Étienne Létourneau, Fabiola Vallejo Castaneda, Nancy El Bared, Danny Duplan, Martin Hinse Presented by: Étienne Létourneau 2015 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?

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 N K from the NRC for a Farmer chamber in the kV range

Methods and Calibration

8x8 cm 2 4x4 cm 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

 These results match similar studies: –Song et al. Med. Phys. 35: (2008) XVI dose from 0.1 to 3.5 cGy/scan Using ionization chamber in phantom –Hyer et al. Appl. Clin. Med. Phys (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). Acquisition Maximum dose (cGy) % of daily treatment dose Ratio with default dose Head0.11/2 Pelvis1.01/4 Chest (low dose)0.51/8 Chest (ultra low dose) 0.21/22

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% Daily100% Weekly97% None95%

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 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?