HDR 192Ir source speed measurements using a high speed video cameraGabriel P. Fonseca, Rodrigo S. S. Viana, Mark Podesta, Rodrigo A. Rubo, Camila P. de Sales, BrigitteReniers, Hélio Yoriyaz, and Frank Verhaegen Citation: Medical Physics 42, 412 (2015); doi: 10.1118/1.4903286 View online: http://dx.doi.org/10.1118/1.4903286 View Table of Contents: http://scitation.aip.org/content/aapm/journal/medphys/42/1?ver=pdfcov Published by the American Association of Physicists in Medicine Articles you may be interested in Measurement of absorbed dose-to-water for an HDR 192Ir source with ionization chambers in a sandwichsetup Med. Phys. 40, 092101 (2013); 10.1118/1.4816673 Radiochromic film dosimetry of HDR 192Ir source radiation fields Med. Phys. 38, 6074 (2011); 10.1118/1.3651482 Absolute depth-dose-rate measurements for an Ir 192 HDR brachytherapy source in water using MOSFETdetectors Med. Phys. 33, 1532 (2006); 10.1118/1.2198168 Dosimetry close to an 192 Ir HDR source using N-vinylpyrrolidone based polymer gels and magneticresonance imaging Med. Phys. 28, 1416 (2001); 10.1118/1.1382603 Determination of the tissue attenuation factor along two major axes of a high dose rate (HDR) 192 Ir source Med. Phys. 26, 1492 (1999); 10.1118/1.598678

HDR 192Ir source speed measurements using a high speed video cameraGabriel P. FonsecaInstituto de Pesquisas Energéticas e Nucleares—IPEN-CNEN/SP, São Paulo 05508-000, Braziland Department of Radiation Oncology (MAASTRO), GROW School for Oncology and Developmental Biology,Maastricht University Medical Center, Maastricht 6201 BN, The Netherlands

Rodrigo S. S. VianaInstituto de Pesquisas Energéticas e Nucleares—IPEN-CNEN/SP, São Paulo 05508-000, Brazil

Mark PodestaDepartment of Radiation Oncology (MAASTRO), GROW School for Oncology and Developmental Biology,Maastricht University Medical Center, Maastricht 6201 BN, The Netherlands

Rodrigo A. Rubo and Camila P. de SalesHospital das Clínicas da Universidade de São Paulo—HC/FMUSP, São Paulo 05508-000, Brazil

Brigitte ReniersDepartment of Radiation Oncology (MAASTRO), GROW School for Oncology and Developmental Biology,Maastricht University Medical Center, Maastricht 6201 BN, The Netherlands and Research Group NuTeC,CMK, Hasselt University, Agoralaan Gebouw H, Diepenbeek B-3590, Belgium

Hélio YoriyazInstituto de Pesquisas Energéticas e Nucleares—IPEN-CNEN/SP, São Paulo 05508-000, Brazil

Frank Verhaegena)

Department of Radiation Oncology (MAASTRO), GROW School for Oncology and Developmental Biology,Maastricht University Medical Center, Maastricht 6201 BN, The Netherlands and Medical Physics Unit,Department of Oncology, McGill University, Montréal, Québec H3G 1A4, Canada

(Received 5 June 2014; revised 22 October 2014; accepted for publication 13 November 2014;published 30 December 2014)

Purpose: The dose delivered with a HDR 192Ir afterloader can be separated into a dwell component,and a transit component resulting from the source movement. The transit component is directlydependent on the source speed profile and it is the goal of this study to measure accurate source speedprofiles.Methods: A high speed video camera was used to record the movement of a 192Ir source (Nucletron,an Elekta company, Stockholm, Sweden) for interdwell distances of 0.25–5 cm with dwell times of0.1, 1, and 2 s. Transit dose distributions were calculated using a Monte Carlo code simulating thesource movement.Results: The source stops at each dwell position oscillating around the desired position for a durationup to (0.026±0.005) s. The source speed profile shows variations between 0 and 81 cm/s with averagespeed of ∼33 cm/s for most of the interdwell distances. The source stops for up to (0.005±0.001) sat nonprogrammed positions in between two programmed dwell positions. The dwell time correctionapplied by the manufacturer compensates the transit dose between the dwell positions leading to amaximum overdose of 41 mGy for the considered cases and assuming an air-kerma strength of 48 000U. The transit dose component is not uniformly distributed leading to over and underdoses, which iswithin 1.4% for commonly prescribed doses (3–10 Gy).Conclusions: The source maintains its speed even for the short interdwell distances. Dose variationsdue to the transit dose component are much lower than the prescribed treatment doses forbrachytherapy, although transit dose component should be evaluated individually for clinical cases.C 2015 American Association of Physicists in Medicine. [http://dx.doi.org/10.1118/1.4903286]

Key words: brachytherapy, Ir-192, source speed, transit dose

1. INTRODUCTION

The transit dose component of a brachytherapy source move-ment from the safe to the dwell positions, between the dwellpositions, and when returning to the safe has been studiedpreviously,1–7 reporting differences up to a factor of 10 forthe source speed for the same afterloader.8 The effect of

these differences on the transit dose component for clinicalbrachytherapy treatments performed with a HDR 192Ir sourcewas evaluated recently.8 The study demonstrated potentiallysignificant dose variations depending on the speed profile.These publications indicate a need to obtain accurate sourcespeed profile measurements. In this work, we report on suchmeasurements for a single type of afterloader.

412 Med. Phys. 42 (1), January 2015 0094-2405/2015/42(1)/412/4/$30.00 © 2015 Am. Assoc. Phys. Med. 412

413 Fonseca et al.: HDR 192Ir source speed measurements 413

Previously reported measurements were performed usinga video camera,1,4 up to 30 frames per second (fps), an ioni-zation chamber,3 oscilloscopes connected to the afterloader,9

optical fibers to detect induced Cerenkov radiation,5 and filmdosimetry.2 This work describes the speed profiles obtainedusing a high speed video camera. Transit dose distributionsand dose reductions due to dwell time corrections were calcu-lated using a Monte Carlo (MC) code, 6 (Monte CarloN-Particle),10 as described in the supplementary material.12

2. METHOD AND MATERIALS

Dwell times and source speed profiles were measuredfor a microSelectron v.3 192Ir source (Nucletron, an Elektacompany, Stockholm, Sweden) using a Sony NEX-FS700(Sony Corporation, Tokyo, Japan) video camera with 960 fpsto register the source trajectory. Time resolution was as-sessed by recording a stopwatch display as (1.04±0.01) ms.Experimental uncertainties are described in the supplemen-tary material.12

For this study, the trajectory of the source inside of a trans-parent channel aligned with 0.05/0.10 cm resolution rulerswas recorded (Fig. 1). The source trajectory was registeredusing: from 1 up to 18 dwell positions; interdwell distancesof 0.25, 0.50, 1.00, 1.50, 2.50, and 5.00 cm; and source stepsizes from 0.25 to 1.00 cm. The higher resolution ruler wasused to calibrate the pixel width (3.78±0.04)×10−3 cm formost of the videos.

Source positions were evaluated frame-by-frame with in-house software developed using version 8.0 (Math-works, Inc., Natick, MA). The region of interest (ROI) thatrepresents the source trajectory was manually assigned andthen the pixel intensity profile, without the source, was recor-ded as the background (BG) profile. Source structures (e.g., thetip and the welding) can be easily identified dividing thepixel intensity profile inside the ROI of each frame by theBG profile as illustrated by the top-right figure in video 1(Ref. 12). This video shows the software used to track thesource, the source movement, and intensity profiles including

F. 1. Experimental setup for source speed measurements. The insert (top-right) was obtained from one of the acquired slow motion videos. The sourceguide was positioned to be clinically relevant avoiding excessive bendingduring the experiments.

reference lines, with fixed distances between them, associatedwith source structures. These lines were included to verifypossible misdetections. Average speeds were calculated overthree measurements.

All speed values were averaged over distance intervals of0.2 cm (v̄0.2), except when otherwise stated. The dwell timecorrection applied by the afterloader was measured for allsetups by comparing the programmed dwell time against thedwell time recorded using the video footage. Dwell timeswere measured after the source comes to rest at the specifieddwell position.

The source trajectory between two consecutive dwell posi-tions starts with the source leaving the first dwell position andends immediately before the source reaching the second dwellposition for the first time. We noticed that, after the sourcereaches a dwell position, it overshoots and oscillates aroundit, an effect that was not considered for the average speeddetermination. The duration the source spends oscillatingaround a dwell position was measured.

3. RESULTS

Figure 2(a) shows the average speed (v̄0.2) for the averageof six measurements, consisting of three arrival profiles andthree return profiles. Video 1 (Ref. 12) shows an exampleof the measurements with one dwell position at the end ofthe trajectory. There is no continuous slowing down sincethe motor stops instantaneously at the dwell position causingsource oscillations around it. No differences were seen forthe arrival and return source speed profile due to the sourcestopping when arriving at a dwell position and acceleratingfrom rest when returning to the safe. This is because thesource acceleration is high and average speeds were obtainedover 0.2 cm reducing the differences in the speed profile nearthe dwell position. The only exception was noted for onepoint at 0.5 cm from the dwell position for which the arrivaland return speed values were added in Fig. 2(a).

Video 2 (Ref. 12) shows the source movement describedin Fig. 2(b) with v̄0.2 values obtained for six dwell positionsequally spaced between 0 and 5 cm. The source speed appearsto exhibit a periodicity. The labeled points in Fig. 2(b) referto the following phases: (1) source accelerating after leavingthe dwell position; (2) source reaches maximum speed; (3)source decelerates reaching the lowest speed around 0.5 cmfrom the dwell position; (4) source accelerating again; (5)source reaches a high speed just before dwelling.

The source stops at nonprogrammed positions for less than(0.005±0.001) s, which can be seen in video 2 (Ref. 12).However, Fig. 2(b) does not show zero source speed atnonprogramed dwell positions since only averaged v̄0.2 valuesare shown. The speed variation appears to reduce with largerdistances as can be observed in video 3 (Ref. 12) that shows thesource movement between two consecutive dwell positions,at 0 and 5 cm, also depicted in Fig. 2(c).

According to the manufacturer, to compensate for thetransit time, the afterloader reduces the dwell time at eachdwell position with the time spent in traveling to it, to a

Medical Physics, Vol. 42, No. 1, January 2015

414 Fonseca et al.: HDR 192Ir source speed measurements 414

F. 2. (a) Source speed profile for the source arriving at the first dwellposition (0 cm) and returning to the safe. Uncertainties Type A and Type B arealmost equivalent so only Type B component is shown. (b) v̄0.2 values wereobtained for six dwell positions equally spaced between 0 and 5 cm (verticaldotted lines). (c) v̄0.2 values were obtained for the source movement betweentwo dwell positions, at 0 and 5 cm. Figures (b) and (c) use the same notationwith Type A and Type B uncertainty components indicated (Ref. 12). Thesource speeds at the dwell positions were not considered, therefore figures donot show speed values equal to zero.

maximum of 0.1 s. The measured mean value of this correc-tion is (0.06±0.03) s, which is in good agreement with Wonget al.1 (0.07±0.01) s. The time correction was measuredfor 43 dwell positions going from (0.030±0.007) s up to(0.096±0.037) s for interdwell distance of 0.25 and 2.5 cm,respectively. In addition, we verified that the source spends upto (0.026±0.005) s oscillating around the dwell positions afterthe motor stops (video 4).12 The amplitude of the oscillationdepends on the interdwell distance which is 0.08 cm for thesource arriving at the first dwell position, 0.15 cm for 0.25 cminterdwell distance (video 4),12 0.05 cm for 0.5 and 1 cminterdwell distances, and 0.08 cm for 2.5 and 5 cm interdwelldistances.

The source step size of the afterloader is either 0.25, 0.50,or 1.00 cm. This did not cause measurable differences forinterdwell source speed profiles for 0.25, 0.50, 1.00, 2.50,and 5.00 cm interdwell distances. On the other hand, thespeed profiles depend on the interdwell distance followingnonuniform movements. Table I shows the average speedobtained in this work compared against literature data.

4. DISCUSSION

The periodic speed variations seem to be independent ofthe interdwell distance. For example, the speed profile for1 cm interdwell distance [Fig. 2(b)] is similar to the speedprofile of the first centimeter obtained with 5 cm interdwelldistance [Fig. 2(c)]. Moreover, the speed profile obtainedwith 5 cm interdwell distance was used to obtain averagespeeds for the first 0.25, 0.50, 1.00, and 2.50 cm of the sourcetrajectory. All values obtained are equivalent to the averagespeed profiles obtained for equivalent interdwell distances(Table I) within uncertainties. The highest average speed forthe 0.5 cm interdwell distance occurs due to the absence ofnonprogrammed dwell positions for this interdwell distance.In addition, the source reaches the maximum speed [Figs. 2(b)and 2(c)] within 0.5 cm interdwell distance and does notreach it for a 0.25 cm interdwell distance, which also explainsa higher average speed for the 0.5 cm than for 0.25 cminterdwell distance.

The complex behavior of the source movement, includ-ing very short stops at nonprogrammed positions, was alsoobserved for another afterloader by Wojcicka et al.2 Onepossible explanation for the observed variations could beattributed to a motor warm-up since the amplitude of speedoscillations reduces with distance [Fig. 2(c)]. It can also bedue to wire spring or another mechanical property of theequipment, which was not evaluated in this study.

The periodic speed variations may explain differences ob-tained in the literature as source speed varies significantlywith source position. Studies performed with a video cam-era with a lower frame rate or detectors at fixed positionswould not have the required spatial/temporal resolution toobserve these effects. This explanation does not apply forintegration methods, which may indicate that differences can

Medical Physics, Vol. 42, No. 1, January 2015

415 Fonseca et al.: HDR 192Ir source speed measurements 415

T I. Average source speed over the interdwell length for interdwell distances of 0.25, 0.50, 1.00, 2.50, and5.00 cm. Uncertainty values were not available for all references. All values were obtained for a Nucletronafterloader (Nucletron, an Elekta company, Stockholm, Sweden), however, the model may change.

Interdwelldistance (cm)

This work(cm/s)

Wong (Ref. 1)(cm/s)

Sahooa (Ref. 3)(cm/s)

Bastin(Ref. 7)

Houdek(Ref. 9)

0.25 32.8 ± 2.7 5.4 ± 2.3 — 23.0 22.70.50 45.8 ± 2.6 7.2 ± 1.6 33.3 25.5 27.11.00 34.9 ± 0.9 23.3 ± 7.3 50.0 — 30.32.50 32.3 ± 0.3 — — — —5.00 32.0 ± 0.2 — 43.5 — 33.1

aTransit time measurement uncertainty is up to 100% for interdwell distances less than 1.00 cm and between 9% and26% for larger distances (Ref. 3).

be due to high uncertainties and/or different behaviors be-tween each piece of equipment or their models. This high-lighted the importance of including transit dose componentmeasurements in quality assurance (QA) tests since low speedsources can lead to high transit doses.8,11 Williamson et al.also described a simple methodology to measure source speedusing an ionization chamber and how to estimate the transitdose that can be employed for QA tests.11

The transit dose does not seem to be significant for theobtained speed profiles. On the other hand, our results showthat the transit dose is not uniformly distributed and that thetransit dose for dwell positions far apart was not fully correctedshowing over and underdoses (supplementary material).12

5. CONCLUSION

A high speed camera allowed a detailed determinationof the source movement, which can be clearly visualizedwith the videos included in this paper. The dwell timecorrection applied by the manufacturer may lead to doses,averaged over the volume, equivalent to the planned dosessince the transit time between the dwell positions may befully compensated reducing the dwell times. This depends onthe distances between dwell positions, though. However, thetransit dose distribution is not uniform and, ideally, shouldbe considered during treatment planning to optimize the dosedistribution. This issue increases in importance for slowermoving brachytherapy sources.

ACKNOWLEDGMENTS

The authors would like to thank Yury Niatsetski and Robvan der Laarse from Nucletron for valuable comments on thiswork. This study was partially supported by Fundação de

Amparo à Pesquisa do Estado de São Paulo (FAPESP), Grantnumbers 2011/01913-4 and 2011/22778-8.

a)Author to whom correspondence should be addressed. Electronic mail:frank.verhaegen@maastro.nl; Telephone: +31 (0)88 4455792.

1T. P. Wong, W. Fernando, P. N. Johnston, and I. F. Bubb, “Transit dose of anIr-192 high dose rate brachytherapy stepping source,” Phys. Med. Biol. 46,323–331 (2001).

2J. B. Wojcicka, R. Yankelevich, F. Trichter, and D. P. Fontenla, “Comparisonof the transit dose components and source kinematics of three high dose rateafterloading systems,” Med. Dosim. 24, 61–65 (1999).

3N. Sahoo, “Measurement of transit time of a remote after-loading high doserate brachytherapy source,” Med. Phys. 28, 1786–1790 (2001).

4A. Palmer and B. Mzenda, “Performance assessment of the BEBIG multi-source high dose rate brachytherapy treatment unit,” Phys. Med. Biol. 54,7417–7434 (2009).

5R. A. Minamisawa, R. A. Rubo, R. M. Seraide, J. R. Rocha, and A. Almeida,“Direct measurement of instantaneous source speed for a HDR brachyther-apy unit using an optical fiber based detector,” Med. Phys. 37, 5407–5411(2010).

6G. P. Fonseca, R. A. Rubo, R. A. Minamisawa, G. R. dos santos, P. C. An-tunes, and H. Yoriyaz, “Determination of transit dose profile for a (192)IrHDR source,” Med. Phys. 40, 051717 (8pp.) (2013).

7K. T. Bastin, M. B. Podgorsak, and B. R. Thomadsen, “The transit dosecomponent of high dose rate brachytherapy: Direct measurements and clin-ical implications,” Int. J. Radiat. Oncol., Biol., Phys. 26, 695–702 (1993).

8G. P. Fonseca, G. Landry, B. Reniers, A. Hoffmann, R. A. Rubo, P. C. G.Antunes, H. Yoriyaz, and F. Verhaegen, “The contribution from transit dosefor 192 Ir HDR brachytherapy treatments,” Phys. Med. Biol. 59, 1831–1834(2014).

9P. V. Houdek et al., “Dose determination in high dose-rate brachytherapy,”Int. J. Radiat. Oncol., Biol., Phys. 24, 795–801 (1992).

10M. J. T. Goorley, T. Booth, F. Brown, J. Bull, L. J. Cox, J. Durkee, J. El-son, M. Fensin, R. A. Forster, J. Hendricks, H. G. Hughes, R. Johns, B.Kiedrowski, R. Martz, S. Mashnik, G. Mckinney, D. Pelowitz, R. Prael, J.Sweezy, L. Waters, T. Wilcox, and T. Zukaitis, “Initial 6 release over-view,” Radiat. Transp. Prot. 180, 298–315 (2012).

11J. F. Williamson, G. A. Ezzell, A. Olch, and B. R. Tomadsen, “Qualityassurance for high dose rate brachytherapy,” in High Dose Rate Brachy-therapy: A Textbook, edited by S. Nag (Futura Pub. Co., New York, 1994),pp. 147–212.

12See supplementary material at http://dx.doi.org/10.1118/1.4903286 fortransit dose component and experimental uncertainty.

Medical Physics, Vol. 42, No. 1, January 2015