Étienne Létourneau, Fabiola Vallejo Castaneda, Nancy El Bared, Danny Duplan, Martin HinsePresented by: Étienne Létourneau2015 Joint Congress, Montreal, Quebec – May 28

CALCULATING DOSAGE FOR CBCT

Overview

Objectives Methods and Calibration Measurements and Results Acquisition adjustments Future work

Objectives

To measure the Cone Beam Computed Tomography (CBCT) dose-to-organs using Optically Stimulated Luminescence (OSL) dosimeters : We want to know what we are giving to the patient!

To reduce the dose by adjusting the CBCT acquisition parameters.

Purpose

In radiation therapy (RT), the CBCT is used for patient positioning prior to treatment.

CBCT dose seems negligible compared to the treatment dose. Why bother?– A.L.A.R.A– CBCT gives dose to healthy tissues– A patient can receive as many CBCT scans as

treatment fractions (sometimes more) – In some cases, the treatment dose to a given organ is

already reaching the recommended limits– The goal of CBCT is matching (most of the time using

bones), and matching doesn’t require high quality

Methods and Calibration

The Elekta X-ray Volumetric Imaging (XVI) system uses divergent x-rays with a pyramid shape.

1. kV X-ray beam reference axis

2. X-ray beam (followed by a collimator cassette)

3. Projected X-ray field

4. kV detector panel

Methods and Calibration

Important CBCT parameters– Energy– # of frames– mA per frame– ms per frame– Start/Stop angles– Filter

All influence the total number of photons. These are the parameters we’ll adjust in order to reduce the dose-to-patient.

Methods and Calibration

Optically Stimulated Luminescence Dosimeters (OSLD) work in a similar way than Thermoluminescent Dosimeters (TLD) using the electrons trapped in a crystalline structure.

Electrons are released using light instead of heat. Landauer Nanodots have a useful energy range

from 5 keV to 20 MeV.

Methods and Calibration

How did we calibrate OSLD?Half Value Layer (HVL) measurements using

aluminum sheets

Air-kerma calibration factor obtained from standard lab

in the kV range for a Farmer ionization chamber

Absolute dose measurement with the

Farmer chamber (AAPM TG 61 in-air method)

Establish a link between the dose read by the chamber

and the number of counts in OSLD

Methods and Calibration

Ok, so where did we start?– Measuring the Half Value Layer (HVL) using :

• Aluminum • RTI Barracuda Dose detector (also estimated kVp and

mAs)

– With the HVL, we get the air-kerma calibration factor NK

from the NRC for a Farmer chamber in the kV range

Methods and Calibration

Using AAPM Task Group 61 in-air method:

• dose at water phantom surface• : corrected free-in-air chamber reading• : air-kerma calibration factor • : mean mass energy-absorption water-to-air ratio• : chamber stem correction factor (1.000 in our case)• : backscatter factor

Methods and Calibration

8x8 cm2

4x4 cm2

100

cm

Measurements and Results

We filled an heterogeneous anthropomorphic adult phantom with OSLs.

Measurements and Results

We measured for the following XVI default acquisitions:

• Head• Chest• Pelvis

Measurements and Results

In a nutshell… The thyroid received the

highest dose (3.6 cGy). The lungs, sternum,

breasts, liver, stomach, spleen and gonads can receive between 2.0 and 3.0 cGy per CBCT (1% of the daily treatment dose).

With one scan per fraction, these organs could receive more than 100 cGy during the whole treatment from CBCT!

Measurements and Results

These results match similar studies:– Song et al. Med. Phys. 35: 480-486 (2008)

• XVI dose from 0.1 to 3.5 cGy/scan • Using ionization chamber in phantom

– Hyer et al. Appl. Clin. Med. Phys. 11 181-97 (2010)• XVI dose from 0.1 to 3.0 cGy/scan • Fiber-Optic-Coupled devices

– Alei, Ding and Guan. Med Phys. 37(1) (2007)• XVI dose from 0.1 to 2.8 cGy/scan • Using TLD and Monte-Carlo simulation

Measurements and Results

These results need to be interpret carefully :– Depending on the patient size: Since the number of

photons are always the same, a larger patient might receive less dose and vice versa.

– OSLD response depends on the radiation angle.– The absorbed dose in a given material depends on the

electronic density and the energy of the interacting photon.

– However, these results still a good approximation

Acquisition adjustments

We’ve adjusted the CBCT acquisition parameters (mA per frame, ms per frame and total number of frames).

AcquisitionMaximum

dose (cGy)% of daily

treatment doseRatio with

default dose

Head 0.1 0.1% 1/2

Pelvis 1.0 0.7% 1/4

Chest (low dose) 0.5 0.4% 1/8

Chest (ultra low dose)

0.2 0.2% 1/22

Acquisition adjustments

We’ve created low dose breast scans:– The imaged breast received a

maximum of 0.7 cGy.– Considering we are giving a

maximum of 25 fractions for a breast treatment (most of time 16), the patient will not receive more than 20 cGy from CBCT which correspond to less than 0.4 % of the total dose.

Acquisition adjustments

A clinical study using the CBCT dose-to-organs…– The maximum dose to the heart by our half CBCT left breast

protocol is 0.6 cGy.– 10 patients with left sided breast cancer receiving 50 Gy in 25

fractions and getting daily HCBCT were randomly chosen and analyzed retrospectively

– Assuming daily HCBCT, the heart will receive a maximum extra dose of 15 cGy.

HCBCT frequency% of the target volume

covered by isodose 95%

Daily 100%

Weekly 97%

None 95%

Future work

Do a similar experiment on CT scan Compared with different CBCT scans such as the

Varian On-Board Imaging (OBI) Encourage other centers to adjust their

acquisition parameters even if they can’t measure the dose-to-organs

Acknowledgement

Martin Hinse, Nancy El Bared, Danny Duplan, Fabiola Vallejo and all the CICL staff. This research is a beautiful example of interdisciplinary team work.

Merci à vous!

Be there for my presentation on dose to cardiac implantable electronic device, Thursday June 11 AM, Dosimetry and Quality Control session

References

[1] C.-M Ma, Chair et al. AAPM protocol for 40-300 kV x-ray beam dosimetry in radiotherapy and radiobiology. Medical Physics, 28(6), June 2001

[2] Hyer, D.E et al. An organ and effective dose study of XVI and OBI cone-beam CT systems. Journal of applied clinical medical physics (2010), 11(2), 3183, April 2010

[3] El Bared, Nancy et al. Daily Half Cone Beam Computed Tomographies for left sided breast cancer: Is the added dose worth it?. CARO-ACRO, St.John’s, August 2014

THANK YOU!

QUESTIONS?