(HDR) brachytherapy quality assurance - Biomedical Imaging and ...

DA Wilkinson. Biomed Imaging Interv J 2006; 2(2):e34 2This page number is notfor citation purposeThe Cleveland Clinic FoundationMANCHESTER TEMPLATEThe Cleveland Clinic FoundationThe Cleveland Clinic FoundationHDR Fletcher-suitGYN TEMPLATEFigure 1 Examples of HDR applicators used for lung, rectal, and gynecologic diseases.(AAPM). The latter are intended to provide the medicalphysicist in the USA (and it is hoped other nations aswell) the proper guidance to ensure that brachytherapyprocedures are carried out safely and with due attentionto these rules. There are two AAPM task group (TG)reports that are particularly relevant to HDR QA. Theyare TG 56: Code of practice for brachytherapy physics[3], and especially TG 59: HDR brachytherapy treatmentdelivery [4]. Another useful reference for brachytherapyquality assurance has been published by ESTRO and isavailable on their website [5]. This paper will providedetails about HDR QA as it is performed in our radiationtherapy department (on a Nucletron Microselectronsystem) and how the QA program is related to the NRCregulations and the task group recommendations.APPLICATOR QAPrior to the initial use of a new (or replacement)applicator, it is necessary to verify that the source dwellpositions correspond to the radiographic markerpositions used in simulation and treatment planning. TG-56 recommends that coincidence of dummy andradioactive sources be checked annually as well. Thereare many standard applicators; photographs of several ofthose used in our institution are shown in Figure 1.The method we employ to verify coincidence ofdwell position and radiographic marker isautoradiography. An applicator is taped securely to asealed film envelope (Figure 2) and the HDR after loaderis programmed to send the source to a few appropriatelychosen dwell positions for less than 1 second (e.g. 0.3 sfor 0.31 GBq source). Next, the film plus applicator istransferred to a diagnostic X-ray source such as asimulator, the dummy source markers are placed in theapplicator and the film exposed and developed (e.g. 125kVp, 125 mAs for Kodak XV film). An example of thisis shown in Figure 3. TG-56 recommends that the

DA Wilkinson. Biomed Imaging Interv J 2006; 2(2):e34 3This page number is notfor citation purpose1713915Figure 2 Photograph of a ring applicator secured to a base platewith film taped securely in place.Figure 3 Autoradiograph of a 3 cm ring showing 5 dwellpositions (1,5,9,13,17) as well as the interveningdummy markers (unnumbered arrows). Dwell positionsare for the 0.5 cm step-size.coincidence of dummy and active sources be within 2mm; the NRC regulations call for ± 1 mm.PERIODIC SPOT-CHECKThe new NRC regulations require a periodic spotcheckof each HDR unit prior to the first use on anygiven day that the after loader is in operation and aftereach new source installation. These spot-checks need notbe done by the authorized medical physicist, but thelatter must review the results and notify the licensee inwriting of his findings. Table 1 lists the checks that mustbe performed at a minimum to assure proper operation ofthe unit according to NRC Regulations 10 CFR Part 35.A convenient way of implementing and recordingthe above quality assurance is by using a checklist suchas the one our clinic uses as shown in figure 4.Certain tests require only a simple inspection toensure that materials are present, viz. User manual,Removal kit, Emergency instructions, Bailout pig,Radiation alarm setting, and Printer paper. Switching onthe system allows the tests in item 7 to be performed.The source activity comparison can be made using atable generated by the medical physicist (Figure 5). Thiswill also satisfy the requirement (see Full Calibrationbelow) for performing decay correction which must bedone by the authorized medical physicist. Agreementshould easily be within 1 percent tolerances.For the remaining tests, the active source will needto be deployed. For this, the system can be programmedmanually each time or a standard program recalled fromthe system memory. A single dwell time of 20 to 30 ssuffices to test the door interlock, the interrupt button,and the emergency off button as well as to verify that theappropriate exposure indicators and radiation monitorsare functioning properly. The spot check form requirestesting of the functioning of the meter in the treatmentroom (Figure 6) under battery power alone. Its alarmsetting of 4 mR/hr was established so as to be aboveexposure levels in the room due to an adjacent linactherapy suite. As an additional safety measure, we have acalibrated GM meter that is carried by hand by personnelupon entering the treatment room. It is checked using a 1mCi Cs-137 source that yields a 10 mR/hr contact value.The remaining item is an estimate of timer accuracy. Forthis, a stopwatch is used to time a 30 s dwell. Typicalerror estimates are well below 1 second.FULL CALIBRATIONA “full calibration” is mandated for several differentcircumstances, e.g. before first medical use, following asource change or any major repair, etc. Since the sourcein most, if not all, modern HDR after loaders is Ir-192with a half-life of approximately 74 days, therequirement for quarterly calibration [2] does notformally apply. However, it is usual to replace an iridiumsource four times a year so as to maintain reasonabledose rates and treatment times. Quarterly QA testing ofHDR after loaders was recommended in the report of TG56. The components of a full calibration as defined inNRC Regulations 10 CFR Part 35 are listed in Table 2.

DA Wilkinson. Biomed Imaging Interv J 2006; 2(2):e34 4This page number is notfor citation purposeFigure 5 Source decay on physicist-generated spreadsheet (left)and printout from the HDR control console (right). Theactual numbers for the particular treatment date are36772 and 36754 mGy m2 h-1 (respectively).Figure 4 Spot-check form used each day of patient treatment.The form we employ for the full calibration is on anExcel spreadsheet (Figure 7) which allows convenientcalculations of source activity and positioning as well astimer accuracy and linearity. The activity of the source ismeasured using a well chamber and electrometer havingcalibrations traceable to the National Institute ofStandards and Technology (within 2 years as indicatedby the dates on the form). The electrometer needs to becalibrated in both current and charge (integral) modes.The source is programmed to go to a series of positionswithin the well chamber and the maximum currentreading is used to calculate the activity in air kerma units.This value is then compared to the manufacturer’s statedactivity decayed to the day of measurement. Agreementis typically within 2%. The regulations allow a 5% range.If equipment calibrations traceable to a national standardare not available, dosimetry system constancy checks canbe performed using a long-lived source such as Cs-137[5]. This is not a desirable substitute for propercalibration.Positioning accuracy is measured using a specialruler supplied by the manufacturer (Figure 8). Theprogrammed position (e.g. 995) is for the center of thesource, hence the correction (one-half of the sourcelength, or 2.15 mm) for the leading edge. The one mmcriterion may not be satisfied if there is much curvaturein the measuring system (see Figure 9). Thus, some caremust be taken to ensure that the transfer tube isreasonably straight and horizontal.We perform the battery back-up test by shutting offthe AC power to the after loader while a source has beendeployed. This makes it a somewhat different testcompared to the one in the spot-check where theemergency off button is pushed. That the source has beenretracted is printed out at the control console and isverified by the radiation monitor indicating exposurerates below the set value (4 mR/hr).Timer error and linearity are measured using atechnique established for teletherapy sources. Charge iscollected and measured in the well chamber for a set ofpredetermined times. The pass/fail criteria we adoptedseem both reasonable and reproducible and well withinthe capability of the system.We test the integrity of the transfer tube/applicatorsystem in three ways.● Once a program has been loaded into thecontrol unit, a transfer tube + applicator isattached to Channel 1 but the indexer ring is notlocked.● The second test has the transfer tube removedfrom Channel 1 and the ring locked.● The final test has the transfer tube inserted intoChannel 1, the ring locked, and an applicatorwith an obstruction in it attached. It should beadded that in the Nucletron system, failure toconnect the transfer tube to the applicatorproperly will usually generate the same errorcode as an obstruction.

DA Wilkinson. Biomed Imaging Interv J 2006; 2(2):e34 5This page number is notfor citation purposeHDR FULL CALIBRATIONNucletron MicroSelecton HDR S/N 9213Date: December 5,2005 Time: 10:201. SOURCE ACTIVITYChamber: Standard Imaging HDR 1000 Plus; S/N A943623Electrometer: CNMC K602; S/N 51090Source # Stated activity Reference date Check date Decayed actD35A-2870 41250 11/17/2005 12/5/2005 34852.6471T = 23.2 C t,p: 1.0188P = 749 Calibration factor: 4921 Calibrated: Apr 05Electrometer factor: 0.981 Calibrated: Oct 04Electr. rdgsPosition Rdg (Amp E-08)940 7.066945 7.102950 7.117955 7.109960 7.077Measured activity (U) 35003.72 8.68 CiRatio: measured/stated activity 1.0043_____ within ± 5%2. SOURCE POSITIONING ACCURACY (Mode 13)Figure 6 Exposure rate meter mounted on a wall in the treatmentroom so as to be visible from the entrance way.Programmed Measured Actual DeviationDistance (mm) Distance (mm) Distance (-2.15mm) (mm)905 907.5 905.35 0.35995 997 994.85 -0.15_____ within 1 mm3. BATTERY BACK-UP_____ Source retracted from treatment position when power to the unit was interrupted_____ Printout indicated failure due to power interruption and gave source out time and position4. LENGTH OF SOURCE TRANSFER TUBES & APPLICATOR(changes < 1 mm)_____ transfer tubes for flexible applicators_____ transfer tubes for rigid applicators_____ transfer tubes for gyn applicators_____ applicators5. TIMER ACCURACY & LINEARITY d = 950mmTREATMENT PLANNING QUALITY ASSURANCEIt is standard practice in external beam radiotherapyto have a second, independent check of the treatmentplan and monitor unit calculations. This may take theform of a simplified algorithm using data from phantommeasurements or dose measurements inside a suitablephantom (especially for IMRT plans). For brachytherapy,the independent check is also desirable (but notmandated by NRC regulation), but there is no generallyaccepted method for doing it. Some characteristicparameter(s) of the plan must be compared to anexpected value; however, what the characteristicparameters should be and how to arrive at the expectedvalue are left to each medical physicist or institution.TG-59 addresses these issues and lists severalapproaches that have appeared in the literature [6-8].Typically, the dose is calculated at representative pointsby the treatment planning system and then compared tothe results from a second independent system (perhaps aspreadsheet or nomogram). It remains unclear whatagreement is acceptable. Our method has been to use aplot of treatment time x source activity/ dose versus theglobal parameter of treatment volume for variousapplicator types. This is similar to the Paterson-Parkertables from the days of radium sources. It has beendescribed previously [9] and will be summarized below.The treatment volume (usually V100 in ourexperience) is obtained from the dose volume histogram(DVH). Several plans were run on both the Plato and asecond treatment planning system and the respectiveDVH’s compared to lend credibility to the use of thisparameter. We, then, used data from 20 to 30 patientplans for each of several applicator types (vaginalcylinder, tandem/ring, endobronchial tube) to constructthe T*A/D vs V100 plots. Several cases for eachTime (s) Q x 10 -7 Slope = ∆Q/∆T Timer Error =(Q2-2Q1)∆T/(Q2-Q1)5 3.75510 7.176 0.6842 -0.488220 14.006 0.683 -0.5066_____ timer error < 1 sFigure 7 Full calibration spreadsheet with actual calibration data.applicator were double planned with a second treatmentplanning system (ROCS or Pinnacle) for verificationpurposes. The data on each graph were then fitted toeither a straight line or a second order polynomial usingstatistical methods. The result was then used forchecking new patient plans to ensure consistency. Asummary of the initial use of this method for two typesof applicators is shown in Table 3.Clearly, the agreement is better when a polynomialis used for fitting the reference data. A similar situationis found for other types of applicators as well.Perhaps an even more important aspect of treatmentplan quality assurance is to have a second trained personinspect the plan and compare it with the written directive.The comparison should include such items as the doseprescription (per fraction and per course of treatment),the step size, dwell positions, etc. A more complete list isto be found in the report of TG-59. A check-list that ispart of the patient’s chart is a practical method to ensurethat this aspect of quality control is performed.Examination of other input data such as simulator filmsand comparison with the treatment plan is also essential.In our institution, specially trained radiation therapytechnologists and the authorized user physician check thetreatment plan.

DA Wilkinson. Biomed Imaging Interv J 2006; 2(2):e34 6This page number is notfor citation purposeFigure 8 Source position ruler showing white plastic indicator (red circle).Figure 9 Source position accuracy test showing a well-aligned ruler, transfer tube, and afterloader (left) and a setupwith a large curvature.TRAINING OF PERSONNELThe clinical personnel involved in an HDR programinclude the authorized user physician, authorized medicalphysicist, radiation safety officer, dosimetrist, nurse, andradiation therapy technologist. Some of these roles maybe combined into one. For example, the medicalphysicist may act also as the radiation safety officer anddo the treatment planning in lieu of a dosimetrist. Theauthorized physician and medical physicist should becertified by the appropriate medical specialties board andhave had special training in brachytherapy. Of primeimportance is the radiation safety training that allpersonnel involved in HDR treatments undergo. This isadministered to new personnel and then annually for allthose in the HDR program. Included is training in theproper response to a major emergency, in particular,failure of the source to be retracted into the after loadersafe upon completion of treatment or upon power outage.The daily spot check should ensure that properequipment (removal kit and bailout pig) is in place andthat simple emergency instructions are posted so as to bereadily available. If the source has to be retractedmanually, the standard precepts of radiation safety, viz.time, distance, and shielding, should be followed. Ifoperation of the hand crank is unsuccessful, then theapplicator containing the stuck source has to be removedfrom the patient. Once again, speed is crucial as ishaving such items as long forceps and a flashlight onhand. With the applicator plus source placed in thebailout pig and the patient and medical personnelremoved from the treatment room, the HDR suite shouldbe secured and a service engineer contacted for repair.We find it useful at the time of the annual training toreview and discuss in detail what each member of ourbrachytherapy team would do in various emergencysituations.REFERENCES1. Corbett PJ. Brachytherapy in carcinoma of the cervix: The state ofthe art. Martinez AA, Orton CG, Mould RF, eds. BrachytherapyHDR & LDR. Columbia: Nucletron, 1990:100-9.2. U.S. Nuclear Regulatory Commission [Web Page]. Available athttp://www.nrc.gov.3. Nath R, Anderson LL, Meli JA, et al. Code of practice forbrachytherapy physics: report of the AAPM Radiation TherapyCommittee Task Group No. 56. American Association ofPhysicists in Medicine. Med Phys 1997;24(10):1557-98.4. Kubo HD, Glasgow GP, Pethel TD, et al. High dose-ratebrachytherapy treatment delivery: report of the AAPM RadiationTherapy Committee Task Group No. 59. Med Phys1998;25(4):375-403.

DA Wilkinson. Biomed Imaging Interv J 2006; 2(2):e34 7This page number is notfor citation purpose5. Venselaar, J, Perez-Calataynd, J, eds. A practical guide to qualitycontrol of brachytherapy equipment [Web Page]. 2004; Availableat http://www.estro.be.6. Kubo HD, Chin RB. Simple mathematical formulas for quickcheckingof single-catheter high dose rate brachytherapy treatmentplans. Endocurietherapy/Hyperthermia Oncology 1992;8:165-9.7. Ezzell GA, Luthmann RW. Clinical implementation of dwell-timeoptimization techniques for single stepping-source remote afterloaders. Williamson JF, Thomadsen BT, Nath R, eds.Brachytherapy Physics. Madison: Medical Physics Pub, 1995:617-40.8. Rogus RD, Smith MJ, Kubo HD. An equation to QA check thetotal treatment time for single-catheter HDR brachytherapy. Int JRadiat Oncol Biol Phys 1998;40(1):245-8.9. Wilkinson DA, Clouser E, Fleming PA. A method for checkingHDR brachytherapy treatment plans. Radiother Oncol2004;71(supp 2):S47.Table 1Mandated periodic spot-checks1. Electrical interlocks at entrance to room2. Source exposure indicator lights on the after loader, control console, and in thefacility3. Viewing and intercom systems4. Emergency response equipment5. Radiation monitors to indicate source position6. Timer accuracy7. Clock (date and time) in unit’s computer8. Decayed source activity in unit’s computerTable 2Full calibration measurements (as applicable).1. Output within ± 5%2. Source positioning accuracy to within ± 1 mm3. Source retraction with backup battery upon power failure4. Length of the source transfer tubes5. Timer accuracy and linearity over the typical range of use6. Length of the applicators7. Function of the source transfer tubes, applicators, and transfer tube-applicatorinterfacesTable 3Percentage difference between newly created plans and our reference data.Applicatorlinearaverage differencepolynomialaverage differencevaginal cylinder 5.45±.06% 2.76±.01% 34tandem/ring 4.56±.02% 2.48±.02% 40n