Ver 2.11 Chapter XIII PDD TAR TMR Dose Free Space Joseph F. Buono RTT Allied Health Science Nassau Community College 1 Education Drive Garden City, NY 11530-6793 phone: 516-572 - 7536 oice - 9460 Secretary email: joseph.buono@ncc.edu website: rtscanner.com page setup 10 7.5 Title
Menu - Objectives - next by deault - Dose in ree space - BackScatter Factor ( BSF ) - Percentage Depth Dose ( PDD ) - Tissue Air Ratio ( TAR ) - Dose to second point ( i.e. - Cord Dose ) or SSD setup - Tissue Maximum Ratio ( TMR ) - Dose to second point ( i.e. - Cord Dose ) or SAD setup - Relationship between BSF, PDD, TAR, TMR - Conversion between PDD & TAR (also TMR) Menu
Objectives 1 - Deine Dose in Free Space. 2 - Deine BackScatter Factor ( BSF ). 3 - State the actors that eect BackScatter Factor ( BSF ). 4 - Deine Percantage Depth Dose ( PDD ). 5 - State the actors that eect Percentage Depth Dose ( PDD ). 6 - Deine Tissue Air Ratio ( TAR ). 7 - State the actors that eect Tissue Air Ratio ( TAR ). 8 - Deine Tissue Maximum Ratio ( TMR ). 9 - State the actors that eect Tissue Maximum Ratio ( TMR ). 10 - State the relationship between TAR, TMR and BSF. 11 - State the relationship between TAR table and BSF. obj 1
Dose in Free Space Dose in Free Space DFS 0
source Dose in Free Space radiation beam ionization chamber build-up cap point in space The exposure measured in Air at the center o the chamber is: X exposure Meter reading C T,P,S N x Exposure measured in Roentgen Correction actors due to Temperature Pressure Stem error Calibration actor DFS 1
source Dose in Free Space The reading rom the ionization chamber gives the X exposure at the center o the beam without the perturbing inluence o the chamber. Next, need to place a small mass o tissue at the center o the beam, who's radius is equal to depth dmax. The dose at the center o the mass o tissue is reerred to as the dose in "ree space" X exposure which is equal to: X exposure Meter reading C T,P,S N x D s X exposure tissue A eq where: END Dose in Free Space tissue is the Roentgen to rad conversion actor or tissue next BackScatter Factor DFS 2
BackScatter Factor BackScatter Factor BSF 0
BackScatter Factor ixed distance (usually machine operating distance) generally 100 cm Linac 80 cm Co60 central axis beam Find dose in "ree space on the central axis. D.s. D.s. Find maximum dose in a phantom on the central axis at a ixed distance. D dmax D dmax BSF 1
BackScatter Factor NOTE: Deinition: D dmax BSF D.s. Since the dose at Dmax (D dmax ) will always be equal to OR greater then the dose in ree space (D.s. ), BackScatter Factors (BSF) will always be equal to OR greater then 1. As the photon energy increases the BSF will become closer to 1. BSF is dependent on: D.s. D dmax 1) Energy increase in energy decrease in BSF (max BSF.6 to.8 mm Cu HVL) depending on ield size can be as large as 1.5 2) Field Size increase in F.S. increases BSF 3) SSD independent o SSD BSF 2
Percentage Depth Dose Percentage Depth Dose PDD 0
Percentage Depth Dose Beam Fixed SSD central axis generally 100 cm Linac 80 cm Co60 Find maximum dose on central axis. D dmax D dmax D d phantom Find dose at some other depth on central axis. D d PDD 1
Percentage Depth Dose Percentage Depth Dose @ depth has been deined as: Dose @ depth PDD d 100% Dose @ Dmax D PDD d d 100% D dmax D dmax D d PDD 2
Percentage Depth Dose PDD is dependent on: Percentage Depth Dose @ depth has been deined as: Dose @ depth PDD d 100% Dose @ Dmax D PDD d d 100% D dmax D dmax D d SSD 1) Energy an increase in energy increases PDD 2) Field Size an increase in F.S. increases PDD (because o an increase in scatter) 3) Depth o Tissue an increase in depth decreases PDD 4) SSD an increase in SSD increases PDD ( due to the inverse square law and the act that PDD is deined at two dierent points ) two dierent distances rom the source SSD + dmax SSD + depth PDD 3
Tissue Air Ratio Tissue Air Ratio TAR 0
Tissue Air Ratio Distance rom source Beam generally 100 cm Linac 80 cm Co60 central axis Find dose in "ree space on the central axis. D.s. D.s. TAR 1
Tissue Air Ratio Distance rom source Beam generally 100 cm Linac 80 cm Co60 central axis d Find dose in "ree space on the central axis. D.s. D.s. Find dose in phantom at depth on the central axis at the same distance rom source. D d D d TAR 2
Tissue Air Ratio Distance rom source Beam generally 100 cm Linac 80 cm Co60 central axis d Deinition: D TAR d d D.s. D.s. TAR is dependent on: 1) Energy an increase in energy increases TAR 2) Field Size an increase in F.S. increases TAR (because o an increase in scatter) 3) Depth o Tissue an increase in depth decreases TAR D d 4) SAD (distance) independent o distance both reading at same distance rom the source D.s. D d TAR 3
Tissue Air Ratio Distance rom source Beam Note: I depth is changed to dmax then: D TAR dmax dmax D BSF dmax D.s. D.s. generally 100 cm Linac 80 cm Co60 central axis d Deinition: D TAR d d D.s. D.s. TAR is dependent on: 1) Energy an increase in energy increases TAR 2) Field Size an increase in F.S. increases TAR (because o an increase in scatter) 3) Depth o Tissue an increase in depth decreases TAR D dmax 4) SAD (distance) independent o distance both reading at same distance rom the source D.s. D d TAR 3
Tissue Maximum Ratio Tissue Maximum Ratio TMR 0
Tissue Maximum Ratio Distance rom source generally 100 cm Linac 80 cm Co60 Beam central axis Place phantom such that the ionization chamber is at depth o maximum dose. depth dmax Ionization Chamber D dmax dmax Turn beam on and record dose reading. D dmax TMR 1
Tissue Maximum Ratio 100 cm Linac 80 cm Co60 central axis Find dose at depth in the phantom at the same distance rom the source. D d central axis d dmax D dmax D d TMR 2
Tissue Maximum Ratio Deinition: 100 cm Linac 80 cm Co60 central axis D TMR d d D dmax central axis d dmax D dmax D d TMR is dependent on: 1) Energy an increase in energy increases TMR 2) Field Size an increase in F.S. increases TMR (because o an increase in scatter) 3) Depth o Tissue an increase in depth decreases TMR 4) SAD (distance) independent o distance both reading at same distance rom the source D dmax. D d TMR 3
Relationship between BSF, PDD, TAR, TMR Relationship between BSF, PDD, TAR, TMR REL 0
Relationship between BSF, PDD, TAR, TMR SOURCE (SAD) IONIZATION CHAMBER BUILDUP CAP BSF REL 1
Relationship between BSF, PDD, TAR, TMR SOURCE (SAD) d 1 BSF TAR d1 REL 2
Relationship between BSF, PDD, TAR, TMR SOURCE (SAD) d 1 BSF TAR d1 TMR.s.1 REL 3
Relationship between BSF, PDD, TAR, TMR SOURCE (SAD) (ODI,SSD) d 1 dmax2 Note: d 1 BSF TAR d1 TMR.s.1 PD REL 4
Relationship between BSF, PDD, TAR, TMR (SAD) d 1 (ODI,SSD) dmax2 Note: d 1 BSF TAR TMR d1 PD.s.1 REL 6
Relationship between BSF, PDD, TAR, TMR (SAD) d 1 (ODI,SSD) dmax2 Note: d 1 BSF TAR TMR d1 PD.s.1 solve BSF equation or solve TAR d1 equation or BSF TAR d1 Both equations are equal to "Dose in ree space", thereore they are equal to each other. REL 7
Relationship between BSF, PDD, TAR, TMR (SAD) d 1 (ODI,SSD) dmax2 Note: d 1 BSF TAR TMR d1 PD.s.1 BSF rearranging terms: TAR d1 REL 8
Relationship between BSF, PDD, TAR, TMR (SAD) d 1 (ODI,SSD) dmax2 Note: d 1 BSF TAR TMR d1 PD.s.1 BSF rearranging terms: TAR d1 TAR d1 BSF TMR BUT: dmax1 thereore: TMR d1 TAR d1 BSF rearranging terms: TAR d1 TMR d1 BSF REL 9
Relationship between BSF, PDD, TAR, TMR (SAD) d 1 (ODI,SSD) dmax2 Note: d 1 BSF TAR TMR d1 PD.s.1 TAR d1 TMR d1 BSF One other important relationship is between TAR's and BSF. TAR d1 I depth d1 is equal to dmax1 then: TAR d1 BUT: Thereore: At depth Dmax TARs are equal to BSFs REL 10
Conversion between PDD & TAR (& TMR) Conversion between PDD & TAR (also TMR) TMR 0
Conversion between PDD & TAR (& TMR) SOURCE (SAD) (ODI,SSD) d 1 dmax2 Note: d 1 D.s.2 TMR 1
Conversion between PDD & TAR (& TMR) (SAD) d 1 (ODI,SSD) dmax2 Note: d 1 D.s.2 TMR 2
Conversion between PDD & TAR (& TMR) (SAD) d 1 (ODI,SSD) dmax2 Note: d 1 D.s.2 BSF 1 note: This is or ield size at distance TMR 3
Conversion between PDD & TAR (& TMR) (SAD) d 1 (ODI,SSD) dmax2 Note: d 1 D.s.2 BSF 1 TAR D 2.s.1 D.s.2 note: This is or depth d2. This is or the ield size at a distance equal to + d2. Which can be said to be an SAD equal to + d2. TMR 4
Conversion between PDD & TAR (& TMR) (SAD) d 1 (ODI,SSD) dmax2 Note: d 1 D.s.2 BSF 1 TAR D 2.s.1 PDD D 2.s.2 TMR 5
Conversion between PDD & TAR (& TMR) (SAD) d 1 (ODI,SSD) dmax2 Note: d 1 D.s.2 Solving this equation or BSF 1 TAR D 2.s.1 PDD D 2.s.2 I 1 I 2 2 d1 2 TAR 2 Substitute this into PDD 2 equation or. D.s.2 PDD 2 TAR 2 D.s.2 At this point need to realize that D max1 and D max2 are related by the "Inverse Square Law" Thus: TMR 6
Conversion between PDD & TAR (& TMR) (SAD) d 1 (ODI,SSD) dmax2 Note: d 1 D.s.2 BSF 1 TAR D 2.s.1 PDD D 2.s.2 I 1 I 2 2 d1 2 PDD 2 TAR 2 D.s.2 2 TMR 7
Conversion between PDD & TAR (& TMR) (SAD) d 1 (ODI,SSD) dmax2 Note: d 1 D.s.2 BSF 1 TAR D 2.s.1 PDD D 2.s.2 I 1 I 2 2 d1 2 ( + dm) 2 2 PDD 2 TAR 2 D.s.2 TMR 8
Conversion between PDD & TAR (& TMR) (SAD) d 1 (ODI,SSD) dmax2 Note: d 1 D.s.2 BSF 1 TAR D 2.s.1 PDD D 2.s.2 I 1 I 2 2 d1 2 PDD 2 TAR 2 D.s.2 ( + dm) 2 solving this equation or: 2 2 ( + dm) 2 Substitute this into PDD 2 equation. PDD 2 TAR 2 D.s.2 TMR 9
Conversion between PDD & TAR (& TMR) (SAD) d 1 (ODI,SSD) dmax2 Note: d 1 D.s.2 BSF 1 TAR D 2.s.1 PDD D 2.s.2 I 1 I 2 2 d1 2 PDD 2 TAR 2 D.s.2 ( + dm) 2 2 2 ( + dm) 2 TAR 2 D.s.2 ( + dm) 2 PDD 2 2 TMR 10
Conversion between PDD & TAR (& TMR) (SAD) d 1 (ODI,SSD) dmax2 Note: d 1 D.s.2 BSF 1 TAR D 2.s.1 PDD D 2.s.2 TAR 2 D.s.2 ( + dm) 2 PDD 2 2 BUT: BSF 1 Solving or: D max1 BSF 1 TMR 11
Conversion between PDD & TAR (& TMR) (SAD) d 1 (ODI,SSD) dmax2 Note: d 1 D.s.2 BSF 1 TAR D 2.s.1 PDD D 2.s.2 TAR 2 D.s.2 ( + dm) 2 PDD 2 2 BUT: BSF 1 Solving or: D max1 TAR 2 D.s.2 ( + dm) 2 PDD 2 BSF 2 1 BSF 1 Substituting or: D max1 TMR 12
Conversion between PDD & TAR (& TMR) (SAD) d 1 (ODI,SSD) dmax2 Note: d 1 D.s.2 BSF 1 TAR D 2.s.1 PDD D 2.s.2 TAR 2 D.s.2 ( + dm) 2 PDD 2 2 TAR 2 D ( + dm) 2.s.2 PDD 2 BSF 2 1 BUT the relationship between and D.s.2 is by the "Inverse Square Law". THUS: substituting into the equation invert I 1 D.s.2 I 2 I 2 2 d1 2 I 1 d 1 2 d2 2 2 ( + ) 2 substituting into the equation TMR 13
Conversion between PDD & TAR (& TMR) (SAD) d 1 (ODI,SSD) dmax2 Note: d 1 D.s.2 BSF 1 TAR D 2.s.1 PDD D 2.s.2 TAR 2 D.s.2 ( + dm) 2 PDD 2 2 TAR 2 PDD 2 BSF 1 2 ( + dm) 2 ( + ) 2 2 D.s.2 2 ( + ) 2 TMR 14
Conversion between PDD & TAR (& TMR) (SAD) d 1 (ODI,SSD) dmax2 Note: d 1 D.s.2 BSF 1 TAR D 2.s.1 PDD D 2.s.2 TAR 2 D.s.2 ( + dm) 2 PDD 2 2 TAR + 2 2 ( + dm) 2 PDD 2 BSF 2 1 ( + ) 2 TAR 2 + dm PDD 2 BSF 1 ( + ) 2 ( ) 2 END TMR 15