IAPM 3rd Annual Scientific Meeting 2012 337

Unravelling Dose Prescription methods in Prostate Seed Br

achytherapy

MARGARET MOOREUniversity Hospital Galway, Ireland

Abstract: Prostate Seed Brachytherapy is a rapidly growingtreatment option for low-risk prostate patients. The treatmenttechnique in its present forms was developed in the 1980s of whichtwo distinct technique “schools” dominate:

� The Seattle Method of pre-planned implants delivered usingstranded seeds.

� The Stock-Stone method using nomograms and single seeddelivery using a MICK applicator.

Other variations on these methods have been developed since butthis paper will not distinguish these. Dose prescription during theimplant varies between the techniques in that one methodprescribes a dose (usually 145Gy) to the prostate gland plusa margin, while the other technique prescribes a dose (usually160Gy) to the prostate gland itself. In each of these cases theprostate gland is contoured using trans-rectal ultrasound imagingobtained just prior to and/or during the implant itself. Publishedoutcomes from prostate seed implant treatments have used postimplant CT-based dosimetry obtained generally one month post

implant. It is known that the prostate gland outlined using the CTimage set post implant will always result in a larger volume ofprostate than that outlined using ultrasound imaging during theimplant. There is controversy whether margins should also be addedat this stage. The dose parameters for describing the implant arethe D90 or dose covering 90% of the volume of the prostate and theV100, which is the volume of the prostate receiving 100% of theprescription dose. Both of these parameters are sensitive to themeans of determining the volume of the gland and the means ofdescribing the dose. Recently MRI imaging has been introduced toimprove the localisation of the prostate gland pre and post implantin conjunction with the ultrasound and CT image data. This paperwill show the variation in prescription dose in published literatureand highlight the difficulties in resolving these difficulties. Usinga data set from prostate seed brachytherapy patients treated atUniversity Hospital Galway, the impact of applying the variousdefinitions will be presented. An example of one case whereMRI imaging was used to supplement the dosimetry will bediscussed also.Keywords: Prostate-seed-brachytherapy, prescription, dosimetry

INVITED SPEAKER

Permanent prostate brachytherapy at the Royal Berkshire Hospital and implications of IPEM Report No. 106

ANDREW DOGGARTRoyal Berkshire Hospital, Reading, Surrey, UK

Abstract: Permanent prostate brachytherapy has been carried out atthe Royal Berkshire Hospital in Reading since 2003, using iodineseeds in the live, intraoperative method. To date 500 patientshave been treated using this method, and the procedure hasundergone several refinements since the programme wasintroduced. The evolution of the procedure at this institution will

be reviewed, in the hope that discussion of some of the issuesencountered will be of use to those who are commencing a similarprogramme. Attention will be paid to the need for a coordinatedapproach to address the particular radiation protection issuessurrounding these patients, and implications surrounding theimplementation of the recent IPEM Report 106 will be reviewed.

Dose build up effects of Med-Tec Carbon Fibre couch insert used in MV photon beams: Treatment Planning System vs.Monte Carlo and Ion Chamber Measurements

EZHILALAN RAMALINGAMCork University Hospital, Ireland

Introduction: The aim of this work is to validate the dose calculatedby the TPS at the dose build-up region when the treatment beamirradiates the patient (skin surface) through the CF couch. Theaccuracy of dose calculated by TPS for these situations depends onthe type of algorithm used.Methods: In order to validate the TPS for the situation mentionedearlier, we have used BEAMnrc MC Simulation and Parallel Plate(PP) chamber measurements as a benchmark. The threemeasurement set-ups used in the MC simulation are (a) A 6MV,10x10 cm2 photon field irradiating a water phantom through a 1.2cm thick CF material. (b) A 6MV, 10x10 cm2 photon fieldirradiating the water phantom without the CF material. (c) A 6MV,10x10 cm2 photon field irradiating the water phantom with the1.2 cm thick CF material replaced by water. In all thesemeasurements, the machine isocentre was kept constant in the

phantom. In addition, PP chamber measurements were done tovalidate the MC simulation explained earlier. In order to recreatethe measurement set-up (outlined earlier) on the TPS, CF couchinsert along with solid water phantom was scanned and importedin to the TPS. Subsequently, depth doses were measured for thethree measurement set-ups and compared against the MCsimulation and chamber measurements.Results: For the measurement set-up (a) MC simulation and chambermeasurement showed an increased surface (0.1 cm deep) dosez80%. However, the PBC algorithm over- estimated the surfacedose as z100% because the PBC algorithm treated the CF materialas water. MC simulated depth dose from set-up (c) was comparedagainst PBC depth dose calculated for set-up (a) and were found ingood agreement. This confirms that for set-up (a) PBC algorithmtreated the CF material as water. Performance of a more