Reference Dosimetry for Megavoltage Therapy Beams: Electrons

Transcription

1 Reference Dosimetry for Megavoltage Therapy Beams: Electrons David Followill Ph.D Radiological Physics Center UT M.D.Anderson Cancer Center Houston TX

2 Protocol for Clinical Reference Dosimetry of High-Energy Photon and Electron Beams. Med. Phys. 26, p (1999) TG-51 Based on an Absorbed Dose to Water (in Water) Standard ( 60 Co): Conversion to Absorbed dose at other energies and modalities (electrons) is by correction factors, k ecal, k R50, and P gr (electrons). Based on Bragg Gray cavity theory. All of the chamber correction factors introduced in TG-21 are incorporated into K ecal, and k R50. Many chamber specific corrections are averaged into classes of chambers, so only a few k R50 values are needed.

3 Protocol for Clinical Reference Dosimetry of High-Energy Photon and Electron Beams. Med. Phys. 26, p (1999) TG-51 Requires Calibration in Water ( at least annually): Plastics may be used for monthly output checks, but must be referenced to the water calibration by a correction factor. Electrons Energy characterized by depth of 50% of max dose, d 50 Reference measurement depth, d ref, is related to d 50.

4 Ideal: Absorbed Dose Calibration Factor D = M N Q D,w Q N D,w Where: is the chamber Absorbed Dose Calibration Factor specific for the energy and modality of the beam being measured. (N K for Kilovolt X-rays) Pragmatic: Too expensive to be practical We understand the behavior of chambers in the megavoltage range. Cobalt 60 is still available, and very reproducible. Cobalt 60 is a near perfect beam to use as the reference energy for chamber calibration. D = M 60 N Co D,w other energy/modality specific corrections.

5 Chamber Calibration Factor Obtain from ADCL Chamber waterproofing material: 1 mm Acrylic (PMMA) wall Provided by ADCL Waterproof chambers

6 Calibrate P-P P Chamber ADCL s: TG-39, in water 60 Co Evidence of problems with k ecal User: (Recommended) TG-39 in water/plastic Cylindrical Chamber is NIST traceable TG-51 In water high energy electron R 50 near 7.5 cm compare with cylindrical chamber

7 Electron Equation k Q D Q w = M P Q gr k ' R50 k ecal N 60 Co D,w [Gy] P Q gr k ' R 50 = k R 50 (Eq 6) P Q gr = Gradient Correction k ' R 50 = non - gradient component of energy correction factor k ecal is conversion of N D,w from 60 Co to electrons

8 For Electrons k = P Q Q gr k R 50 (Eq 4) Q P gr = k = k ' R R50 50 ecal correction for gradient at the point of calibration (d ref ) k (Eq 5) P Q gr = ( ) ref + 0.5rref M ( d ) M d raw ref (Eq 21)

9 TG-51 Measurements Electrons: Look up k ecal for your chamber. Search for Imax and I50 (use 0.5 r cav shift) Determine R 50 Determine d ref and k R50 Move chamber to physical d ref (no shift) Measure P pol and P ion Move chamber to d ref r cav Calculate the gradient correction, P gr

10 k Electrons ecal k ecal dummy factor carries into Q N D,w N Co D, w Q D,w ( ecal = dose calib factor at reference electron energy d 50 = 7.5cm) Simplifies the calculations Allows for a specific electron calibration factor to be introduced later. Includes L/ρ and P repl for R 50 = 7.5 cm and L/ρ, P repl, P wall for Cobalt 60. N ecal

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12 TG-51 Measurements Electrons: Look up k ecal for your chamber. Search for Imax and I50 (use 0.5 r cav shift) Determine R 50 Determine d ref and k R50 Move chamber to physical d ref (no shift) Measure P pol and P ion Move chamber to d ref r cav Calculate the gradient correction, P gr

13 Electrons: Reference Conditions Field Size: assure full lateral scatter for R cm x 10 cm for R 50 < 8.5 cm 20 cm x 20 cm for R 50 > 8.5 cm Reference Depth, d ref d ref = 0.6R [cm] (Eq 18) Nominal SSD

14 Electrons Clinical % dd Measure % ionization Shift to effective point of measurement Convert ionization to dose -- using TG-25 (revised L/ρ) Use %dd to shift from d ref to d max, not %dion Parallel Plate and diode need no shift

15 Effective point of measurement Electron source x r cav Center of chamber Effective pt of measurement d eff = d meas -f (r cav ) f = 0.5 r cav for electrons f = 0 for parallel plate inner surface of front window

16 Effective Point of Measurement Water surface Effective depth Physical depth X X Cylindrical Parallel plate

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18 TG-51 Measurements Electrons: Look up k ecal for your chamber. Search for Imax and I50 (use 0.5 r cav shift) Determine R 50 Determine d ref and k R50 Move chamber to physical d ref (no shift) Measure P pol and P ion Move chamber to d ref r cav Calculate the gradient correction, P gr

19 Beam Quality Specification for Electron Beams Specified by R 50 R 50 = depth (cm) at which dose falls to 50% of max for a 10 cm x 10 cm field. (20cm x 20cm for R 50 > 8.5cm) R 50 R 50 = I (cm) [I 50 < 10cm] = I (cm) [I 50 >10cm] (Eq 16) (Eq 17) {I 50 = depth of 50% ionization}

20 TG-51 Measurements Electrons: Look up k ecal for your chamber. Search for Imax and I50 (use 0.5 r cav shift) Determine R 50 Determine d ref and k R50 Move chamber to physical d ref (no shift) Measure P pol and P ion Move chamber to d ref r cav Calculate the gradient correction, P gr

21 Reference depth: dref d ref = 0.6R [cm]

22 Energy Dependent Factor - chamber specific (Includes ratio of L/ρ P rep P cel for arbitrary electron energy to that for R 50 = 7.5 cm) ' well behaved (observed ) k R50 ' k R50 ' k R50 (cyl) = e -( R50/3.67) (Eq 19) (cyl cham for 2cm < R 50 < 9cm), 0.2% error for Farmer chambers. Fig 5 & 7 (cyl) Fig 6 & 8 (pp)

23 For Electrons For Electrons For Electrons 60 Cobalt evaluated at cel wall repl water air 7.5 energy R evaluated at cel repl water air ecal P P P L P P L k 50 ρ ρ = = energy R at evaluated cel repl water air 50 energy R the at evaluated cel repl water air 50 R P P L P P L k' = ρ ρ =

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25 TG-51 Measurements Electrons: Look up k ecal for your chamber. Search for Imax and I50 (use 0.5 r cav shift) Determine R 50 Determine d ref and k R50 Move chamber to physical d ref (no shift) Measure P pol and P ion Move chamber to d ref r cav Calculate the gradient correction, P gr

26 Phantoms Water only Annual calibration Plastics: Weekly/Monthly Compare with water at annual calibration.

27 Chamber Protection Waterproof Chambers no protective sleeve needed Other Chambers 1 mm thick Acrylic protective sleeve

28 Chambers: parallel plate vs cylindrical Electrons: Parallel Plate Chambers: RECOMMENDED Cylindrical Chambers: ACCEPTABLE P-P Chamber required for R cm

29 Chamber Position Cylindrical Chamber (e - ) Clinical depth dose data: effective pt of measurement Beam quality specification: effective pt of measurement Calibration: Physical center of chamber gradient correction included in P gr

30 M = P ion P TP P elec P pol M raw [C or rdg] (Eq 8) P ion = Collection efficiency correction P TP = Temp-Press correction P elec = Electrometer factor P pol = Polarity Correction M raw = uncorrected charge reading

31 TG-51 Measurements Electrons: Look up k ecal for your chamber. Search for Imax and I50 (use 0.5 r cav shift) Determine R 50 Determine d ref and k R50 Move chamber to physical d ref (no shift) Measure P pol and P ion Move chamber to d ref r cav Calculate the gradient correction, P gr

32 P Pulsed ion Beam V H = normal operating potential of chamber V L = reduced potential on chamber - H M raw L M raw P ion ( V ) H = VH 1.00 VL H M raw V L M raw V = raw reading with full potential = raw reading with reduced potential H L V L < V H /2 (Eq 11)

33 1.005 Response (Norm to 1st full bias) d Monitor Δ NEL, NEL' X PTW, PTW' Cap-G, Cap-C 6 dm Sequence #

34 P ion using the equations in TG-51 For V H /V L = 2 continuous radiation pulsed radiation P io n M H /M L

35 Polarity Correction P pol M + = raw raw (Eq 9) 2M - M raw M + (M - ) is the charge collected with positive (negative) polarity on the collector M raw = charge collected with normal polarity

36 TG-51 Measurements Electrons: Look up k ecal for your chamber. Search for Imax and I50 (use 0.5 r cav shift) Determine R 50 Determine d ref and k R50 Move chamber to physical d ref (no shift) Measure P pol and P ion Move chamber to d ref r cav Calculate the gradient correction, P gr

37 For Electrons Q P gr = gradient correction P Q gr ( ref + 0.5rcav ) M ( d ) M d = (Eq 21) raw ref Q P gr = M (dref + 0.5rcav) M raw(dref)

38 Whole equation with %dd% D Q Q ' Co = Mraw PTP Pelec Pion Ppol Pgr kr50 k ecal N 60 w D,w (1/pdd) [Gy]

39 Implementation

40 Current Implementation Status Current Implementation Status TOTAL 1494 of 1623 ACTIVE INSTITUTIONS (92%) Only 8% to go Nov-99 May-00 Nov-00 May-01 Nov-01 May-02 Nov-02 May-03 Nov-03 May-04 Nov-04 May-05 Nov-05 May-06 Nov-06 May-07 Nov-07 May-08 Nov-08 May-09

41 What s s the Holdup? New Air Kerma standard TG-51 TG-21 depending on the chamber Time and effort required (everyone is very busy) New equipment requirements (chambers and phantoms)

42 Equipment Needs Properly sized liquid water phantom (30x30x30 cm 3 ) Don t use the scanning tank Adequate scatter conditions Easy reproducible setup

43 Chamber Holder and Positioner Holder Versatile to hold different chambers Rigid (sensitive volume perpendicular to water surface) No lateral displacement with depth Accurate sub-millimeter placement at any depth Verify accuracy prior to initial use Remote electronic control is nice

44 Ion Chambers TG-51 ion chambers vs NEW ion chambers Most are similar in design but now waterproof 1. Wall material 2. Radius of air cavity 3. Presence of Al electrode 4. Wall thickness AAPM working group to determine the k R50, k ecal for new chambers

45 Ion Chambers - Electrons Parallel-plate or cylindrical chambers okay Cylindrical for energies > 6 MeV per protocol (R cm) Cylindrical = parallel plate if care in placement Always use a parallel plate chamber for 4 MeV beams Caution as to where the inside surface of the front window is located

46 Ion Chambers - Electrons All chambers must have an ADCL calibration coefficient EXCEPT PARALLEL PLATE CHAMBERS AAPM recommendation is to cross calibrate parallel plate chamber with cylindrical chamber in a high energy electron beam (worksheet C a la TG-39) ADCL N D,w good TG-51 k ecal bad Use of (N D,w k ecal ) results in an error of 1-2% ONE EXCEPTION Exradin P11 seems to be okay AAPM working group determining new k ecal values

47 Measurement Techniques Accurate placement of cylindrical ion chamber at depth Whether manual or electronic motor driven there must be a starting reference point 1. Surface method Two techniques Air Correct Position Water

48 Measurement Techniques Cut ruler down 2. Cowboy method to minimize surface area U-shape Water plastic surface attached flush with end of ruler Accuracy depends on cutting ruler Used for reference starting point Periodic check of depth Ion chamber Cut ruler by the chamber radius and wall thickness weights

49 Measurement Techniques Parallel plate ion chambers 1. Flat surface makes it easy to measure depth 2. Accurate ruler needed 3. Must know where the inside surface of the front window is located

50 Beam Quality Conversion Factors Electrons k R 50 Only small figures, no tables Good figures at: apers/tg51_figures.pdf

51 Beam Quality Conversion Factors Electrons 4 MeV beams (R 50 < 2.0 cm) Only use parallel plate chamber Need to extrapolate curve Equation good down to 1 cm 1

52 Charge Measurements M = P ion P TP P elec P pol M raw P TP correction factor Mercury thermometers and barometers most accurate (but they are no longer kosher) Hg barometers T&G corrections needed Quality aneroid or digital can be used Check annually against a standard Digital purchased with a calibration does not mean accurate but rather what it read at certain pressures or temperatures

53 Charge Measurements P elec correction factor ADCL calibration for each scale needed P pol correction factor Change polarity requires irradiation (600 to 800 cgy) to re-equilibrate chamber Use of eq 9 in TG-51 requires that you preserve the sign of the reading or P pol = M + raw 2 raw raw P pol should be near unity for cylindrical chambers and slightly larger correction for parallel plate chambers + M M

54 Charge Measurements A-12' P ion correction Δ NEL'' factor Response norm to 1st rdg (-300V) for a beam High dose rate capabilities result in higher P ion Change in bias requires irradiation ( cgy) to reequilibrate chamber. PTW'' Monitor X PTW'' 6 d10 A-12 NEL'' 18 x 6 d10 Cl 2100CD (210 5) data (75 mu) dated 19-Feb-00 Monitor's drift due to Ktp & machine fluctuation (All other chamber data are norm to monitor) d5 Sequence # 6 dm 20 dm

55 Charge Measurements P ion correction factor Use eqs. 11 and 12 to calculate P ion As a check if using V H /V L = 2 (within 0.1%) Pulsed beam : P ion = M H /M L if M H /M L < 1.02 Continuous beam : P ion = {(M H /M L 1)/3}+1 P ion depends on chamber, beam energy, linac and beam modality Tends to increase with energy

56 Charge Measurements Electron beam gradient (P gr ) correction factor Only for cylindrical ion chambers Ratio of readings at two depths ( d 0.5r ) ( d ) The reading at d ref +0.5r cav should have the same precision as the reading at d ref since: Dose = M(d ref ) (many factors) M(d ref +0.5r cav ) P gr M ref + = M raw ref cav M(d ref )

57 Charge Measurements Electron beam gradient (P gr ) correction factor E < 12 MeV; P gr >1.000 E 12 MeV; P gr Why? Because for low electron energies d ref = d max and this places the eff. pt. of measurement in the buildup region thus a ratio of readings greater than At higher electron energies d ref is greater than d max and as such the eff. Pt. of measurement is on the descending portion of the depth dose curve thus a ratio of readings less than

58 Charge Measurements Physical depth Percent Dose x x 6 MeV Dose Depth (cm) Effective depth ( d 0.5r ) M ref + M raw ( d ) ref cav

59 Charge Measurements Physical depth 80 Percent Dose Effective depth Depth (cm) x d ref x d ref+0.5rcav ( d 0.5r ) M ref + M raw ( d ) ref cav

60 Clinical Depth Dose For electrons depth dose correction for 16 MeV is significant (~98.5% - 16 MeV and ~95.5% - 20 MeV) Caution!!! Super big problem if you use % depth ionization data (3-5% error for high energy electron beams)

61 Summary Implementation is straightforward 1. Must read the protocol and follow the prescriptive steps 2. Many suggestions to clarify confusion have been made 3. RPC will assist you and answer questions Differences between TG-51 and other protocols such as TG-21 and TRS 398 are minimal.