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Angela Kempen
Treatment Planning Project
2/25/2012
Homogeneity versus Heterogeneity in Treatment Planning
Introduction
The use of ionizing radiation in medicine has proven beneficial, particularly in patients
diagnosed with cancer. Conformal radiation treatment requires accurate dose
calculations at a specific point, in addition to accurate dose distribution throughout the
planning target volume (PTV) and within the critical structures in the path of the beam.
Radiation and the equipment utilized to produce x-rays are calibrated using homogenous
unit density phantoms made up of water-equivalent densities. The phantoms produce
even beam flatness and symmetry; however, in treatment planning this is not the case.
The human body is not completely comprised of water-equivalent tissues so it does not
exhibit homogenous unit densities; therefore, optimal benefit of treatment involves
treatment planning that correctly accounts for the inhomogeneity of a patient’s many
different tissue types and organs. Absorbed dose may be accurately calculated when
these different tissue densities are accounted for. This concern is prominent throughout
the thoracic region where tissue densities change dramatically. Inhomogeneity
corrections, “which take into account that the patient consists of heterogeneous tissue
densities instead of a homogeneous tank of water1” are incorporated into treatment
planning in radiation oncology departments. Inhomogeneity correction is greatest at
lower photon energies and smaller field sizes.2 The radiation oncologist and medical
dosimetrist work together to create a treatment plan that is truly representative of the dose
distribution depending on tumor location, extent of disease and its proximity to critical
structures, in addition to any tissue inhomogeneities within the path of the beam.
Methods and Materials
This patient had a lung cancer located in the posterior lobe of the right lung. I contoured
the right lung, left lung, spinal cord, PTV tumor, heart and patient external contour. I
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started by placing an anterior beam and added a block around the PTV tumor plus a 1.5
cm margin. I copied and opposed this beam in order to create the posterior beam. At this
point, I turned the heterogeneity correction off and computed the beams. The anterior
and posterior beams were weighted 43% and 57% respectively. The plan was prescribed
to the 98% isodose line to achieve 100% of the dose covering 95.4% of the PTV tumor.
The monitor units given to achieve this dose distribution was 102 MU anteriorly and 119
MU posteriorly. The depth of the point used to calculate the MU was a physical depth of
13.02 cm anteriorly and 10.06 cm posteriorly. This homogenous plan was copied and
saved. On the copied trial, I turned heterogeneity correction factors on and reviewed the
plan. The coverage had decreased to 100% of the dose covering 63.7% of the PTV
tumor, with 90 MU given anteriorly and 120 MU posteriorly. The depth of the point used
to calculate the MU was the effective depth of 7.38 cm anteriorly and 8.25 cm
posteriorly. Both of the plans were calculated in the Pinnacle treatment planning system
using the adaptive collapsed cone convolution algorithm.
Results
The homogeneous lung plan displayed a conformal dose distribution throughout the
entire treatment volume. The isodose lines did not display any bowing in throughout the
lung region, as shown in Figure 1. The plan was normalized to the 98% isodose line,
which gave 100% coverage to 95.4% of the PTV tumor. The heterogeneity lung plan
was not delivering proper dose coverage to the PTV tumor. Only 63.7% of the PTV
tumor was receiving 100% of the dose. The isodose lines illustrated a bowing in through
the lung region, as displayed in Figure 4.
Discussion
Medical physicists measure and calibrate radiation with tissue-equivalent phantoms,
usually water, at a certain depth. These measurements create isodose distributions and
depth dose charts, which assume homogenous density material. However, when
treatments are delivered to patients, the assumption of homogenous material is no longer
valid. When entering and exiting through a patient, the radiation beam may intersect
different layers of densities including bone, muscle, fat, air passages, cavities and lung.
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These different tissue types contain different electron densities, making the human body
heterogeneous. A heterogeneous material affects a beam in two major ways including,
through differential attenuation or absorption of the primary beam and by energy and
scattering differences depending on field size.1 The presence of these inhomogeneities
will cause changes in isodose distributions, with direct dependence on the thickness of
the material encountered and on the quality of the radiation beam.2 The penetration of
the beam and the scattering characteristics will be affected.3 The degree of this effect
depends on the size and shape of the volume and the density (g/cm3) of the
inhomogeneity, in addition to the energy of the beam.3
Water and soft tissue in the body has an electron density of 1.0 g/cm3, whereas lung has a
lower density varying between 0.25-1.0 g/cm3 with the most common densities being
0.25-0.33 g/cm3, depending on the amount of air. Having a lower density results in lung
tissues attenuating less of the beam than the equal thickness of soft tissue. Bone, on the
other hand, is given an electron density of approximately 1.8 g/cm3, which may fluctuate
depending on whether it is hard, or soft spongy bone. Based on these densities, bone will
attenuate more of the primary beam than the equal thickness of lung or soft tissue. The
degree of attenuation is equal to the number of photons transmitted through the material.
Therefore, the number of photons transmitted through lung tissue is higher than it is for
soft tissue or bone, allowing more photons to reach a greater depth in the patient and
increasing dose to the tissues beyond the lung. Heterogeneity corrections and effective
depth account for this difference between tissue electron densities.
A homogenous plan is assuming all densities that the beam encounters in its path are 1.0
g/cm3, therefore electrons are interacting with tissues in the body to contribute to the
dose. The dose distribution is homogenous without any bulging of the isodose curves.
When the heterogeneity is accounted for in regards to a lung plan, there is less electrons
interacting with tissue simply because there is a greater volume of air present in the
lungs, making it less dense than tissue, resulting in less backscatter and dose buildup
from phantom scatter. The effective depth increases when the beam penetrates through
lung tissue because of less attenuation than an equal thickness of soft tissue or bone.
Three factors cause this phenomenon. First, the most prominent interaction with
megavoltage (MV) photon energies is the Compton effect. Compton attenuation is
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related to electron density, so when the material in the path of the beam is less dense,
fewer electrons are available to interact with, therefore decreasing dose. Secondly, as the
beam re-enters a higher density, such as from lung to soft tissue interface, electron
equilibrium is restarted. Electronic equilibrium at the interface causes an under dose to
the surface of the structure laying distal to the interface, since dmax is not located on the
surface. Lastly, the penumbra is larger causing a broadening or bowing in of the isodose
lines. This is the direct effect of the probability of interactions decreasing, in addition to
less scatter. The differences found in beam fringe and penumbra width (20-80% isodose
lines) increase with increasing beam energies.4 For these reasons, it is important to use
modern algorithms, such as Monte Carlo, which take these factors into consideration
when calculating dose. The pencil beam algorithm does not consider electronic
equilibrium or the increased penumbra in its calculations.
If not taken into consideration when planning, tissue inhomogeneities can have an effect
on the planning treatment volume. At the interfaces of different tissue types, problems
can occur by build-up and build-down effects.1 If a target volume is located within lung
tissue, the volume may be under dosed due to decreased backscatter from decreased
electron density of the lung. The opposite result will occur if the target volume is located
near or in bone, or any density greater than 1.0 g/cm3. The volume will receive more
than the prescribed dose due to increased backscatter.
The history of radiation oncology and all past clinical experience is based on and has
been gained from dose calculations assuming homogeneity. Older planning systems did
not have an option to account for heterogeneity. Most protocols and critical structure
tolerances used today are derived from planning systems assuming all the different
tissues as having an equal water-equivalent density. Historically, the calculations of dose
changes due to inhomogeneities was complicated due to the variations of density within
the inhomogeneity and uncertainties in the three dimensional shape. The increased
popularity of computed tomography (CT) has improved dose calculation methods
accounting for inhomogeneities considerably.3 A correction factor is applied for different
tissue densities when calculating dose.
With the lung plan I constructed assuming all tissues to be homogenous, it is shown that
102 MU were delivered from the anterior beam. The tumor lies in the posterior part of
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the lung. When the heterogeneity was accounted for in the second plan, the MU from the
anterior beam decreased to 90 MU. The heterogeneity correction made this correction
because the radiation beam was less attenuated when penetrating through the lung, a less
dense material, than it would have been passing through soft tissue as was assumed on
the homogenous plan. The MU given from the posterior beam remained fairly equal on
both plans, at 119 and 120, because of the tumors posterior location. The beam is passing
through the same thickness of soft tissue and bone on the homogenous and heterogenous
plans. The plan taking heterogeneity into its calculations showed some bowing in of the
isodose lines in the lung region, so I would have prescribed to a lower isodose line in
order to provide adequate coverage to the PTV tumor.
When planning, certain techniques can be applied to optimize the heterogeneous plan to
look similar to the homogenous plan. Prescribing to a lower isodose line will help bring
the dose to the prescription dose. Depending on the location of the tumor, weighting of
the beams, compensators or wedges can additionally optimize the plan.
Conclusion
A plan optimized without the heterogeneity correction factors illustrates better dose
coverage; however, this is not a true representation of what is happening in the patient’s
body. Inhomogeneities have a direct impact on isodose curves and the dose distribution.
It is pivotal to account for these changes in order to assure proper prescription dose
coverage of the tumor volume. AAPM TG No. 85 recommends that “heterogeneity
corrections be applied to treatment plans and dose prescriptions, with the provision that
the algorithms used for the calculations are reviewed and tested by the Medical
Physicist.5” All individuals involved in the treatment planning process need to be aware
and understand the effects of inhomogeneities on dose distributions and the importance of
accounting for those effects.
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Figure 1: Axial, Sagittal and Coronal views of homogenous lung plan.
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Figure 2: Monitor Unit sheets for the Anterior and Posterior homogeneous lung fields.
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Figure 3: Dose Volume Histogram of the homogenous lung plan
Figure 4: Axial, Sagittal and Coronal views of the heterogeneous lung plan
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Figure 5: Monitor Unit sheets for the Anterior and Posterior heterogeneous lung fields
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Figure 6: Dose Volume Histogram of heterogeneous lung plan
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Figure 7: Comparison of Dose Volume Histogram (homogenous versus heterogeneous)
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REFERENCES:
1. Saxena, R, Higgins P. Measurement and Evaluation of Inhomogeneity
Corrections and Monitor Unit Verification for Treatment Planning. Med
Dosimetry. 2010;35(1):19-27. http://www.meddos.org/article/S09583947(09)00002-8/fulltext. Accessed February 29, 2012.
2. Khan, FM. Treatment Planning in Radiation Oncology. 2nd Ed. Philadelphia,
PA: Lippincott Williams & Wilkins; 2007.
3. Bentel, G. Radiation Therapy Planning. 2nd ed. New York, NY: McGraw-Hill;
1996.
4. Engelsman, M, Damen E, Koken P, van’t Veld A, van Ingen, K, Mijnheer, B.
Imapct of simple tissue inhomogeneity correction algorithms on conformal
radiotherapy of lung tumours. Radiotherapy & Oncol. 2001;60(3):299-309.
http://www.thegreenjournal.com/article/S0167-8140(01)00387-5/abstract.
Accessed February 29, 2012.
5. Papanikolaou N, Battista JJ, Boyer AL, et al. Tissue Inhomogeneity Corrections
for Megavoltage Photon Beams. Madison WI: Medical Physics Building; 2004.
AAPM Report No. 85.