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11.1 - Isodose Chart Example: Co-60 beam normalized at dmax, used for normalized at a fixed depth, SSD setup used for SAD setup 2
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11.1 - Isodose Chart Example: a linac beam “horn” (usually at shallow depth) The flattening filter is designed to produce a ‘flat’ dose distribution at a selected depth, typically, at 10 cm. As a result, the dose distributions at shallow depths are over-compensated, exhibiting the ‘horn’ effect. 3
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‘horn’ diminishes with depth‘horn’ diminishes with depth, because (1) greater scatter on the central axiswith depth, and (2) more penetrating, i.e., harder beam, on the central axisdue to the larger thickness of the flattening filter in the center than off-center 4
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11.1 - Isodose Chart Beam profile 90-20% penumbra widthField width defined Scatter in theat 50% dose level medium and head/ collimator leakage For Co-60, the penumbra is due to the ‘geometric penumbra’ (1-2cm) and the scatter in the medium. For linac, the ‘geometric penumbra’ is small (~2mm), the penumbra is mostly due to the scatter in the medium. 5
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11.1 - Isodose Chart Isodose curves on a plane perpendicular to the beam central axis. (The field edge is typically 5mm outside the 90% lines. In the penumbra region, the dose gradient is approx. 8-10% / mm.) 6
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11.2 – Measurement of Isodose Curves A B Dose measurement in a water phantom: B. Moving probe (central axis depth dose, beam profile) C. Fixed reference probe (position fixed, located in field) D. Relative dose = A/B 7
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11.3 – Parameters of Isodose Curves (Beam quality) 200 kVp, bulging Co-60, penumbra penumbra due to primarily due to greater scatter of source sizelow energy photons. (geometric penumbra) 4 MV, small 10 MV, penumbra penumbra, due to greater than that of 4small photon scatter MV, due to greater and short electron electron range range 8
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11.3 – Parameters of Isodose Curves(source size, source-to-surface distance,source-to-diaphragm distance- the penumbra effect) s geometric penumbra: SDD s ( SSD + d − SDD) SSDPd = SDD To reduce the geometric d penumbra, trimmers can be used to increase SDD. Pd 9
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11.3 – Parameters of Isodose Curves (collimation and flattening filter) primaryBeam profiles are measured at several depths. collimatoAt shallow depths, the beam profile is lower on- raxis and higher off-axis (so-called ‘horns’).At depth about 10 cm, the beam profile is nearly secondaryflat (due to the design of the flattening filter). collimato rAt larger depths, the beam profile sags off-axis.Why?1. Central axis beam is ‘harder’ than off-axisbeam, hence more penetrating.2. More scatter contribution on-axis then off-axis. horn(note: inverse square is not a factor.)The ‘flatness’ of beam profile is measured within d ~ 10 cm80% of the field size, or up to 1-cm away fromthe field edge, to stay away from the penumbra. large depth 10
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11.3 – Parameters of Isodose Curves(field size)Field size is determined based on dosimetric coverage, not geometriccoverage.For small field (< 6-cm), a large portion of the field is in the penumbraregion, hence beam profile tends to be bell-shaped.For Co-60 beams, no flattening filter is used. Consequently, for large field orelongated field, the beam profile is higher on central axis (due to largerscattered dose), and lower off-axis (due to reduced scattered dose andoblique incidence). 11
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11.4 – Wedge Filters (wedge angle) Normalized to dmax without wedge Normalized to dmax with wedge (open field)d = 10 cm wedge wedge angle angle Physical wedge: use metal, such as steel and Pb. 12 Dynamic wedge: independently move one collimator while the beam is on.
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11.4 – Wedge Filters (wedge factor) wedge factor : DwWF (d , W × H ) = Do Dw Do Measured at some depth beyond the dmax, usually 10 cm. Old Co-60 isodose curves were normalized to the dmax without the wedge. the isodose curves already included the wedge factor. With advent of TPS, we normally input the isodose curves normalized to the dmax with the wedge, and introduce a wedge factor to account for the transmission factor of the wedge. Wedge factor depends on the depth and field size. 13
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11.4 – Wedge Filters (wedge systems) WF WF (universal wedge) (individualized wedge) Center of wedge fixed at beam Thin edge of wedge aligns with the axis. Wedge factor varies weakly field edge. Wedge factor varies with field size. strongly with field size, (greater wedge factor with smaller field size), resulting in more efficient use of beam-on-time 14
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11.4 – Wedge Filters (effect on beam quality)For Co-60, photons are nearly mono-energetic, wedge filter does not havemuch effect on beam quality.For linac-produced photon beams, thewedge filter preferentially attenuates low-energy photons. As a result, the wedged- dosebeam is ‘harder’ than open field beam,that is, more penetrating. wedgeThus, the wedge factor, which is the ratio openof wedge-field-dose to open-field-dose,increases with the depth. depth Backscatter factors are assumed to be not affected by the wedge filter. 15
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2 11.4 – Wedge Filters (design of wedge filters)1. Draw a horizontal line at the reference depth (e.g. 10 cm)2. Construct fanlines (e.g. 1-cm 4 1 intervals)3. Construct parallel wedged isodose 3 lines, use same values as open field4. At each intersection point, find dose values for non-wedge & wedge fields5. Find ratio of wedge/open dose values6. Normalize max ratio to 1007. Get thickness by attenuation coeff. A B C E G I K M O Q S T U Nonwedge isodose 40 55 62 65 67 68 68 68 67 65 62 55 40wedge isodose 35 39 41 47 53 60 68 76 86 95 105 110 1155 ratio .875 .710 .660 .720 .790 .880 1.00 1.12 1.28 1.46 1.70 1.20 2.886 transmission - - .387 .425 .462 .515 .590 .660 .750 .860 1.00 - -7 mm Pb - - 15.2 13.6 12.2 10.5 8.3 6.5 4.5 2.3 0.0 - - penumbra region 16
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11.5 – Combination of Radiation FieldsSingle field treatment can be used, if:2. Dose distribution inside tumor reasonably uniform (e.g. ±5%) – tumor volume cannot be too big3. Maximum dose to normal tissue cannot be too large (e.g.110%) – tumor must be relatively shallow4. Dose to normal tissue within tolerance – no critical organs in the beam 17
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11.5 – Combination of Radiation Fields (parallel opposed beams) SSD setup SAD setup Each beam delivers 100 to isocenter Each beam delivers 100 to dmax 18
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11.5 – Combination of Radiation Fields(patient thickness vs dose uniformity) The larger the patient thickness, the greater the energy is needed to produce more uniform dose in the tumor and less dose in normal tissues. 19
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11.5 – Combination of Radiation FieldsEdge effect, lateral tissue damage ?When multiple fields are used, each field should be treated each day!This will minimize damage to the normal tissues outside the tumor region.(this result is based on cell-survival calculation. For details, see Johns &Cunningham, chapter 17)Integral dose ( = mass × dose ) (Σ = 1.44 D0 Ad1 2 1 − e 1 + ) 2.88d1 2 − 0.693d / d1 2 SSD Σ = integral dose, D 0 = peak dose along central axisA = field area, d = patient thicknessd1 2 = half - value depth (depth of 50% dose)SSD = source - surface distance 20
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11.5 – Combination of Radiation Fields (multiple fields) Increase the ratio of tumor dose to normal tissue dose 21
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11.6 – Isocentric Techniques (rotation therapy) 4 MV Field Size = 7x12 cm Past pointing 100° arc 180° arc Diso = Dref × T Diso = D0 × S c × S p × TMR 360° full rotation 23
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11.6 – Isocentric Techniques (example)Rotation therapy, 4 MV, field size 6 × 10-cm, SAD = 100 cm.Given: TMR = 0.746, Sc(6x10)=0.98, Sp(6x10)=0.99, dose rate = 200 MU/min To deliver a prescription dose of 250 cGy to the isocenter, what is the number of MUs ? Diso = D0 × S c × S p × TMR Diso = 200 × 0.98 × 0.99 × 0.746 = 144.8 cGy / min 250 cGy treatment time = = 1.73 min 144.8 cGy/min MU = 200 MU/min × 1.73 min = 345 MU 24
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11.7 – Wedge Field Techniques 180 160 140 90 90 80 70 Uniform120 80 dose 60 70 50 60 Orthogonal pair – open field Orthogonal pair – wedged field 25
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11.7 – Wedge Field Techniques (hinge angle and wedge angle) φ = hinge angle θ θ = wedge angle φ 90 90 θ = 90 − 2 80 80 φ 70 70In clinical reality, this relationshipisn’t true in general, due to surface 60 60curvature.Computer treatment plan is neededto find the optimal wedge angle 26
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11.7 – Wedge Field Techniques (open and wedge fields combinations) 27
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11.8 – Tumor Dose Specification for External Photon Beams (ICRU Report No.50 and No.62) Clinical tumor volume (CTV) Gross tumor volume GTV + subclinical (CTV-I) (GTV) palpable or imaged; multiple CTV’s possible gross demonstrable extent (CTV-II, CTV-III,…) and Additional volumes with location of the malignant presumed subclinical spread growth; consists of primary (e.g. Regional lymph nodes) tumor (GTV Primary) and Planning tumor volume (PTV) possibly metastatic CTV + margin lymphadenopathy (GTV (internal+setup); nodal) or other metastases includes CTV with an IM and (GTV M).Internal tumor volume (ITV) as a set-up margin (SM) forICRU 62 recommends that patient movement and set-upan internal margin (IM) be uncertainties. To delineateadded to CTV to account PTV, IM and SM are notfor internal physiological added linearly but aremovements and variation in combined subjectively. Treated volume: The volume enclosed by an isodosesize, shape, and position of surface which adequately covers the PTV.the CTV during therapy. Irradiated volume: The volume that receives a 28 significant dose (e.g., ≧ 50% of the target dose)
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11.8 – Tumor Dose Specification for External Photon Beams(ICRU Report No.50 and No.62) PRV OAR Planning Organ at Risk Volume (PRV) The organ at risk (OAR) needs adequate protection just as CTV needs adequate treatments. Once the OR is identified, margins need to be added to account for its movements, internal as well as set-up. 29
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11.8 – Tumor Dose Specification for External Photon Beams(ICRU Report No.50 and No.62) most frequently occurred dose 1 mean dose = ∑ Di n i∈target half of the target points with dose < median dose < half of the target points with dose maximum target dose minimum target dose Hot spot: an area outside the target that receives a higher dose than the prescribed target dose; clinically meaningful only if the area > 2 cm2 30
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11.8 – Tumor Dose Specification for External Photon Beams(specification of target dose) The ICRU Reference Point • The point should be selected so that the dose at this point is clinically relevant and representative of the dose throughout the PTV • The point should be easy to define in a clear and unambiguous way • The point should be selected where the dose can be accurately calculated • The point should not lie in the penumbra region or where there is a steep dose gradient 31
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11.8 – Tumor Dose Specification for External Photon Beams(specification of target dose) Stationary Photon Beams • For single beam, the TD should be specified in the central axis of the beam placed within the PTV • For parallel opposed, equally weighted beams, the point of TD specification should be on the central axis midway between the beam entrances • For parallel opposed, unequally weighted beams, the TD should be specified in the central axis placed within the PTV • For any other arrangement of two or more intersecting beams, the point of TD specification should be at the intersection of the central axes of the beams placed within the PTV 32
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11.8 – Tumor Dose Specification for External Photon Beams(specification of target dose) Rotation Therapy • For full rotation or arcs of at least 270 degrees, the TD should be specified at the center of the rotation in the principal plane. • For smaller arcs, the TD should be stated in the principal plane, first at the center of rotation and, second, at the center of the target volume. This dual point specification is required because in small arc therapy, ‘past pointing’ techniques are used that give maximum absorbed dose close to the center of the target area. 33
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11.8 – Tumor Dose Specification for External Photon Beams(specification of target dose) Additional Information • The specification of TD is only meaningful if sufficient information is provided regarding the irradiation technique. • Radiation quality, SSD or SAD, field sizes, beam modification devices, beam weighting, correction for inhomogeneities, dose fractionation and patient positioning should be included as well. 34