with the beam. The level of electron contamination is dependent on the beam energy and the size of the treatment field. Common beam modifiers employed during radiation therapy include; blocks and stationary multi-leave collimators (MLCs) which affect the beam fluence, hard wedges which serve as sources of wedge scatter and determine the beam configuration, and dynamic wedges which affect primary radiation, scatter components and collimator backscatter. Dynamic wedges also modulate the beam based on the motion of the MLCs.Other beam modifiers such as compensators and dynamic MLC also affect the fluence of the beam. Dynamic MLCs are modelled as pseudo-filters, where the head scatter effects are taken into account by using a finite source located at the bottom surface of the flattening filter. Here electron contamination is dependent on the photon beam fluence shape. The thickness of the compensator matrix and its effective linear attenuation coefficient modifies the fluence of the beam (3).

Beam configuration is important because it directly influences the quality of the treatment beam, which determines the level of success in relation to tumour shrinkage and cancer cell remission. Beam configuration is usually done through three main processes:

Optimizing source parameters

As outlined in (3), the initial optimization of the source parameters include; determining the mean energy spectrum, intensity profile and secondary source parameters while ignoring the dose build-up region. Optimization of the electron contamination parameters based on measured and calculated percentage depth doses (PDD's) is also done and refining optimization of the source parameters taking into account the dose build-up region.

Absolute calibration

The dose distribution is initially calculated in units of Gy/particle and then converted to Gy/Mu by a conversion factor 1/dref × (Gyref/ Muref) where;
-dref : dose at calibration point for calibration geometry, absolute dose scaling factor.
-Gyref/ Muref: known Gy/ Mu ratio at the calibration point for the calibration geometry (3).

Collimator backscatter factor

This is based on output factor measurements and calibration calculations made for the reference field size. The backscatter factor is determined in the monitor chamber by the following;

  • x,y - collimator setting, (X= x2 - x1 ) and (Y= y2 - y1)
  • CBSF (x,y) - collimator back scatter factor for an open field with the same collimator setting
  • OFref - output factor table value for reference field size
  • OF(x,y) - output factor table value for field size (x,y)
  • D' (x,y) - dose at the reference point calculated for the field size (x,y) and reference geometry.

D'ref - dose calculated by the algorithm for the reference condition in the reference geometry(3):
The parameters calculated were obtained from charge measurements recorded from the Dose 2 Electrometer and the Farmer ion chamber. All calculations were done using the parameters stipulated by the protocol and evaluated with a 3% room for error. Ideally, calculations should prove that 1mu delivered by the treatment unit is equivalent to 1 cGy of radiation. As shown, all the stipulated variables from the calculations were within the stipulated 3% tolerance. It thus follows that beam contamination within this unit is very small. The absolute dose at Dmax for both photon energies were 1.03 cGy respectively. Slight variations in the positioning of the ion chamber can yield substantial deviations in the measurements obtained exacerbating the overall deviation in the calculation of the absolute dose. Due care was taken to ensure the chamber was parallel with the coronal wall lasers simultaneously being perpendicular to the saggital roof laser. The isocenter of the beam, indicated by the intersection of all room lasers and the collimator crosshair was aligned to the active volume of the ion chamber. The average charges obtained for both photon energies are shown in Table 2. As illustrated the variations between the charges for each photon was very small, as the experimental set up was not disturbed between readings and the ion chamber was also discharged after each measurement.

 

Conclusion

Due to the complexity of photon production and interactions and the need to precisely treat a tumour volume, beam quality verification becomes paramount. Beam verification should be conducted on a daily basis before treatment begins. Quality assurance check should also be conducted on treatment plans to ensure the beam data and energy fluence being delivered accurately reflects the beam data and energy fluence calculated by the treatment planning system. As shown, all the stipulated variables from the calculations for this test were within the stipulated 3% tolerance. It thus follows that beam contamination within this unit is very small. In the event where quality assurance checks reveal beam contamination or a deviation from the expected absolute dose the unit should be removed from clinical use. The aim of all radiation procedure is to keep the radiation dose As Low As Reasonable Achievable (ALARA).

Synopsis

This paper was derived from quality assurance testing to determine beam quality of a C-Series linear accelerator. Beam quality verification is an integral part of the quality assurance system utilized during radiation therapy. The quality of the beam will determine the rate of tumour regression and the subsequent radiation dose to the skin and healthy organs. Due to the nature of particle production, and delivery in radiation therapy, beam contaminants are produced through interaction with air or components of the gantry. It is thus important to determine the level of contamination and take corrective actions when beam quality falls below the accepted standard.

References