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Radiation Dosimetry Parameters
and Isodose Curves
Presenter: Dheeraj Kumar
MRIT, Ph.D. (Radiology and Imaging)
Assistant Professor
Medical Radiology and Imaging Technology
School of Health Sciences, CSJM University, Kanpur
Bragg-Peak
Definition: The Bragg-peak is a
characteristic feature of charged
particle beams, such as protons
or heavy ions, where there is a
sudden increase in the deposited
dose near the end of the
particle's range in a medium.
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Radiation Dosimetry Parameters and Isodose Curves By-Dr.
Dheeraj Kumae
2
Explanation
As charged particles travel through matter,
they lose energy continuously via ionization
and excitation processes. The rate of energy
loss increases as the particle slows down,
reaching a maximum just before coming to
rest. This results in a peak in the dose
deposition profile known as the Bragg-
peak.
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Radiation Dosimetry Parameters and Isodose Curves By-Dr.
Dheeraj Kumae
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Significance
The Bragg-peak phenomenon is exploited in
radiation therapy, particularly in proton
therapy, due to its ability to deliver a high dose
to the target volume while sparing surrounding
healthy tissues. The depth of the Bragg-peak
can be precisely controlled by adjusting the
energy of the particle beam, allowing for
targeted treatment of tumors located at various
depths within the body.
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Example: Proton beam with an initial energy of 200 MeV is used to treat a tumor located at a depth of 10 cm in tissue.
Calculate the depth of the Bragg-peak for this proton beam.
Solution
Using the Bragg equation: R = R0​ +
𝑠
ρ
Where:
• R = Range of the particle in the medium
• R0​ = Range of the particle in the medium at the reference depth
• S = Stopping power of the medium
• ρ = Density of the medium
• Assume a stopping power of 2 MeV/cm for tissue (S) and a density of 1 g/cm³ (ρ).
• At the reference depth (R₀), let's assume the range of the particle is 20 cm.
Plugging in the values:
R =20 +
𝑆
ρ
=20 +
1
10
=30 cm
The Bragg-peak for the given proton beam occurs at a depth of 30 cm in tissue, making it suitable for targeting the tumor
located at a depth of 10 cm.
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Radiation Dosimetry Parameters and Isodose Curves By-Dr.
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Percentage Depth Dose (PDD)
Definition: Percentage Depth Dose
(PDD) is a fundamental dosimetric
parameter that describes the percentage
of absorbed dose at a certain depth in
tissue relative to a reference depth. It
provides valuable information about
how the dose varies with depth in a
radiation field.
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Explanation
As a radiation beam penetrates tissue, the
amount of energy deposited decreases with
increasing depth due to interactions with
the tissue. PDD quantifies this dose
reduction at various depths, which is
crucial for treatment planning and ensuring
that the prescribed dose reaches the target
volume while minimizing damage to
surrounding healthy tissues.
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Radiation Dosimetry Parameters and Isodose Curves By-Dr.
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Significance
PDD curves are used extensively in radiation therapy to characterize the
dose distribution in tissue and determine the depth at which the
maximum dose occurs. This information helps clinicians optimize
treatment plans to deliver the desired dose to the tumor while sparing
critical structures.
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Radiation Dosimetry Parameters and Isodose Curves By-Dr.
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Given Data: Radiation beam: 10 MeV electron beam
Reference depth (D₀): 3 cm
Depth of interest (D): 8 cm
Dose at the reference depth (D₀): 300 cGy
Dose at the depth of interest (D): 250 cGy
Solution:
PDD= (
𝐷
𝐷0
) ×100%
Where: D = Dose at the depth of interest (8 cm)
D0​ = Dose at the reference depth (3 cm)
PPD= (
250
300
) ×100% = (0.833)×100% =83.3%
The Percentage Depth Dose (PDD) at a depth of 8 cm for the given 10 MeV electron beam is 83.3%. This indicates that 83.3% of the prescribed dose is
delivered at a depth of 8 cm, providing essential information for treatment planning and ensuring adequate dose coverage to the target volume.
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Radiation Dosimetry Parameters and Isodose Curves By-Dr.
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Peak Scatter Factor (Scp)
Definition: The Peak Scatter Factor (Scp) is a
dosimetric parameter used in radiation therapy to
account for the increase in scatter radiation at the
depth of maximum dose compared to a reference
depth.
Explanation: As a radiation beam penetrates
tissue, it undergoes interactions that result in both
primary and scattered radiation. The Scp
quantifies the ratio of scatter radiation at the
depth of maximum dose to that at a reference
depth, providing insight into the distribution of
scatter within the patient's body.
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Radiation Dosimetry Parameters and Isodose Curves By-Dr.
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Significance:
• Scp helps in accurately determining the dose delivered
to the target volume by accounting for the contribution
of scatter radiation.
• It assists in treatment planning by adjusting the
prescribed dose to ensure adequate coverage of the
target while minimizing dose to healthy tissues.
Calculation of Scp:
• Scp is typically calculated by measuring or simulating
the dose at the depth of maximum dose and a reference
depth, usually in a water phantom.
• The ratio of scatter doses at these two depths gives the
Scp value for a specific beam energy and field size.
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Radiation Dosimetry Parameters and Isodose Curves By-Dr.
Dheeraj Kumae
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Clinical Application:
• Scp values are incorporated into treatment planning systems to optimize dose distributions and
ensure accurate dose delivery.
• By considering Scp along with other dosimetric parameters, clinicians can customize treatment
plans tailored to individual patient anatomy and tumor characteristics.
Example:
• For a 6 MV photon beam with a field size of 10x10 cm, the Scp may be determined experimentally
or calculated using Monte Carlo simulations.
• Suppose the measured scatter dose at the depth of maximum dose is 150 cGy, and at the reference
depth, it is 100 cGy.
• The Scp for this beam would be 150/100 = 1.5
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Radiation Dosimetry Parameters and Isodose Curves By-Dr.
Dheeraj Kumae
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Tissue-Air Ratio (TAR)
Definition: Tissue-Air Ratio (TAR) is a dosimetric
parameter used in radiation therapy to quantify the
ratio of absorbed dose in tissue to dose in air at a
specific depth and position within the patient's body.
Explanation: TAR provides insight into how
radiation is attenuated as it travels through tissue
compared to air. It is essential for accurate treatment
planning, particularly when dealing with tissue
inhomogeneities.
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Radiation Dosimetry Parameters and Isodose Curves By-Dr.
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Significance:
• TAR helps in adjusting the prescribed dose to account
for tissue density variations within the patient's
anatomy.
• It ensures that the calculated dose accurately reflects
the actual dose delivered to the target volume,
considering the tissue's radiation absorption
characteristics.
Calculation of TAR:
• TAR is typically calculated by measuring or simulating
the dose at a specific depth in tissue and dose in air at
the same position.
• The ratio of absorbed dose in tissue to dose in air gives
the TAR value for that particular depth and position.
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Radiation Dosimetry Parameters and Isodose Curves By-Dr.
Dheeraj Kumae
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Example:
Consider a 6 MV photon beam used in radiation therapy.
Let's calculate the TAR at a depth of 5 cm in tissue.
Assume the absorbed dose in tissue at 5 cm depth is 180 cGy, and the dose in air at the same depth is 200 cGy.
Solution:
Using the formula for TAR =
𝑫𝒕𝒊𝒔𝒔𝒖𝒆
𝑫𝒂𝒊𝒓
Where:
Dtissue​ = Absorbed dose in tissue at the depth of interest (5 cm)
Dair​ = Dose in air at the same depth (5 cm)
Plugging in the given values: TAR =
180
200
= 0.9
The Tissue-Air Ratio (TAR) at a depth of 5 cm in tissue for the given 6 MV photon beam is 0.9. This indicates that the
absorbed dose in tissue at that depth is 90% of the dose measured in air, providing essential information for treatment
planning and dose optimization.
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Radiation Dosimetry Parameters and Isodose Curves By-Dr.
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Tissue Maximum Ratio (TMR)
Definition: Tissue Maximum Ratio (TMR) is a dosimetric
parameter used in radiation therapy to quantify the ratio of
the dose at a specific depth in tissue to the dose at a
reference depth for a given field size.
TMR provides valuable information about the dose
distribution within tissue at various depths, relative to a
reference depth. It helps in understanding how the dose
attenuates as the radiation beam penetrates deeper into the
tissue.
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Radiation Dosimetry Parameters and Isodose Curves By-Dr.
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Calculation of TMR:
• TMR is typically determined experimentally or calculated
using treatment planning systems.
• The ratio of dose at the depth of interest to the dose at the
reference depth yields the TMR value for a specific beam
energy and field size.
Clinical Application:
• TMR values are incorporated into treatment planning
algorithms to optimize dose calculations and ensure
accurate dose delivery.
• They guide clinicians in selecting treatment parameters to
achieve desired dose distributions while considering
patient-specific factors and tumor characteristics.
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Radiation Dosimetry Parameters and Isodose Curves By-Dr.
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Example
• For a 10x10 cm field size and a 6 MV photon beam, TMR values may
be measured or calculated at various depths within tissue.
• Suppose at a depth of 10 cm, the dose is 80% of the dose at the
reference depth of 5 cm.
• This indicates that the TMR at 10 cm depth is 0.8 for this beam and
field size combination.
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Radiation Dosimetry Parameters and Isodose Curves By-Dr.
Dheeraj Kumae
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Scatter-Air Ratio (SAR)
Definition: The Scatter-Air Ratio (SAR) is a dosimetric parameter used in
radiation therapy to quantify the ratio of scatter dose in air at a specific point
to the dose at a reference point.
SAR provides insight into the scatter radiation characteristics of different
beams and field sizes. It helps in understanding how scatter radiation
contributes to the overall dose distribution in the treatment area.
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Radiation Dosimetry Parameters and Isodose Curves By-Dr.
Dheeraj Kumae
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Significance:
• SAR assists in accurately estimating scatter radiation doses, which is crucial for
treatment planning and dose optimization.
• It helps in assessing the potential impact of scatter radiation on healthy tissues
surrounding the treatment area.
Calculation of SAR:
• SAR is typically determined experimentally or calculated using Monte Carlo
simulations.
• The ratio of scatter dose in air at the point of interest to the dose at a reference point
gives the SAR value for a specific beam and field size.
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Radiation Dosimetry Parameters and Isodose Curves By-Dr.
Dheeraj Kumae
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Example
• Consider a 6 MV photon beam with a field size of 10x10 cm.
• SAR values may be measured or calculated at various points within the
treatment area.
• Suppose at a specific point, the scatter dose in air is 20% of the dose at a
reference point.
• This indicates that the SAR at that point is 0.2 for this beam and field size
combination.
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Radiation Dosimetry Parameters and Isodose Curves By-Dr.
Dheeraj Kumae
21
Isodose Curves
Definition: Isodose curves are graphical representations
that depict lines connecting points of equal dose within a
radiation field. These curves provide a visual depiction
of the dose distribution within the patient's anatomy
during radiation therapy.
Isodose curves illustrate how the dose varies spatially
within the treatment area, showing regions receiving
different levels of radiation dose. Each isodose curve
represents a specific dose level, with higher doses
typically depicted by curves closer to the radiation
source.
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Radiation Dosimetry Parameters and Isodose Curves By-Dr.
Dheeraj Kumae
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Significance:
• Isodose curves are essential for treatment planning,
allowing clinicians to visualize the spatial distribution of
dose within the patient's body.
• They help in identifying areas receiving the prescribed
dose, as well as regions receiving higher or lower doses,
aiding in dose optimization and plan evaluation.
Interpretation of Isodose Curves:
• Higher dose regions are represented by isodose curves
closer to the radiation source, while lower dose regions
are depicted by curves farther away.
• Clinicians can assess dose coverage of the target volume
and evaluate dose sparing to surrounding healthy tissues
by analyzing the distribution of isodose curves.
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Radiation Dosimetry Parameters and Isodose Curves By-Dr.
Dheeraj Kumae
23
Clinical Application:
• Isodose curves are used extensively in treatment planning to optimize dose distributions and
ensure accurate dose delivery to the target volume.
• They provide valuable information for plan evaluation and quality assurance, helping
clinicians make informed decisions about treatment parameters and techniques.
Example
• For a given radiation therapy plan, isodose curves may be generated to visualize the dose
distribution within the patient's anatomy.
• Higher dose regions corresponding to the target volume are represented by isodose curves of
the prescribed dose, while lower dose regions depict dose sparing to critical structures.
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Radiation Dosimetry Parameters and Isodose Curves By-Dr.
Dheeraj Kumae
24
Radiation Penumbra
Definition: Radiation penumbra refers to the gradual dose fall-off at the
edges of the radiation field, resulting in a transition zone between areas
receiving full dose and those receiving little to no dose.
When a radiation beam is delivered to a target volume, the dose distribution is
not perfectly sharp at the field edges. Instead, there is a region where the dose
decreases gradually, known as the penumbra. This phenomenon occurs due to
factors such as beam divergence, collimator scatter, and geometric effects.
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Radiation Dosimetry Parameters and Isodose Curves By-Dr.
Dheeraj Kumae
25
Significance:
• Understanding the radiation penumbra is crucial for
accurate treatment planning and delivery in radiation
therapy.
• Minimizing the penumbra ensures that the target
volume receives the intended dose while sparing nearby
healthy tissues from unnecessary radiation exposure.
Factors Affecting Penumbra:
• Beam energy: Higher energy beams tend to produce
sharper penumbra due to reduced scattering.
• Collimator design: Collimators with sharper edges can
help minimize penumbra.
• Source-to-surface distance (SSD): Penumbra width
increases with increasing SSD.
• Field size: Larger fields tend to have broader penumbra
due to increased scatter.
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Radiation Dosimetry Parameters and Isodose Curves By-Dr.
Dheeraj Kumae
26
Clinical Application:
• Radiation oncologists and medical physicists utilize knowledge of the penumbra to design
treatment plans that optimize dose distribution and minimize dose to critical structures.
• Advanced techniques such as intensity-modulated radiation therapy (IMRT) and volumetric-
modulated arc therapy (VMAT) are employed to shape the dose distribution and reduce
penumbra effects.
Example:
• Isodose curves may be generated to visualize the radiation penumbra in a treatment plan.
• Clinicians can evaluate the width of the penumbra and adjust treatment parameters to
minimize its impact on healthy tissues.
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Radiation Dosimetry Parameters and Isodose Curves By-Dr.
Dheeraj Kumae
27
References
1. Bushberg, J. T., Seibert, J. A., Leidholdt Jr, E. M., & Boone, J. M. (2011). The Essential Physics of Medical Imaging. Lippincott
Williams & Wilkins.
2. Rehani, M. M., & Szczykutowicz, T. P. (Eds.). (2012). Radiation Dose Management in the Nuclear Industry: An Integrated
Approach. Springer Science & Business Media.
3. The International Electrotechnical Commission. (2017). IEC 61223-3-5: Medical electrical equipment - Characteristics of digital X-
ray imaging devices - Part 3-5: Determination of the detective quantum efficiency - Detectors used in mammography. IEC.
4. Shrader, J. A., Casarella, W. J., & Ritenour, E. R. (2016). Introduction to Health Physics. CRC Press.
5. Valentin, J. (2007). Radiation and Your Patient: A Guide for Medical Practitioners. International Atomic Energy Agency.
13-04-2024
Radiation Dosimetry Parameters and Isodose Curves By-Dr.
Dheeraj Kumae
28

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Radiation Dosimetry Parameters and Isodose Curves.pptx

  • 1. Radiation Dosimetry Parameters and Isodose Curves Presenter: Dheeraj Kumar MRIT, Ph.D. (Radiology and Imaging) Assistant Professor Medical Radiology and Imaging Technology School of Health Sciences, CSJM University, Kanpur
  • 2. Bragg-Peak Definition: The Bragg-peak is a characteristic feature of charged particle beams, such as protons or heavy ions, where there is a sudden increase in the deposited dose near the end of the particle's range in a medium. 13-04-2024 Radiation Dosimetry Parameters and Isodose Curves By-Dr. Dheeraj Kumae 2
  • 3. Explanation As charged particles travel through matter, they lose energy continuously via ionization and excitation processes. The rate of energy loss increases as the particle slows down, reaching a maximum just before coming to rest. This results in a peak in the dose deposition profile known as the Bragg- peak. 13-04-2024 Radiation Dosimetry Parameters and Isodose Curves By-Dr. Dheeraj Kumae 3
  • 4. Significance The Bragg-peak phenomenon is exploited in radiation therapy, particularly in proton therapy, due to its ability to deliver a high dose to the target volume while sparing surrounding healthy tissues. The depth of the Bragg-peak can be precisely controlled by adjusting the energy of the particle beam, allowing for targeted treatment of tumors located at various depths within the body. 13-04-2024 Radiation Dosimetry Parameters and Isodose Curves By-Dr. Dheeraj Kumae 4
  • 5. Example: Proton beam with an initial energy of 200 MeV is used to treat a tumor located at a depth of 10 cm in tissue. Calculate the depth of the Bragg-peak for this proton beam. Solution Using the Bragg equation: R = R0​ + 𝑠 ρ Where: • R = Range of the particle in the medium • R0​ = Range of the particle in the medium at the reference depth • S = Stopping power of the medium • ρ = Density of the medium • Assume a stopping power of 2 MeV/cm for tissue (S) and a density of 1 g/cm³ (ρ). • At the reference depth (R₀), let's assume the range of the particle is 20 cm. Plugging in the values: R =20 + 𝑆 ρ =20 + 1 10 =30 cm The Bragg-peak for the given proton beam occurs at a depth of 30 cm in tissue, making it suitable for targeting the tumor located at a depth of 10 cm. 13-04-2024 Radiation Dosimetry Parameters and Isodose Curves By-Dr. Dheeraj Kumae 5
  • 6. Percentage Depth Dose (PDD) Definition: Percentage Depth Dose (PDD) is a fundamental dosimetric parameter that describes the percentage of absorbed dose at a certain depth in tissue relative to a reference depth. It provides valuable information about how the dose varies with depth in a radiation field. 13-04-2024 Radiation Dosimetry Parameters and Isodose Curves By-Dr. Dheeraj Kumae 6
  • 7. Explanation As a radiation beam penetrates tissue, the amount of energy deposited decreases with increasing depth due to interactions with the tissue. PDD quantifies this dose reduction at various depths, which is crucial for treatment planning and ensuring that the prescribed dose reaches the target volume while minimizing damage to surrounding healthy tissues. 13-04-2024 Radiation Dosimetry Parameters and Isodose Curves By-Dr. Dheeraj Kumae 7
  • 8. Significance PDD curves are used extensively in radiation therapy to characterize the dose distribution in tissue and determine the depth at which the maximum dose occurs. This information helps clinicians optimize treatment plans to deliver the desired dose to the tumor while sparing critical structures. 13-04-2024 Radiation Dosimetry Parameters and Isodose Curves By-Dr. Dheeraj Kumae 8
  • 9. Given Data: Radiation beam: 10 MeV electron beam Reference depth (D₀): 3 cm Depth of interest (D): 8 cm Dose at the reference depth (D₀): 300 cGy Dose at the depth of interest (D): 250 cGy Solution: PDD= ( 𝐷 𝐷0 ) ×100% Where: D = Dose at the depth of interest (8 cm) D0​ = Dose at the reference depth (3 cm) PPD= ( 250 300 ) ×100% = (0.833)×100% =83.3% The Percentage Depth Dose (PDD) at a depth of 8 cm for the given 10 MeV electron beam is 83.3%. This indicates that 83.3% of the prescribed dose is delivered at a depth of 8 cm, providing essential information for treatment planning and ensuring adequate dose coverage to the target volume. 13-04-2024 Radiation Dosimetry Parameters and Isodose Curves By-Dr. Dheeraj Kumae 9
  • 10. Peak Scatter Factor (Scp) Definition: The Peak Scatter Factor (Scp) is a dosimetric parameter used in radiation therapy to account for the increase in scatter radiation at the depth of maximum dose compared to a reference depth. Explanation: As a radiation beam penetrates tissue, it undergoes interactions that result in both primary and scattered radiation. The Scp quantifies the ratio of scatter radiation at the depth of maximum dose to that at a reference depth, providing insight into the distribution of scatter within the patient's body. 13-04-2024 Radiation Dosimetry Parameters and Isodose Curves By-Dr. Dheeraj Kumae 10
  • 11. Significance: • Scp helps in accurately determining the dose delivered to the target volume by accounting for the contribution of scatter radiation. • It assists in treatment planning by adjusting the prescribed dose to ensure adequate coverage of the target while minimizing dose to healthy tissues. Calculation of Scp: • Scp is typically calculated by measuring or simulating the dose at the depth of maximum dose and a reference depth, usually in a water phantom. • The ratio of scatter doses at these two depths gives the Scp value for a specific beam energy and field size. 13-04-2024 Radiation Dosimetry Parameters and Isodose Curves By-Dr. Dheeraj Kumae 11
  • 12. Clinical Application: • Scp values are incorporated into treatment planning systems to optimize dose distributions and ensure accurate dose delivery. • By considering Scp along with other dosimetric parameters, clinicians can customize treatment plans tailored to individual patient anatomy and tumor characteristics. Example: • For a 6 MV photon beam with a field size of 10x10 cm, the Scp may be determined experimentally or calculated using Monte Carlo simulations. • Suppose the measured scatter dose at the depth of maximum dose is 150 cGy, and at the reference depth, it is 100 cGy. • The Scp for this beam would be 150/100 = 1.5 13-04-2024 Radiation Dosimetry Parameters and Isodose Curves By-Dr. Dheeraj Kumae 12
  • 13. Tissue-Air Ratio (TAR) Definition: Tissue-Air Ratio (TAR) is a dosimetric parameter used in radiation therapy to quantify the ratio of absorbed dose in tissue to dose in air at a specific depth and position within the patient's body. Explanation: TAR provides insight into how radiation is attenuated as it travels through tissue compared to air. It is essential for accurate treatment planning, particularly when dealing with tissue inhomogeneities. 13-04-2024 Radiation Dosimetry Parameters and Isodose Curves By-Dr. Dheeraj Kumae 13
  • 14. Significance: • TAR helps in adjusting the prescribed dose to account for tissue density variations within the patient's anatomy. • It ensures that the calculated dose accurately reflects the actual dose delivered to the target volume, considering the tissue's radiation absorption characteristics. Calculation of TAR: • TAR is typically calculated by measuring or simulating the dose at a specific depth in tissue and dose in air at the same position. • The ratio of absorbed dose in tissue to dose in air gives the TAR value for that particular depth and position. 13-04-2024 Radiation Dosimetry Parameters and Isodose Curves By-Dr. Dheeraj Kumae 14
  • 15. Example: Consider a 6 MV photon beam used in radiation therapy. Let's calculate the TAR at a depth of 5 cm in tissue. Assume the absorbed dose in tissue at 5 cm depth is 180 cGy, and the dose in air at the same depth is 200 cGy. Solution: Using the formula for TAR = 𝑫𝒕𝒊𝒔𝒔𝒖𝒆 𝑫𝒂𝒊𝒓 Where: Dtissue​ = Absorbed dose in tissue at the depth of interest (5 cm) Dair​ = Dose in air at the same depth (5 cm) Plugging in the given values: TAR = 180 200 = 0.9 The Tissue-Air Ratio (TAR) at a depth of 5 cm in tissue for the given 6 MV photon beam is 0.9. This indicates that the absorbed dose in tissue at that depth is 90% of the dose measured in air, providing essential information for treatment planning and dose optimization. 13-04-2024 Radiation Dosimetry Parameters and Isodose Curves By-Dr. Dheeraj Kumae 15
  • 16. Tissue Maximum Ratio (TMR) Definition: Tissue Maximum Ratio (TMR) is a dosimetric parameter used in radiation therapy to quantify the ratio of the dose at a specific depth in tissue to the dose at a reference depth for a given field size. TMR provides valuable information about the dose distribution within tissue at various depths, relative to a reference depth. It helps in understanding how the dose attenuates as the radiation beam penetrates deeper into the tissue. 13-04-2024 Radiation Dosimetry Parameters and Isodose Curves By-Dr. Dheeraj Kumae 16
  • 17. Calculation of TMR: • TMR is typically determined experimentally or calculated using treatment planning systems. • The ratio of dose at the depth of interest to the dose at the reference depth yields the TMR value for a specific beam energy and field size. Clinical Application: • TMR values are incorporated into treatment planning algorithms to optimize dose calculations and ensure accurate dose delivery. • They guide clinicians in selecting treatment parameters to achieve desired dose distributions while considering patient-specific factors and tumor characteristics. 13-04-2024 Radiation Dosimetry Parameters and Isodose Curves By-Dr. Dheeraj Kumae 17
  • 18. Example • For a 10x10 cm field size and a 6 MV photon beam, TMR values may be measured or calculated at various depths within tissue. • Suppose at a depth of 10 cm, the dose is 80% of the dose at the reference depth of 5 cm. • This indicates that the TMR at 10 cm depth is 0.8 for this beam and field size combination. 13-04-2024 Radiation Dosimetry Parameters and Isodose Curves By-Dr. Dheeraj Kumae 18
  • 19. Scatter-Air Ratio (SAR) Definition: The Scatter-Air Ratio (SAR) is a dosimetric parameter used in radiation therapy to quantify the ratio of scatter dose in air at a specific point to the dose at a reference point. SAR provides insight into the scatter radiation characteristics of different beams and field sizes. It helps in understanding how scatter radiation contributes to the overall dose distribution in the treatment area. 13-04-2024 Radiation Dosimetry Parameters and Isodose Curves By-Dr. Dheeraj Kumae 19
  • 20. Significance: • SAR assists in accurately estimating scatter radiation doses, which is crucial for treatment planning and dose optimization. • It helps in assessing the potential impact of scatter radiation on healthy tissues surrounding the treatment area. Calculation of SAR: • SAR is typically determined experimentally or calculated using Monte Carlo simulations. • The ratio of scatter dose in air at the point of interest to the dose at a reference point gives the SAR value for a specific beam and field size. 13-04-2024 Radiation Dosimetry Parameters and Isodose Curves By-Dr. Dheeraj Kumae 20
  • 21. Example • Consider a 6 MV photon beam with a field size of 10x10 cm. • SAR values may be measured or calculated at various points within the treatment area. • Suppose at a specific point, the scatter dose in air is 20% of the dose at a reference point. • This indicates that the SAR at that point is 0.2 for this beam and field size combination. 13-04-2024 Radiation Dosimetry Parameters and Isodose Curves By-Dr. Dheeraj Kumae 21
  • 22. Isodose Curves Definition: Isodose curves are graphical representations that depict lines connecting points of equal dose within a radiation field. These curves provide a visual depiction of the dose distribution within the patient's anatomy during radiation therapy. Isodose curves illustrate how the dose varies spatially within the treatment area, showing regions receiving different levels of radiation dose. Each isodose curve represents a specific dose level, with higher doses typically depicted by curves closer to the radiation source. 13-04-2024 Radiation Dosimetry Parameters and Isodose Curves By-Dr. Dheeraj Kumae 22
  • 23. Significance: • Isodose curves are essential for treatment planning, allowing clinicians to visualize the spatial distribution of dose within the patient's body. • They help in identifying areas receiving the prescribed dose, as well as regions receiving higher or lower doses, aiding in dose optimization and plan evaluation. Interpretation of Isodose Curves: • Higher dose regions are represented by isodose curves closer to the radiation source, while lower dose regions are depicted by curves farther away. • Clinicians can assess dose coverage of the target volume and evaluate dose sparing to surrounding healthy tissues by analyzing the distribution of isodose curves. 13-04-2024 Radiation Dosimetry Parameters and Isodose Curves By-Dr. Dheeraj Kumae 23
  • 24. Clinical Application: • Isodose curves are used extensively in treatment planning to optimize dose distributions and ensure accurate dose delivery to the target volume. • They provide valuable information for plan evaluation and quality assurance, helping clinicians make informed decisions about treatment parameters and techniques. Example • For a given radiation therapy plan, isodose curves may be generated to visualize the dose distribution within the patient's anatomy. • Higher dose regions corresponding to the target volume are represented by isodose curves of the prescribed dose, while lower dose regions depict dose sparing to critical structures. 13-04-2024 Radiation Dosimetry Parameters and Isodose Curves By-Dr. Dheeraj Kumae 24
  • 25. Radiation Penumbra Definition: Radiation penumbra refers to the gradual dose fall-off at the edges of the radiation field, resulting in a transition zone between areas receiving full dose and those receiving little to no dose. When a radiation beam is delivered to a target volume, the dose distribution is not perfectly sharp at the field edges. Instead, there is a region where the dose decreases gradually, known as the penumbra. This phenomenon occurs due to factors such as beam divergence, collimator scatter, and geometric effects. 13-04-2024 Radiation Dosimetry Parameters and Isodose Curves By-Dr. Dheeraj Kumae 25
  • 26. Significance: • Understanding the radiation penumbra is crucial for accurate treatment planning and delivery in radiation therapy. • Minimizing the penumbra ensures that the target volume receives the intended dose while sparing nearby healthy tissues from unnecessary radiation exposure. Factors Affecting Penumbra: • Beam energy: Higher energy beams tend to produce sharper penumbra due to reduced scattering. • Collimator design: Collimators with sharper edges can help minimize penumbra. • Source-to-surface distance (SSD): Penumbra width increases with increasing SSD. • Field size: Larger fields tend to have broader penumbra due to increased scatter. 13-04-2024 Radiation Dosimetry Parameters and Isodose Curves By-Dr. Dheeraj Kumae 26
  • 27. Clinical Application: • Radiation oncologists and medical physicists utilize knowledge of the penumbra to design treatment plans that optimize dose distribution and minimize dose to critical structures. • Advanced techniques such as intensity-modulated radiation therapy (IMRT) and volumetric- modulated arc therapy (VMAT) are employed to shape the dose distribution and reduce penumbra effects. Example: • Isodose curves may be generated to visualize the radiation penumbra in a treatment plan. • Clinicians can evaluate the width of the penumbra and adjust treatment parameters to minimize its impact on healthy tissues. 13-04-2024 Radiation Dosimetry Parameters and Isodose Curves By-Dr. Dheeraj Kumae 27
  • 28. References 1. Bushberg, J. T., Seibert, J. A., Leidholdt Jr, E. M., & Boone, J. M. (2011). The Essential Physics of Medical Imaging. Lippincott Williams & Wilkins. 2. Rehani, M. M., & Szczykutowicz, T. P. (Eds.). (2012). Radiation Dose Management in the Nuclear Industry: An Integrated Approach. Springer Science & Business Media. 3. The International Electrotechnical Commission. (2017). IEC 61223-3-5: Medical electrical equipment - Characteristics of digital X- ray imaging devices - Part 3-5: Determination of the detective quantum efficiency - Detectors used in mammography. IEC. 4. Shrader, J. A., Casarella, W. J., & Ritenour, E. R. (2016). Introduction to Health Physics. CRC Press. 5. Valentin, J. (2007). Radiation and Your Patient: A Guide for Medical Practitioners. International Atomic Energy Agency. 13-04-2024 Radiation Dosimetry Parameters and Isodose Curves By-Dr. Dheeraj Kumae 28