MOHAMMAD HUSSIEN, NICHOLAS SPYROU & ANDY MA ANTHROPOMORPHIC VOXEL PHANTOM EFFECTIVE NEUTRON DOSES OUTSIDE A LINAC ROOM Department of Physics, University of Surrey, Guildford GU2 7XH, United Kingdom presented on behalf of by Mary Chin

reproduced from Chin MPW 1999 MSc Dissertation, University of Surrey PROBLEM STATEMENT RADIOTHERAPY DELIVERED IN THE ROOM STAFF WORK HERE POSSIBLY PUBLIC AREA WE NEED TO ESTIMATE THE DOSE

CURRENT STANDARDS = HOMOGENEOUS ELLIPSOIDS

THIS WORK = ANTHROPOMORPHIC PHANTOM Zubal G et al 1994 Med. Phys Alghamdi A A et al 2005 Radiat. Prot. Dosimetry WE GET TO KNOW NOT ONLY THE EFFECTIVE DOSE, BUT ALSO THE ABSORBED DOSE ORGAN-BY-ORGAN

Chin MPW 1999 MSc Dissertation, University of Surrey * 252 Cf neutron source METHOD: A 2-STAGE SIMULATION STAGE #1 FIND NEUTRON SPECTRUM AT EACH OF THESE LOCATIONS ACTUAL LINAC & PHOTONEUTRON PRODUCTION NOT MODELLED JUSTIFIED IN Chin, Spyrou & Tomlinson 2007 Trans ANS

METHOD: A 2-STAGE SIMULATION STAGE #2 FOUND NEUTRON SPECTRUM SURFACE SOURCE ORIENTATION REPEATED FOR: LEFT-TO-RIGHT RIGHT-TO-LEFT FRONT (ANTERIOR-TO-POSTERIOR) BACK (POSTERIOR-TO-ANTERIOR) DOSE ESTIMATES AT 5 POSITIONS REPEATED FOR: ICRP PUBLICATION 60 (1991) ICRP RECOMMENDATIONS 2005 ICRP RECOMMENDATIONS 2007

E = EFFECTIVE DOSE W T = TISSUE WEIGHTING FACTOR W R = RADIATION WEIGHTING FACTOR D T, R = AVERAGE ABSORBED DOSE Most significant changes in w T between 1990 and 2007 Gonads from 0.2 to 0.08 Breast from 0.05 to 0.12 Liver, thyroid, bladder & oesophagus from 0.05 to 0.04 Salivary glands introduced at 0.01 Prostate, lymph nodes & gall bladder added to the ‘remainder’ section

E = EFFECTIVE DOSE W T = TISSUE WEIGHTING FACTOR W R = RADIATION WEIGHTING FACTOR D T, R = AVERAGE ABSORBED DOSE DOSE ESTIMATES AT 5 POSITIONS REPEATED FOR: ICRP PUBLICATION 60 (1991) ICRP RECOMMENDATIONS 2005 ICRP RECOMMENDATIONS 2007 FOR THERMAL NEUTRONS W R = 5 W R = 1 W R = 2.5 THE NEED TO REDUCE W R =5 WHEN NEUTRONS INTERACT, CONTRIBUTION FROM SECONDARY PHOTONS INCREASES WITH DECREASING NEUTRON ENERGY Alghamdi A 2006 PhD Thesis University of Surrey

ADDITIONAL SHIELDING CONCERNS RADIATION BUNKER WALL PHOTONUCLEAR REACTION 1.PHOTON (TREATMENT) BEAM 2.NEUTRON CONTAMINATION NEUTRON REACTIONS 3.NEUTRON- INDUCED PROMPT RADIATION 4.NEUTRON- INDUCED DELAYED RADIATION

6 MV 15 MV UNWANTED HOT-SPOTS SPARES HEALTHY TISSUES MORE PENETRATING FOR PELVIS / BIG PATIENTS FOR HEAD&NECK / SMALL PATIENTS

15 MV MORE PENETRATING IN BODY TISSUES CONCRETE WALL ALSO MORE PENETRATING IN BUNKER WALLS RADIATION PATIENT WE WANT TO TREAT HENCE ADDITIONAL SHIELDING CONCERNS    STAFF / PUBLIC WE MUST PROTECT

Organ Absorbed Dose

SHIELDED BY

ORGAN SHIELDING EFFECTS BY ANOTHER ORGAN ORGANS ON THE LEFT vs ORGANS ON THE RIGHT ORGANS ON THE ANTERIOR vs ORGANS ON THE POSTERIOR RESULTS CONSISTENT WITH EARLIER FINDINGS Tzortzis M MSc Thesis University of Surrey

Effective Doses

Comparison between ICRP 1990 and 2007 draft using w R = 1 but different w T

Comparison between ICRP 1990, 2005 draft and 2007 draft W R = 5 W R = 1 W R = 2.5 FROM 1990 TO 2005 ICRP CHANGED ITS MIND BY 500% FROM 2005 TO 2007 ICRP CHANGED ITS MIND BY 250% QUESTION DO THE REVISIONS MAKE US MORE, OR LESS, CONFIDENT?

linac operates at 4 Gy / min typical prostate patient receives 2 Gy per visit 10 patients treated per day on a linac radiographer works 230 days per year effective dose per year = mSv using current ICRP 1990 (6.78 mSv using 2007 draft) due to neutrons only. WORST-CASE SCENARIO

no breasts, salivary glands & thymus in the phantom only one adrenal gland is included in the phantom a male phantom was used UK statistics: 9/10 radiotherapy nurses are female w T for breast increased from 0.05 to 0.12 in 2007 draft POINTS TO NOTE

MCNPX DOES NOT SIMULATE DELAYED GAMMAS SIMULATION WAS DONE USING MCNPX ALTHOUGH IN SOME CASES ARTIFICIAL LINES HAVE BEEN INCLUDED IN THE CROSS SECTIONS [MCNPX manual] BETA VERSIONS 26C/D DO SIMULATE DELAYED GAMMAS HOWEVER, DUE TO MULTIPLE GLITCHES WE FOUND THE CAPABILITY NOT READY FOR OUR USE [Chin & Spyrou Monte Carlo simulation of ( γ, n) and (n, γ ) activations: a multi-code comparison with theory. 12th Int Conf Modern Trends in Activation Analysis. Tokyo, ] Note RESULTS SHOWN HERE EXCLUDE DELAYED GAMMAS ATOMIC NUMBER MASS NUMBER LIBRARY SPECIFIER

eg. ZUBAL‘s VERY DETAILED INDEED … BUT MONTE CARLO CODES AREN’T GOING TO MAKE MUCH SENSE OUT OF IT EACH TISSUE/ORGAN TYPE MUST BE MAPPED INTO ELEMENTAL (IF NOT ISOTOPIC) COMPOSITION TEDIOUS SEGMENTATION WORK… ASK KRAMER et al ANTHROPOMORPHIC VOXEL PHANTOMS WE KNOW SOME (eg Spyrou et al papers) BUT NOT ALL

IN FACT, VOXEL PHANTOMS ARE ROUTINELY USED FOR RADIOTHERAPY PLANNING / VERIFICATION CHIN MPW et al 2006 Scientific Computing INDIVIDUAL PATIENT INSTEAD OF CADAVER / VOLUNTEER DOSE TO THE INDIVIDUAL PATIENT INSTEAD OF PUBLIC RISK ASSESSMENT VOXELS ATTRIBUTES: HOUNSFIELD UNITS INSTEAD OF TISSUE / ORGAN TYPES THE ISSUE OF MAPPING

VOXELS ATTRIBUTES: HOUNSFIELD UNITS INSTEAD OF TISSUE / ORGAN TYPES MORE MEANINGFUL IN TERMS OF RADIATION PROPERTIES BUT NOT AS STRAIGHT-FORWARD AS WE WISHED … BELGIUM USA UK Germany Switzerland Italy Spain Portugal THE ISSUE OF MAPPING

MATHEMATICAL / MIRD–TYPE PHANTOMS Downloaded from ZUBAL / FAX / MAX / … VOXEL PHANTOMS WE STILL HAVE A LONG WAY TO GO FAITHFUL REPRESENTATION OF THE INDIVIDUAL / VOLUNTEER / CADAVER BEFORE DEBATING ON GENDER / SIZE / ETHNIC DIVERSITIES WE HAVE GONE A LONG WAY