Treatment Planning I:
Isodose Distributions
References: Physics of Radiation Therapy, Chapter 11, ICRU Reports
Handout available at: http://www.tc.umn.edu/~alaei001/5173.htm
Introduction
• The central axis depth dose distribution is not sufficient to characterize a radiation beam that produces a 3D dose distribution
• Thus, in order to represent dose, isodose curves, lines passing through points of equal dose, are utilized
ISODOSE CHART
3
Isodose Chart
• An isodose chart for a beam consists of a family of isodose curves usually drawn at equal increments of absorbed dose, representing the variation of dose as a function of depth and transverse distance from the central axis
Isodose Chart
• The isodose curves:
– Are normalized at the point of maximum dose (SSD case, most common) – Are normalized at isocenter beyond dmax, corresponding to axis of rotation of unit (SAD case)
• Or:
– Are specified as absolute doses
Isodose Chart
s
SSD-Normalized to dmax
SAD-Normalized to dmax
SAD-Absolute dose
Isodose Chart
Fixed SSD or isocentric
SSD=100 cm
SAD=100 cm
Isodose Chart
General Properties
•
Dose greatest on the CA at any depth and decreases toward the edges of the beam with the exception of LINAC beams with horns at shallow depths
•
Horns created by flattening filter which is designed to overcompensate near the surface in order to obtain flat isodose curves at depth (10 cm)
Flattening Filter
Horns
Dmax
10 cm
10 cm Field Size
Beam profiles, LINAC vs. Cobalt 60
Isodose Chart
General Properties ‐ Penumbra
• Near the edge of the beam dose rate decreases rapidly as a function of lateral distance from the beam axis
• The falloff near the edge of the beam is caused by geometric penumbra and reduced side scatter
Isodose Chart
General Properties ‐ Penumbra
•
This falloff can be described using physical (total) penumbra:
• Physical penumbra is defined as lateral distance between two specified isodose curves at a specified depth (e.g., lateral distance between 90% and 20% isodose lines at the depth of Dmax).
Isodose Chart
General Properties ‐ Penumbra
•
•
Physical penumbra consists of:
–Geometric penumbra (due to finite source size)
–Transmission penumbra (due to transmission through jaws)
–Scatter penumbra (due to in‐phantom scatter)
And depends on:
–beam energy, source size, SSD, source to collimator distance and depth in phantom
Isodose Chart
General Properties ‐ Penumbra
Penumbra
Penumbra
Isodose Chart
General Properties
•
Outside the geometric limits of the beam and the penumbra, the dose variation is the result of side scatter from the field and both leakage and scatter from the collimator system
Beam limits
MEASUREMENT OF ISODOSE CURVES
16
Measurement of Isodose Curves
• Ionization chambers
– Most reliable because of their flat energy response and precision
• Solid state detectors (diodes)
– Small, ideal for electron measurements
• Newer detectors for small fields
– Micro ion chambers and edge detectors
• Film – Can be used but not the easiest, need characteristic curve to convert optical density to dose (outdated)
Measurement of Isodose Curves
• Two detector scanning:
– Scanning detector (A) ‐ to move in the tank of water to sample the dose rate
– Reference detector (B) ‐ fixed at some point in the field to monitor the beam intensity over time
– The final response A/B is independent of fluctuations in output
Measurement of Isodose Curves
Reference detector
Scanning detector
Measurement of Isodose Curves
Measurement of Isodose Curves
6 MV, Dmax depth
6 MV, 5 cm depth
Measurement of Isodose Curves
6 MV, 10 cm depth
6 MV, 20 cm depth
Measurement of Isodose Curves
Sources of Isodose Curves
• Collected by the physicist at the time of commissioning of the linac
• Provided by the manufacturer (golden data):
• Verify the central axis depth dose data corresponds with PDD data measured independently in a water phantom
• A deviation of 2% or less in local dose is acceptable up to depth of 20 cm
• For selected field size and depths, an agreement within 2 mm in the penumbra region is acceptable
PARAMETERS OF ISODOSE CURVES
25
Parameters of Isodose Curves
Beam Quality
• Central axis depth dose distribution depends on beam energy, thus the depth of a given isodose curve increases with beam quality
• Beam quality also affects the shape of isodose curves near the field borders and the physical penumbra
Parameters of Isodose Curves
Beam Quality
Higher beam energy  increase in depth of a given isodose curve
6 MV
10 MV
18 MV
25 MV
Parameters of Isodose Curves
Beam Quality
200 kVp, SSD=50 cm
Increased side-scatter
causes bulging of isodose
curves
28
60Co,
SSD=80 cm
Larger penumbra due
to side-scatter and
source size
4 MV, SSD=100 cm 10 MV, SSD=100 cm
Parameters of Isodose Curves
Source Size and SSD
• Source size and SSD affect the isodose curve shape by affecting the geometric penumbra
• SSD affects the percentage depth dose and depth of the isodose curves
Parameters of Isodose Curves
SSD
100 cm SSD
120 cm SSD
Parameters of Isodose Curves
Collimation and Flattening Filter
• Collimators, flattening filter and other absorbers affect the shape of isodose curves
• Flattening filter has the greatest effect:
– If no filter used, the result is conical field distribution
– Makes field uniform across whole width, with the same intensity
– Flattening filter thickest in the middle and tapers to sides
– Filter causes change in beam quality
– Different photon spectrum at edge than in middle, harder in middle
Parameters of Isodose Curves
Collimation and Flattening Filter
Effect of flattening filter:
“Not flattened” beam
“Flattened” beam
Parameters of Isodose Curves
Collimation and Flattening Filter
• Flatness: The variation of dose relative to the central axis over the central 80% of the field size at 10 cm depth in a plane perpendicular to the central axis. A dose variation of ±3% is considered acceptable
• Symmetry: The variation of the dose at any pair of points situated symmetrically with the central ray. It should not differ by more than 2% for any pair of points
Parameters of Isodose Curves
Flatness and Symmetry
Horns
Dmax
10 cm
Parameters of Isodose Curves
Field Size
• According to the ICRU, the geometric field size is defined as “the projection of the distal end of the machine collimator onto a plane perpendicular to the central axis of the radiation beam as seen from the front center of the source.”
• The dosimetric field size (also called the physical field size) is defined by the intercept of a given isodose surface (usually 50% but can also be up to 80%) with a plane perpendicular to the central axis of the radiation beam at a defined distance from the source.
35
Parameters of Isodose Curves Field Size
• One of the most important parameters in treatment planning
• Tumor coverage must be determined dosimetrically, not geometrically
• Small field sizes may result in large part of the beam be in penumbra region
Parameters of Isodose Curves Field Size
6 MV profiles at 5 cm depth, 2x2-40x40 cm2 field size
Parameters of Isodose Curves Field Size
5x5
10x10
20x20
WEDGE FILTERS
39
Wedge Filters
• The most commonly used beam modifying device
• A wedge‐shaped absorber made of a dense material, such as lead or steel, that causes a progressive decrease in the intensity across the beam, resulting in a tilt of the isodose curves from their normal position
Wedge Filters
• The isodose curves are tilted and the degree of tilt depends on the slope of the wedge filter
Heel
Toe
No wedge
60o wedge
Wedge Filters
No wedge
15 degree wedge
30 degree wedge
Wedge Filters
No wedge
45 degree wedge
60 degree wedge
Wedge Filters
• The wedge is usually mounted on transparent plastic tray (with the exception of Elekta universal wedge) and there is a limit on the field size a wedge can be used for, which varies by wedge angle
• It is ideal to have a minimum 15 cm distance between the wedge and the skin to preserve skin‐
sparing effect
Wedge Filters Wedge Angle
CA
• Wedge angle: the angle through which an isodose curve is titled at the central ray of a beam at a specified depth (typically 10 cm)
• Or, the angle between the isodose curve and the normal to the central axis
Wedge Angle
Wedge Filters Wedge Angle
• The angle of isodose tilt decreases with increasing depth in the phantom due to presence of scattered radiation
Wedge Filters
Wedge Systems
• Individualized wedge system
– A separate wedge for each beam width to minimize the loss of beam output
– Thin end of the wedge aligned with the border of the light field
– “Used to be” used in 60Co units
• “Universal”* wedge system
– A single wedge for all beam widths
– Fixed centrally in the beam
– Always used in LINACs
*Not to be confused with Elekta’s single 60o internal wedge
Wedge Filters Wedge Types
• Physical Wedges:
– Individual – Typically four wedge angles (15, 30, 45, 60 degrees)
– Universal – One 60 degree wedge, mix open and wedge fields to obtain desirable angle
30o steel wedge (Varian)
60o universal wedge (Elekta)
Wedge Filters Wedge Types
• Virtual Wedges ‐ Enhanced Dynamic Wedge (Varian), Omni Wedge (Elekta), Virtual Wedge (Siemens):
– No physical wedge in the beam, movement of one of the collimator jaws mimics the dose distribution of a wedge
– theoretically, any wedge angle is possible, in practice, a set number of wedge angle deliveries are allowed (e.g. for Varian LINACs 10, 15, 20, 25, 30, 45, 60 degrees)
Wedge Filters Wedge Factor
• The presence of a wedge filter decreases the output of the machine and hardens the beam
• This is accounted for by wedge factor: – The ratio of doses with and without the wedge, at a point in phantom along the central axis of the beam
– Measured at a suitable depth beyond dmax (5 to 10 cm)
• Wedge factor = Dose with wedge / Dose without wedge
Wedge Filters Wedge Factor
• Variation of wedge factors with:
– Depth
• Often negligible variation, due to beam hardening
– Field Size
• Larger for larger field sizes, variations of up to ~6% for physical wedges observed
• Smaller for larger field sizes, large variations by a factor of 2 observed, also dependent on independent jaws (for dynamic wedges)
Wedge Filters Effect on Beam Quality
• Physical wedges alter the beam quality by attenuating the lower‐energy photons (beam hardening), and by Compton scattering which results in energy degradation (beam softening)
• The beam hardening affects the depth dose distribution, thus wedged PDD should be measured
• This does not apply to dynamic wedges, the PDD for dynamic wedge is identical to that of open field
6 MV, 10 x 10 cm2
field
6 MV, 10 x 10 cm2
field with 60o wedge
COMBINATION OF RADIATION FIELDS (TWO FIELD, MULTIPLE FIELD, WEDGE FILED)
54
Combination of Radiation Fields
Single Field Technique
• Treatment by a single photon beam is seldom used except when tumor is superficial, there are no normal critical structures in the beam, a reasonable uniform dose distribution can be achieved, and there is no excessive maximum dose (hot spot)
– Examples: Spinal cord and supraclavicular treatments
– Skin lesions usually treated using electrons
Combination of Radiation Fields
Single Field Technique
Combination of Radiation Fields
Single Field Technique
Fixed SSD vs. Isocentric • The isocenter is the point of intersection of the collimator, couch, and gantry axis of rotation (100 cm from the source)
Isocenter
Fixed SSD vs. Isocentric • Fixed SSD technique
– Set SSD = 100 cm for each field
– Commonly used for single field treatments
– Used for extended SSD (>100 cm) treatments
– Also used for all electron treatments
Fixed SSD vs. Isocentric • Isocentric technique:
– Placing the isocenter at a depth with the patient and directing the beams from different directions
– SSD = SAD‐d
– No need to shift patient between fields
Combination of Radiation Fields
Parallel Opposed Fields
• Pair of beams directed along the same axis from opposite sides of treatment volume
• Advantages:
– Simple and reproducible
– Less chance of geometric miss
– Fairly homogenous dose through the tumor
• Disadvantages:
– Excessive dose to normal tissue between the two beams, often more than the tumor dose
Combination of Radiation Fields
Parallel Opposed Fields
Isocentric
SSD=100
Combination of Radiation Fields
Parallel Opposed Fields
Combination of Radiation Fields
Parallel Opposed Fields
Combination of Radiation Fields
Tissue Lateral Effect
• Equally weighted parallel opposed beams usually result in uniform dose distribution within irradiated volume, however, the uniformity of dose distribution depends on:
– Patient Thickness
– Beam Energy
– Beam Flatness Combination of Radiation Fields
Tissue Lateral Effect
• As the patient thickness increases or the beam energy decreases:
– The central axis maximum dose near the surface increases relative to the midpoint dose
• This is called tissue lateral effect
Combination of Radiation Fields
Tissue Lateral Effect
Depth dose curves for parallel opposed
fields normalized to midpoint value
Combination of Radiation Fields
Tissue Lateral Effect
The ratio of maximum peripheral dose to
midpoint dose as a function of patient thickness
Combination of Radiation Fields
Edge Effect
• For parallel opposed beams, treating with one field per day produces greater biological damage to normal subcutaneous tissue than treating with two fields per day
• Alternating days is not common practice Combination of Radiation Fields
Integral Dose
• A measure of the total energy absorbed in the treated volume
• For a uniform dose, the integral dose is the product of mass and dose
• In practice, the absorbed dose in tissue is non‐
uniform, so the solution is complex
• It is not commonly use clinically for treatment planning
Combination of Radiation Fields
Multiple Fields
• Goals of treatment planning:
– Maximum dose to the tumor with minimum dose to the surrounding tissues
– Uniform dose within the tumor
– Sparing of critical organs
Combination of Radiation Fields
Multiple Fields
• Strategies used to achieve the above:
– Using fields of appropriate size
– Increasing the number of fields or portals
– Selecting appropriate beam directions
– Adjusting beam weights
– Using appropriate beam energy
– Using beam modifiers (wedges, compensators)
– Employ electronic compensation or IMRT
Combination of Radiation Fields
Multiple Fields
Four field prostate
Combination of Radiation Fields
Multiple Fields
Two field tangential breast with wedge
Combination of Radiation Fields
Multiple Fields
Five field prostate
Combination of Radiation Fields
Multiple Fields
Three field non-coplanar brain
Combination of Radiation Fields
Multiple Fields
Three field non-coplanar brain
Combination of Radiation Fields
Multiple Fields
Three field non-coplanar brain
Wedge Field (Wedge Pair) Techniques
• Relatively superficial tumors can be irradiated by two wedged beams directed from the same side of the patient
• By inserting appropriate wedge filters in the beams and positioning them with the thick ends adjacent to each other, the distribution can be made fairly uniform
Wedge Field (Wedge Pair) Techniques
• The relationship between the wedge angle and the hinge angle (angle between the beams) to provide most uniform dose distribution is:
–  = 90º‐ Φ/2
–  = the wedge angle
– Φ= the hinge angle
– S = the separation
Wedge Field (Wedge Pair) Techniques
Rotation Therapy
• The beam moves continuously about the patient
• May be used for small, deep‐seated tumors
• Factors affecting dose distribution in rotational treatment include external patient contour, location of isocenter, energy, penumbra, field width
• The beam is pointed beyond the tumor thus the term “past pointing”
Rotation Therapy
Rotation Therapy
• Classic rotation therapy not utilized any longer
BUT
• Current Intensity‐Modulated Radiation Therapy techniques make use of arcs in combination with moving MLCs (chapter 20)
TUMOR DOSE SPECIFICATIONS
86
Rational for universal tumor dose specification
• The results of treatments can be meaningfully interpreted only if sufficient information is provided regarding the irradiation technique and the distribution of dose in space and time. In the absence of this information, recording of the “tumor dose” serves little purpose.
Tumor Dose Specification for External Photon Beams – ICRU recommendations
• The International Commission of Radiation Units and Measurements (ICRU) recognized the need for a universal dose‐specification system.
• ICRU Reports No. 50, 62, and 83 define and describe several target and critical structure volumes to aid in the treatment planning process and to provide a basis for comparison of treatment outcomes.
• They also define dose specification reference points.
Tumor Dose Specification for External Photon Beams – ICRU recommendations
• ICRU‐29: Dose specification for reporting external beam therapy with photon and electron beams (1978)
• ICRU 50: Prescribing, Recording, and Reporting Photon Beam Therapy (1993)
• ICRU 62: Prescribing, Recording and Reporting Photon Beam Therapy (1999)‐suppl. to report 50
• ICRU 83: Prescribing, Recording, and Reporting Intensity‐Modulated Photon‐Beam Therapy (IMRT) (2010)
89
Tumor Dose Specification for External Photon Beams – ICRU recommendations
• Prescribing, Recording, and Reporting Photon Beam Therapy (ICRU Report 50, 1993)
– This Report seeks to promote the use of a
common language for specifying and reporting the doses in radiation therapy, as well as the volumes in which they are prescribed. Report 50 completely supersedes and updates ICRU Report 29 (published in 1978).
Tumor Dose Specification for External Photon Beams – ICRU recommendations
• Prescribing, Recording, and Reporting Photon Beam Therapy (ICRU Report 62, 1999) (Supplement to ICRU Report 50)
– ICRU Report 62 provides recommendations on the volumes and absorbed doses that are important in prescribing, recording and reporting photon beam therapy, utilizing the new and improved irradiation techniques that have become available since the publication of ICRU Report 50 in 1993.
Tumor Dose Specification for External Photon Beams – ICRU recommendations
• Prescribing, Recording, and Reporting Intensity‐
Modulated Photon‐Beam Therapy (IMRT) (ICRU Report 83, 2010)
– In comparison with three‐dimensional conformal radiation therapy (3D‐CRT) it is now possible with IMRT to escalate the absorbed dose in the target volume for the same normal tissue dose and/or to reduce the normal tissue dose for the same tumor dose, resulting in improved tumor control and/or less normal tissue complications. IMRT is accomplished by the sequential isocentric delivery of multiple small beams typically of non‐uniform intensity that can lead to the generation of very steep dose gradients.
Tumor Dose Specification for External Photon Beams – ICRU recommendations
• Prescribing, Recording, and Reporting Intensity‐
Modulated Photon‐Beam Therapy (IMRT) (ICRU Report 83, 2010)
– ICRU Report 83 provides the information necessary to standardize techniques and procedures and to harmonize the prescribing, recording, and reporting of IMRT where possible with those of other modalities.
ICRU Nomenclature
•
•
•
•
•
•
•
•
•
•
•
94
GTV
CTV
IM
ITV
SM
PTV
OR
PRV
TV
IR
RVR
Gross Tumor Volume
Clinical Target Volume
Internal Margin
Internal Target Volume
Set up Margin
Planning Target Volume
Organ at Risk
Planning organ at Risk Volume
Treated Volume
Irradiated Volume
Remaining Volume at Risk
Tumor Dose Specification for External Photon Beams – ICRU recommendations
• Gross Tumor Volume (ICRU 50)
Gross Tumor Volume (GTV) is the gross palpable or visible/demonstrable extent and location of malignant growth. The GTV may consist of primary tumor, metastatic lymphadenopathy, or other metastases. Tumor Dose Specification for External Photon Beams – ICRU recommendations
• Gross Tumor Volume (ICRU 50)‐Cont’d
The GTV corresponds usually to those parts of the malignant growth where the tumor cell density is largest. No GTV can be defined if the tumor has been removed, e.g., by previous surgery.
Tumor Dose Specification for External Photon Beams – ICRU recommendations
• Clinical Target Volume (ICRU 50)
Clinical target volume (CTV) is a tissue volume that contains a demonstrable GTV and/or sub‐clinical microscopic malignant disease, which has to be eliminated. This volume thus has to be treated adequately in order to achieve the aim of therapy, cure or palliation. Tumor Dose Specification for External Photon Beams – ICRU recommendations
• Internal Margin (ICRU 62)
Internal margin (IM) is the margin added to the CTV to compensate for expected physiologic movements and variations in size, shape, and position of the CTV during therapy in relation to an Internal Reference Point and its corresponding coordinate system.
Tumor Dose Specification for External Photon Beams – ICRU recommendations
• Internal Margin (ICRU 62)‐Cont’d.
Internal Margin, commonly asymmetric around the CTV, is intended to compensate for all movements and all variations in site, size, and shape of the organs and tissues contained in or adjacent to the CTV. They may results, e.g., from respiration, different fillings of the bladder and rectum, swallowing, heart beat, movements of the bowel.
Tumor Dose Specification for External Photon Beams – ICRU recommendations
• Internal Margin (ICRU 62)‐Cont’d.
These internal variations are physiological and result in changes in site, size, and shape of the CTV and cannot be easily controlled. They do not depend on external uncertainties in beam geometry, but could depend on patient day‐to‐day set‐up. Tumor Dose Specification for External Photon Beams – ICRU recommendations
• Internal Target Volume (ICRU 62)
Internal Target Volume (ITV) represents the volume encompassing CTV and the Internal Margin.
Tumor Dose Specification for External Photon Beams – ICRU recommendations
• Set‐up Margin (ICRU 62)
Set‐up Margin (SM) accounts for uncertainties in patient positioning and alignment of the beams during treatment planning and delivery.
Tumor Dose Specification for External Photon Beams – ICRU recommendations
• Planning Target Volume (ICRU 50)
Planning Target Volume (PTV) is a geometrical concept, and it is defined to select appropriate beam arrangements, taking into consideration the net effect of all possible geometrical variations, in order to ensure that the prescribed dose is actually absorbed in the CTV.
Tumor Dose Specification for External Photon Beams – ICRU recommendations
• Organs at Risk (ICRU 50)
Organs at Risk (OAR) are normal tissues whose radiation sensitivity may significantly influence treatment planning and/or prescribed dose. As is the case when defining the Planning Target Volume, any possible movement of the organ at risk during treatment, as well as uncertainties in the set up during the whole treatment course, must be considered.
Tumor Dose Specification for External Photon Beams – ICRU recommendations
• Planning Organ at Risk Volume (ICRU 62)
Planning Organ at Risk Volume (PRV) is the volume containing a specific OR and a safety margin around it to account for its anatomical and geometrical variability.
Tumor Dose Specification for External Photon Beams – ICRU recommendations
• Treated Volume (ICRU 50)
Treated Volume is the volume enclosed by an isodose surface, selected and specified by the radiation oncologist as being appropriate to achieve the purpose of treatment (e.g., tumor eradication, palliation).
Tumor Dose Specification for External Photon Beams – ICRU recommendations
• Irradiated Volume (ICRU 50)
Irradiated Volume is that tissue volume which receives a dose that is considered significant in relation to normal tissue tolerance. The Irradiated Volume depends on the treatment technique used.
Tumor Dose Specification for External Photon Beams – ICRU recommendations
• Remaining Volume at Risk (ICRU 83)
Remaining Volume at Risk (RVR) is the difference between the volume enclosed by the external contour of the patient and that of the CTV and OAR’s on the slices that have been imaged. This is a measure of all the normal tissues that could potentially receive significant radiation.
Tumor Dose Specification for External Photon Beams – ICRU recommendations
• Conformity Index (ICRU 50)
Conformity Index (CI)=Treated Volume /Planning Target Volume
CIiso=Treated Volume covered by % Isodose/Planning Target Volume
Tumor Dose Specification for External Photon Beams – ICRU recommendations
From ICRU 50
From ICRU 62
Variation in Treated Volume (50% line)
with the choice of beam arrangement
for treating the same PTV
Tumor Dose Specification for External Photon Beams – ICRU recommendations
• Dose Specification
A clearly defined prescription or reporting point along with detailed information regarding total dose, fractional dose and total elapsed treatment days allows for proper comparison of outcome results. Several dosimetric end points have been defined in ICRU Reports No. 23 and 50 for this purpose.
Tumor Dose Specification for External Photon Beams – ICRU recommendations
• Dose Specification‐Cont’d
– Minimum target dose from a distribution or a dose‐volume histogram (DVH)
– Maximum target dose from a distribution or a DVH
– Mean target dose: the mean dose of all calculated target points
– The ICRU reference point
Tumor Dose Specification for External Photon Beams – ICRU recommendations
• As a general principle, the present system of recommendations for reporting doses is based on the selection of a point within the PTV, which is referred to as the ICRU Reference Point.
Tumor Dose Specification for External Photon Beams – ICRU recommendations
• The ICRU Reference Point shall be selected according to the following general criteria:
(1) the dose at the point should be clinically relevant;
(2) the point should be easy to define in a clear and unambiguous way;
(3) the point should be selected so that the dose can be accurately determined;
(4) the point should be in a region where there is no steep dose gradient.
Tumor Dose Specification for External Photon Beams – ICRU recommendations
• These recommendations will be fulfilled if the ICRU Reference Point is located:
– always at the center (or in a central part) of the PTV,
– and when possible, at the intersection of the beam axes,
• The dose at the ICRU Reference Point is the ICRU Reference Dose
Tumor Dose Specification for External Photon Beams – ICRU recommendations
• Specific recommendations with regard to the position of the ICRU reference point:
• For a single beam: –the point on the central axis at the center of the target volume
• For parallel opposed equally weighted beams: –the point on the central axis midway between the beam entrance points
Tumor Dose Specification for External Photon Beams – ICRU recommendations
• Specific recommendations with regard to the position of the ICRU reference point‐Cont’d:
• For parallel opposed unequally weighted beams: the point on the central axis at the centre of the target volume
• For other combinations of intersecting beams: the point at the intersection of the central axes (insofar as there is no dose gradient at this point)
Tumor Dose Specification for External Photon Beams
• In summary, the main objectives of a dose specification and reporting system are to achieve uniformity of dose reporting among institutions, to provide meaningful data for assessing the results of treatments, and to enable the treatment to be repeated elsewhere without having recourse to the original institution for further information.