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Ryan Salem
Treatment Planning Project / Treatment Plan in Med Dos
Due: April 22nd, 2018
Effects of Tissue Inhomogeneities in Lung Treatment Planning
Introduction:
Today, treatment planning in radiation therapy is more accurate than ever. In recent
history, the field of radiation oncology has seen great improvements in imaging procedures,
beam modulation techniques, immobilization, high definition multi-leaf collimation (MLC),
intensity modulated radiation therapy (IMRT) and volumetric arc therapy (VMAT), and dose
calculation algorithms. Treatment planning accuracy in large part has improved due to
inhomogeneity correction algorithms within the treatment planning system (TPS).
Modern radiation therapy requires accurate dose calculation at dose specification points
within the planning target volume (PTV) and organs at risk (OARs).1 Linear accelerator beam
dosimetry data are obtained under standard conditions. This means there is a homogeneous unit
density phantom, like a water phantom, perpendicular beam incidence, and a symmetrical
surface for the beam to enter. During treatment, however, the patient may have an irregular
surface where the beam enters the body and have different tissues with different densities near
the target of the dose. Previously, isodose charts were corrected for contour irregularities by
manual methods such as the effective source to surface distance (SSD), tissue-air ratio, and
isodose shift methods.2 Since the use of computer tomographic (CT) scans for radiation
treatment planning, tissue inhomogeneity within a patient is calculated without the use of manual
methods. By obtaining the relationship between CT Hounsfield units and electron densities of
tissues within a patient, precise treatment planning is possible.3 This project will compare the
effects of tissue inhomogeneity corrections on similar treatment plans within the TPS used at my
clinical site, Eclipse version 13.6.
Methods:
A lung cancer patient was scanned on a CT simulator in the position for the treatment of
stereotactic body radiation therapy. The patient was positioned supine with her arms up in a
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vacuum bean bag. The bag was on top of an arm shuttle with her hands around a t-grip bar. A
knee sponge was used for patient comfort. Physician drawn structures included the PTV internal
target volume (ITV), and the gross tumor volume (GTV). Organs at risk that were contoured
included the right lung, left lung, spinal cord, tumor, heart, esophagus, and rib. For use in this
assignment, two treatment plans were created using the Eclipse TPS for comparison. Both plans
utilized an anteroposterior (AP) and posteroanterior (PA) beam arrangement weighted 62%:38%
respectively. Both plans utilized 6 mega-voltage (MV) energy for both beams. The fractionation
schedule in both plans was to deliver 180 centigray (cGy) per fraction for 35 fractions to a total
dose of 6300. Each plan was normalized so that 95% of dose was delivered to 100% of the PTV.
The original plan, AP/PA_TxPlan, used eclipse AAA algorithms with heterogeneity corrections
turned on. The second plan, AP/PA_HomOFF, used the same algorithms with the corrections
turned off. These plans were not used in the treatment of this patient, but for the purpose of
comparing dose distribution and delivery with and without heterogeneity corrections turned on.
Results:
The combined dose volume histogram (DVH) of both plans presents very important
information. Because bone has a higher electron density than regular tissue, it absorbs more dose
as beams travel through it. Due to lung having a density close to air, it hardly absorbs dose at all
as a beam traverses it.2 The DVH shows us that the PTV, which is in lung, and the rib both
receive a much higher mean and max dose due to this (Figure 1). The DVH shows us that in
order to cover the PTV with 95% isodose, the nearby OAR tissues must receive higher dose.
With corrections turned off and the PTV being located so anterior, the regular homogenous dose
lines are able to adequately cover the PTV (Figures 2 & 3). This alone shows the sheer
importance of heterogeneity correction factors. To cover the PTV with 95% isodose coverage, it
took the original corrected plan 139 AP MUs and 102 PA MUs. With corrections turned off, it
took 124 AP MUs and 114 PA MUs to cover the PTV with 95% dose. The maximum hot spot
for the corrected plan was 120.1% with a maximum spot in the PTV being 117.4%. In the
uncorrected plan, the maximum hot spot for the plan was 110.1% and the maximum hot spot in
the PTV was 102.8% (Figures 1,2, & 3).
Discussion:
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All around, dose distribution is strongly affected by the differing tissue densities in a
patient’s body. The location of the target area in relationship to structures with different densities
can determine the way dose is deposited throughout the patient. Overall dose distribution with
isodose lines from 25% to 110% can be seen in Figures 2 & 3. It is strongly evident that the
tissue densities affect the way higher isodose lines, especially the 95% line, are represented in
the anatomical cross sections. As dose is not heavily deposited in the lung while it is deposited
more in bones, you can see the 95% line being blocked out by the ribs while it is constricted
throughout the lung in the corrected plan. In the uncorrected plan, the 95% isodose line is not
changed because of structures in the body. (Figures 2 & 3). The MU differences in the plans is
also representative of how the isodose lines are formed in each plan. Due to having more anterior
rib, the AP beam in the corrected plan needs significantly more MUs to achieve dose coverage.
Conversely, although traveling a longer distance, the PA beam needs less MUs due to the large
distance traveled through lung to get to the PTV. In the uncorrected plan, the beams have much
closer amounts of MUs despite the large weighting difference. This is because the PA beam must
traverse much more tissue that the planning systems sees at the same density (Figures 2 & 3).
A third plan was added to further demonstrate how MUs are changed in heterogeneity
corrections. In Figure 4, I adjusted the plan with corrections turned on to deliver the same AP
and PA MUs as the uncorrected plan. The field weighting was changed to represent the closer
amount of MUs delivered by each respective beam. This change also changed the isodose lines.
The PTV is no longer covered by the 95% isodose line, but is covered by 92.9% isodose.
Additionally, the max hot spot of the plan increased to 122.3% and is in the back tissue (Figure
4). This change in coverage and max dose is mainly due to the increased MUs in the PA beam.
The PTV is now receiving more dose from the PA beam, but coverage is decreased because the
AP beam was better utilized in the original plan due to its location in the lung.
Conclusion:
This project demonstrates the importance of heterogeneity corrections in radiation
therapy treatments to the lung. As shown in the study by Engelsman et al1, inhomogeneity
correction algorithms are essential in the accuracy of treatment plans. Without proper correction
algorithms, target structures and OARs cannot be correctly evaluated for dose received in the
treatment planning system. In the project, heterogeneity corrections could have saved this patient
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from adverse radiation effects. Without proper evaluation of PTV coverage and lung and rib
dose, the treatment would not have met its goals. On one hand, the PTV could have been
drastically under-dosed, resulting in disease remaining in the patient. Additionally, the rib and
lungs could have been highly overdosed without the physician knowing. This could lead to
secondary malignancies or serious radiation pneumonitis. All things considered, it is evident that
planning with heterogeneity corrections is essential in treatment planning.
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References
1. Engelsman M, Damen E, Koken P, et al. Impact of simple tissue inhomogeneity
correction algorithms on conformal radiotherapy of lung tumours. Radiotherapy &
Oncology. 2001;60(3):299-309.
https://doi.org/10.1016/S0167-8140(01)00387-5
2. Khan FM, Gibbons JP. The Physics of Radiation Therapy. 5th ed. Philadelphia, PA:
Lippincott Williams & Wilkins; 2014.
3. Schneider U, Pedroni E, Lomax A. The calibration of CT Hounsfield units for
radiotherapy treatment planning. Physics in Medicine & Biology. 1996;41(1):111-124
https://doi.org/10.1088/0031-9155/41/1/009
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Figures
Figure 1. DVH plan comparison of the plan with and without inhomogeneity corrections turned
on. The lines with triangles represent the corrections on, and the squares represent the corrections
turned off. Notice how the rib and PTV get a much higher dose with corrections turned on.
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Figure 2. The axial view of isodose coverage in both plans. Notice the conformal shape of the plan with corrections turned off. The dose is evenly distributed no matter what type of tissue the beams are traversing.
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Figure 3. Isodose coverage of both plans in the sagittal view. Notice in the plan with corrections turned on that the anterior rib absorbs most of the dose and causes the 95% line to dip inferiorly compared with the 95% line of the uncorrected plan.
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Figure 4. An axial view of isodose coverage for the heterogeneity corrected plan when the MUs are set to the values from the uncorrected plan. Notice the significant change in beam weightings and how the PTV is no longer covered by 95% isodose.
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