# AL305-Physics II

AL305-Physics II Meeting #2 Photon Beam Dosimetry John M. Kratina, BMEd, RT(R)(T) LRT - Instructor Copyright 2009 John M. Kratina Calibrated Isocenter Distance All Linacs are Calibrated to a Defined Isocenter Distance. This distance comes into play when making Tx calcs. Older Tx Machines, Co60, 4MV and 6MV are calibrated at an *Isocenter Distance of 80 cm.*

Newer Tx Machines have a Calibrated Isocenter Distance of 100 cm. Calibrated Isocenter Distance See Stanton page 338-Appendix 5: View that the calibrated Isocenter Distance for 4MV is 80cm. See Stanton page 340: View that the Newer

10x (or MV) Linacs have an Isocenter Calibration of 100cm. The Isocenter Calibration will effect how we do Tx calculations. Maximum Dose: Dmax The maximum dose (Dmax) occurs at the point at which the energy of the electrons coming to rest equals the E of electrons being set into motion by new photon interactions. This point at which equal numbers of electrons are being

stopped and driven forward or where Kerma equals Dose is called Electron Equilibrium. *Other Equivalent Terms include: Incident Dose, Given Dose, Dose to Dmax and Dose to Dmax Zone of Intensity all refer to the same thing: *DMAX.* Dmax Dose Dose at maximum Depth , for a single field treatment with a megavoltage beam, is the point where the highest dose occurs. In a single field tx, the Tumor Dose is less the the Dmax Dose. The depth of Dmax increases with Beam E.

*Remember, Depth of Dmax increases as Beam Energy Increases.* Dmax Dose Dmax So, the Depth of Dmax of a 15MeV Beam is Greater than a 6MeV Beam.

Skin sparing effects increases (on the entry side of the Beam) as the Beam Energy (E.) increases. Thus, a 10 MEV beam has more , beam entry side, skin sparing effects than a 4MeV Beam. View page 100, Table 8.2 to see this. See how the Depth of Dmax for a 4Mev Beam is 1.0 cm while the Depth of Dmax for a 10 Mev beam is 2.5cm? Please memorize this chart. Exit dose High E. beams always have an Exit dose. We know that as Beam E. increases, the dose to the skin, on the entry side of the beam, decreases. Thus the skin sparing effect increases. ^E. = ^skin

sparing. Now, as the Beam E. increases, the Exit Dose increases. See page 338, table 2 and page 341, table 5. Remember: * As the Beam E. increases, skin sparing increases on the beam entry side =and= the Exit Dose increases on the Beam Exit side.* Scatter The radiation treatment beam is composed of both primary and scatter radiation.

Any interaction of the primary beam may result in scatter.Radiation that is scattered back towards the surface of the patient is called Backscatter. Scatter THE PHOTON BEAM CONSISTS OF 1- PRIMARY BEAM AND 2- SCATTER COMPONENT THE SCATTER PART HAS SEVERAL FACTORS TO CONSIDER: FIELD SIZE (FS), FD SHAPE, BEAM E., AND FIELD BLOCKING.

SCATTER PLAYS A ROLE IN PERCENTAGE DEPTH DOSE (PDD) AS WELL AS TMR. PLEASE NOTE: FOR A GIVEN DEPTH, AS FD SIZE INCREASES, THE VALUE OF PDD(%DD) OR TMR INCREASES.---THIS IS BECAUSE OF SCATTER. Back Scatter Factor BSF= Dose at dmax in phantom --------------------------Dose without backscatter The ratio of the dose at a point in dmax

including backscatter to the dose at the same point in air without backscatter. This is also known as peak scatter factor. BSF Backscatter is Dependent on Beam Energy Scatter Beam Geometry BSF BSF INCREASES AS E. INCREASES UP *UNTIL* 62

KEV. THEN COMPTN SCATTER TAKES OVER. AT 2MEV, SCATTER IS VIRTUALLY FORWARD SCATTER. *SEE STANTON PAGE 177, FIG 12.3 AT HIGH E., THE BSF IS ALSO CALLED THE PEAK SCATTER FACTOR AND IT'S VALUE APPROACHES "AROUND 1.0"

MU/Time Calculation Factors Cfs =field size correction factor or- output factor for field sizes other than 10X10cm. Ccal =machine calibration output or dose rate, usually 1.00cGy/MU for LinAcs @ 10 X 10 f.s. Cattn = transmission factor for block trays and /or wedges. Output Factor or- Cfs OUTPUT FACTOR, STANTON PAGE 178 REMEMBER:

OUTPUT FACTOR = Cfs THE STANDARD CFS OR OUTPUT FACTOR IS "1" AT 10 X 10 CM FS AT DMAX DEPTH. Output Factor or- Cfs YOU WILL WANT TO REMEMBER THAT THE *OUTPUT FACTOR IS LESS THAN "1" AT A F.S. **LESS***THAN 10 X 10.

THUS, A CFS OR OUTPUT FACTOR IS GREATER THAN "1" WHEN THE F.S. IS GREATER THAN 10 X 10 NOTE THAT GREATER THE FS, THE GREATER THE OUTPUT DUE TO INCREASED BACKSCATTER TO DMAX FROM PHANTOM OR PATIENT -AND- FORWARD SCATTER OF THE JAWS CHANGES BACKSCATTER TO THE MONITOR CHAMBER.****THIS IS ALL VERY IMPORTANT TO CONSIDER****

Output factor = Cfs SO, REMEMBER THAT Cfs=FIELD SIZE CORRECTION FACTOR = *OUTPUT FACTOR* CFS = DOSE @ DMAX FOR A F.S. / DOSE @ DMAX FOR A STANDARD *10 X 10 cm* F.S. CFS FOR 10 X10 CM = 1.0

Cfs or Output factor Calc SEE PAGE 179 (Please Note: that there is a Typo on Stanton page 179. The Cfs and Ccal values are incorrectly reversed.) ***HERE IS THE CORRECT CALCULATION:*** SEE THE CALCS HERE TO FIND OUTPUT AT 160 SSD SO, OUTPUT 160 SSD = OUTPUT @ 80 SSD X

INVERSE SQUARE FACTOR Output calc OUTPUT 160 SSD = OUTPUT @ 80 SSD X INVERSE SQUARE FACTOR (this time, please note Dmax =1cm as stated.) = OUTPUT @ 80SSD X (80 +1 /160+1)*2 =Cfs X Ccal X (81/161)*2 =1.00 (the correct Cfs) x 1.03 cGy/mu (the correct Ccal) x (0.05031055)*2 =1.00 x 1.03 x 0.2531151 =0.2607085 cGy/mu =0.261 cGy/mu (rounded up.)

Output Calc Please note that we are to use the Standard, 10 x 10 F.S. unless otherwise stated. We will be given the Ccal and Cfs in all equations on the Exam. Problem #1, Stanton text,page198 Remember we have a Dmax of 1.5cm We will use the Standard 10 x 10 F.S. so our

Cfs =1. We have been given the Ccal of 1.0 cGy/mu. Plug this in your calculator Problem #1, Stanton text,page 198 Cfs x Ccal x (101.5/201.5)*2 = 1 x 1 x 0.254 = 0.254 cGy/MU What was your answer on your calculator?

Equivlent Square Fields In order to use the square field data tables, we must convert non-square fields to Equivalent Sqaure Fields. ESF= 4 X Area/Perimeter So, ESF = 4 x L x W / 2(L + W) ESF will have the same Cfs, scatter effects and depth dose of a rectangular field. ESF = 4 x A/P See Stanton, page 179-180. THE ABOVE FORMULA CAN BE USED WHEN A CHART IS NOT AVAILABLE. Page 180 shows a computer generated field that is more accurate. TURN TO QUESTION 8 IN THE STANTON CHAPTER 12 PROBLEMS ON PAGE 198.

First, we need the ESF FOR A 25CM X 10CM FD ESF= 4 X A/P ESF = 4 X L x W / 2(L + W)

=4 X 25X10 / 2(25+10) = =4 X 250/70 =4 X 3.57 =14.285714

=14.3 ESF (rounded up.) (please note we got 14.3 ESF, while the computer chart shows 13.6) Cattn factor NEXT, WE HAVE EQUIP. ATTEN FACTORS OF BLOCKS AND TRAYS TO TAKE IN ACCOUNT. THIS IS WHERE Cattn comes in. Cattn = DOSE With DEVICE IN BEAM / DOSE

Without DEVICE IN RAD BEAM PLEASE SEE TABLE 12.3 IN STANTON PAGE 181. HERE WE CAN ALSO SEE FOR THE E.'S WE ARE WORKING WITH, THE Cattn IS "USUALLY ABOUT 1" Patient Attenuation Factors Percent Depth Dose

Tissue-Air Ratio Tissue-Maximum Ratio PDD PERCENT DEPTH DOSE IS USED *ONLY* FOR SSD OR NON-ISOCENTRIC CALCS. PDD = ABSORBED DOSE @ DEPTH ---------------------------ABSORBED DOSE @ DMAX X 100% PDD IS DEPENDENT ON THE CALIBRATED

ISOCENTER DISTANCE OF THE MACHINE. PDD PDD IS DEPENDENT ON THE MACHINE ISOCENTER DISTANCE OF THE MACHINE. PDD WILL BE LESS FOR A GIVEN F.S. AND DEPTH FOR AN 80 CM ISOCENTER DISTANCE MACHINE VERSES A 100CM ISOCENTER DISTANCE MACHINE. (FOR A VISUAL SEE PAGE PAGE 499 IN THE WASHINGTON TEXTBOOK. )

PDD Increases as E. increases. PDD increases AS FS INCREASES, BECAUSE OF SCATTER. PDD DECREASES AS DEPTH IN TISSUE INCREASES. PDD- SEE PAGE 499 IN THE WASHINGTON TEXTBOOK

TAR TAR IS ONLY FOR SAD OR ISOCENTRIC SET-UPS FOR 4MV OR COBALT 60 UNITS. TAR = DOSE @ GIVEN DEPTH IN PHANTOM @SAD ----------------------------------DOSE Without PHANTOM @SAD. TAR VISUAL IS FOUND ON PAGE 500 OF WASHINGTON.

TAR VISUAL IS FOUND ON PAGE 500 OF WASHINGTON. TMR TMR IS USED FOR 6MV OR HIGHER E. SAD MACHINES. TMR= DOSE @ DEPTH d ------------------DOSE @ DMAX

SEE PAGE 501 IN WASHINGTON FOR A TMR VISUAL. PLEASE REMEMBER BOTH TAR AND TMR ARE *INDEPENDENT* OF THE MACHINE ISOCENTER DISTANCE. TMR VISUAL - PAGE 501 IN WASHINGTON. MU/Time Calculation Factors Patient Attenuation Factors:

For Non-isocentric set-ups: %DD For Isocentric set-ups: TAR or TMR Tumor Dose = Dose precribed per field. Nmu = number of monitor units. NMU calcs. Here we go! SEE WRITTEN ASSIGNMENT KEY -PHOTON BEAM Dosimetry MU PROB 1 AS FOUND IN BUONO WKBK PAGE 57. WE ARE IN THE MIDDLE OF THE PAGE OF THE WRITTEN ASSIGNMENT KEY LOOKING AT MU CALCS PROBLEM 1. (ON BUONO PAGE 55 WE LOCATED THE OUTPUT FACTOR OF 0.930)

THE PDD IS FOUND ON BUONO WKBK PAGE 54: Using Sp (or PSF) and Sc (or COF) factors for MU Calcs with TMR Some treatment centers use more than one output factor. Here we see the usage of: Sp (or PSF) and Sc (or COF.) Sp (or PSF) = phantom scatter factor Sp (or PSF) is used to determine the scatter from the patient or phantom device. Sc (or COF) = collimator output factor Sc (or COF) is used to measure scatter from the coll. An Inverse Square Correction Factor (ISCF or SAD factor)

is used. (usually around 1 when the Machine Calibration Distance is the same as the SAD or the Isocenter ) Reference: Washington and Leaver, 3rd Ed. p. 497, 510-511. (2nd Ed.,p.475, 490-1) Using Sp and Sc factors for MU Calcs with TMR For an unblocked single field: NMU= Rx Dose -------------------------------- x ISCF RDR

x Sc x Sp x TMR (or output) (CS) (EFS) (EFS) (Reference: Washington/Leaver, 3rd Ed., p.511512) Using Sp and Sc factors for MU Calcs with TMR

A Radiation Oncologist wants a patient treated with a single open field, using a 6MV LinAc Isocentric (SAD) set-up at 100cm SAD. The set-up SSD is 95cm. (that tells that we have a Tx depth of 5cm)

FS=15 x15cm open field No block No Tray Inverse Square Correction Factor (since the machine calibration distance is at SAD) = 1 Using Sp (or PSF) and Sc (or COF) factors for MU Calcs with TMR RDR (or Dose Rate or Output) = 1.0cGy/MU Sc Factor (or Collimator Scatter Output Factor or COF)(for a 15 x 15 cm field) = 1.021(Washington, p. 517) Sp Factor (or Phantom Scatter Factor or PSF) (for a 15 x15 cm field) = 1.014 (Washington, p. 517) TMR(depth of 5cm,15x15f.s.)=0.937 (Washington, Table 24-9, p. 523)

Dose per fraction = 310 cGy. The Inverse Square Correction Factor (ISCF or SAD factor) since the machine calibration distance is at SAD) = 1 What is the MU setting using TMR? Using Sc and Sp factors for MU Calcs with TMR NMU= 310cGy ------------------------------ X 1.0 1.0cGy/MU X 1.021 x 1.014 x 0.937 (dose rate) (Sc) (Sp) (TMR)

(ISCF) NMU= 319.55 or 320 MU Using Sp and Sc factors for MU Calcs with TMR For blocked field: NMU= Rx Dose --------------------------------- X ISCF Outpt x Sc(openFS) x Sp(blkFS) xTMRxTF Using Sp and Sc factors for MU Calcs with TMR A Radiation Oncologist wants to treat a

patient with POP AP and PA fields with a dose of 4000cGy at a depth of 10cm in 20 fractions. The physician wants to use a 6MV LinAC isocentric (SAD) delivery at 100cm SAD. The tx set-up SSD is at 90cm. FS= 15 X15. Thus, the Sc Factor (or COF) is 1.021. The fields are blocked to an 8 X 8 equivalent square field. Thus, the Sp Factor (or PSF) is 0.992. What is the MU/fraction needed, using TMR? Factors needed include:

Sc (or COF) for a 15 X 15 field size) = 1.021 (Washington, Table 24-9, p. 517) Sp (or PSF) for a 8 X 8 equivalent square field = 0.992 ( Washington, Table 24-9, p.517) TMR(10,8) = 0.775 ( Washington, Table 24-9, p. 523) Inverse Square Correction Factor (since calibration distance is at SAD) = 1 Tray Factor = 0.97 Dose per port =100cGy/port

Dose rate or Output =1.0cGy/MU What is the MU setting for one tx fraction using TMR? Using Sp and Sc factors for MU Calcs with TMR NMU= 100cGy __________________________________ X 1.0 1.0 cGy/MU x 1.021 x 0.992x 0.775 x 0.97 (Output) (Sc)

(Sp) (TMR) (TF) (ISCF) NMU= 131 MU Sp and Sc factors WHEN USING A THE Sp AND Sc FACTORS IN THE FORMULA, YOU WILL ALWAYS USE THE INVERSE SQUARE CORRECTION FACTOR. AS WE HAVE DISCUSSED BEFORE, IN MANY CASES THE INVERSE SQUARE CORRECTION FACTOR( or also known as the SAD factor) IS USUALLY AROUND 1.

THIS IS TRUE WHEN THE MACHINE CALIBRATION POINT IS AT THE SAD. FOR EXAMPLE, THE MACHINE IS CALIBRATED AT 100CM AND THE SAD IS ALSO 100CM. THUS, THE INVERSE SQUARE CORRECTION FACTOR ( or SAD factor) WOULD BE AROUND 1. IN THIS CLASS, WE ARE USING SEPERATE Sp AND Sc FACTORS FOR TAR AND TMR CALCS AS INSTRUCTED BY DR. KHAN AND WASHINGTON/LEAVER. ABOUT THE EARLY LINACS: STANTON AND STINSON TELLS US THAT IN THE 1990S, SOME CENTERS JUST USED A GENERAL Field Size Correction Factor or Cfs. PLEASE NOTE THAT THE INVERSE SQ. CORR. FACTOR WAS USED FOR TAR AND TMR CALCS TO FIND MU.

ANYONE HAVE ANY QUESTIONS OR NOT CLEAR ABOUT THESE VARIATIONS? Sp and Sc factors THIS IS SIMILAR TO THE VARIATIONS OF THE MAYNEORDS FACTOR FORMULAS---I GAVE YOU 3 DIFF. FORMULAS AND THEY ALL WORK.

IT JUST DEPENDS ON THE TX CENTER AND WHAT IS USED THERE. PLEASE REVIEW PAGES IN 3rd Ed. 497, 510-511. (2nd Ed.,p.475, 490-1) THAT WE DISCUSSED IN WASHINTON AND LEAVER, AS WELL AS, REVIEW CH 12 IN STANTON AS YOU PREPARE FOR YOUR DOSIMETRY ROTATIONS AT CLINICAL/WORK SITE. YOU WILL BE READY TO START DOSIMETRY ROTATIONS WITH EASE.

NMU Calc For 4MV; 4X4 FS; DEPTH AT 5CM = 81.1% **VERY IMPORTANT** NOW WE KNOW THAT THE PDD WE NEED TO USE IN OUR EQUATION NEEDS TO BE LESS THAN ONE--AND THAT THE PDD IS A PERCENTAGE--*SO WE NEED TO MOVE THE DECIMAL PT OVER 2 PLACES TO THE LEFT TO SEE %DD= .811 Nmu calc PLUGGING THIS IN WE HAVE : Nmu = Tumor dose / Ccal x PDD X Cfs X Cattn (formula from Stanton page 185) 125CGY/ 1.00CGY/MU X PDD X OUTPUT FACTOR

=125/1.0 X .811 X.93 =125/ 0.75423 =165.73193 MU =166 MU Written Assigment Key,Problem #2 LETS LOOK AT PROBLEM #2IN THE WRITTEN ASSIGMENT KEY. BY LOOKING UP THE PDD AND OUTPUT FACTOR IN BUONO, AS WE DID FOR QUESTION 1 WE SEE: 150 CGY/ 1.00CGY/MU X 0.671 X .985 = 150 CGY/ .660935 =226.95121 MU =227 MU (rounded up)

DID YOU GET THAT ON YOUR CALCULATOR? Written Assignment Key, Separation Problems MOVING ON, LETS LOOK AT THE SEPARATION PROBLEMS AT THE BOTTOM OF THE WRITTEN ASSIGNMENT KEY PAGE. WE ARE LOOKING AT BUONO PAGE 57,PROBLEMS 5.2 OVER POP FD'S

FIRST WE HAVE TO REMEMBER THAT WE ARE TALKING ABOUT 2 POP FD'S ***SO, WE WILL NEED TO DIVIDE THE TOTAL TUMOR DOSE BY 2. AND WE WILL NEED TO DIVIDE THE TOTAL PT. SEPERATION BY 2. Separation problem #1 PROB #1: 200 CGY IS THE TOTAL SO, WE MUST DIVIDE: 200/2 =100CGY PER FD

18 IS TOTAL SEPERATION SO: 18/2 = 9 SEP PER FD PER FD= 100CGY/1.00CGY/MU X .621 X .93 = 173 MU NowPlease try it on your calculator WORKING IT OUT WITH YOU: 11CGY/ 1.00 X.621X.93 = 100/.57753 =173.15117 MU =173MU

WE GOT THE PDD FROM PAGE 54 IN BUONO, DEPTH (SEPERATION )9CM **REMEMBER, SINCE THAT'S A PERCENTAGE, WE MOVED THE DECIMAL POINT OVER 2 POINTS *TO THE LEFT* TO GET: .621 OUTPUT FACTOR FOR 4X4 4MV FD IS SAME AS BEFORE: .93 AS FOUND ON page 55 OF The BUONO WORKBOOK.

Separation Problem #2 LETS WORK THROUGH ONE MORE PROBLEM: SAME SET OF PROBS, PROBLEM #2 POP FD'S 8X8 FS *TOTAL*SEPARATION 20CM TOTAL TUMOR DOSE: 300 CGY AS SEEN ON BUONO PAGE 57, PROBLEMS 5.2 QUESTION #2.

SINCE WE ARE LOOKING AT POP FDS WHAT DO WE HAVE TO DO? DIVIDE WHAT BY WHAT???? ***What do you do?***

If you said divide tumor dose and separation by 2, you are CORRECT! Separation problem #2 Our DOSE PER FD =150 CGY. SEPARATION PER FD IS 10CM.

SO, WE LOOK UP THE VALUE FOR THIS SEPARATION ON BUONO PAGE 54 AND FIND? AND SINCE PDD IS A PERCENTAGE AND WE NEED A VALUE LESS THAN ONE WE NEED TO DO WHAT WITH THE DECIMAL POINT? Separation problem #2 YES, WE MOVE THE DECIMAL OVER TO THE

LEFT 2 PLACES. WHAT IS OUR OUTPUT FACTOR? REMEMBER, CFS =OUTPUT FACTOR YOU FIND .985 on table 5.5pg.55 for 8x8 f.s. Seperation problem #2 NOW WE CAN PLUG IT ALL IN: 150CGY /1 cGy/mu 150/.621535

=241.33797 MU =241 MU X .631 X .985 = Practice, practice,practice Please Work through all of the Written Assignment Problems, as well as, the Stanton Ch 12 Problems.

Please send me an e-mail or give me a call if you have any questions. I'm here to help. Copyright 2009 John M. Kratina Practice makes perfect! Reference and Illustration credits: Stanton and Stinson text, Radiation Oncology Physics (used with permission.) Washington and Leaver text, Principles and Practice of Radiation Therapy (used with permission.) Buono text, Primer in Radiation Therapy

Physics (used with permission.) My lecture notes Copyright 2009 John M. Kratina

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