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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.