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Kayla Tedrick

Treatment Planning Project

April 22, 2018

The Effects of Heterogeneities in Lung Treatment Planning

Introduction: The presence of tissue inhomogeneities produces changes in the interactions of

the radiation beam as it travels through the patient. Fat, bone, lung, and air must be taken into

account based on their ability to affect the absorption of the primary beam and alter secondary

electron fluence.1 Due to the fact standard isodose charts and depth dose tables are based off of a

patient with uniform electron density like water, corrections must be made in order to accurately

represent those interactions with other various tissues. In the past, either no corrections were

made or the methods of correction lacked in precision and time efficiency. However, computed

tomography (CT) has allowed improvements in these corrections by reconstructing the planning

image into voxels that are assigned a certain CT number related to their attenuation coefficient.

The current treatment planning systems contain algorithms that take these attenuation values and

create an accurate 3-dimensional representation of the radiation beams traveling through the

patient’s tissue inhomogeneities. The purpose of this project was to evaluate how treatment

planning with inhomogeneity correction algorithms compares to treatment planning without any

corrections in treatment of a lung tumor.

Process: Utilizing the Varian Eclipse treatment planning system, two treatment plans were

created using a previously treated patient’s CT scan. The patient had a tumor in the anterior

portion of the right lung and received stereotactic body radiation therapy (SBRT). Positioning

consisted of a full-body vacbag with arms up above the head holding onto hand pegs attached to

a wingboard. These devices along with the indexing table top were excluded from the body

contour for this project to enhance the focus on the effect of the patient’s tissue inhomogeneities.

The planning medical dosimetrist contoured various organs at risk (OARs) including the heart,

left and right lung, spinal cord, and chest wall. The doctor contoured the planning target volume

(PTV) which was utilized in the following two plans created for this project.

In each plan, the isocenter was placed in the center of the PTV. Two identical beams

were placed on each plan, an anterior beam with the gantry placed at 0 degrees and a posterior

beam with the gantry placed at 180 degrees, applying a parallel-opposed approach for simplicity.

Multileaf collimators (MLCs) were fit to the PTV with a 0.5 cm margin. Both beams used a low

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energy of 6 MV. The plans were each normalized to achieve 100% at the isocenter, with a dose

prescription of 60 Gy to be delivered in 30 fractions, or 200 cGy per fraction.

The first plan, named Hetero_Off, did not employ any correction algorithms. Hetero_Off

assumed a homogeneous density relative to water. The second plan was named Hetero_On_MU

and did use correction algorithms. The monitor units (MUs) for each beam were adjusted on this

plan to match the MUs of the Hetero_Off plan (Figures 1-2). Hetero_On_MU presents the

treatment the patient actually would have experienced undergoing the Hetero_Off plan.

Results: Differences between the two plans became immediately apparent when comparing them

in the plan evaluation window (Figures 3-5). In appearance, the isodose lines of the Hetero_Off

plan seemed smoother and had a clear path. On the other hand, the idodose lines of

Hetero_On_MU were jagged and had a more random path throughout the patient’s tissue. Also

evident on the Hetero_On_MU plan was the extreme narrowing of the 50%-100% idodose lines

in the central right lung. However, in comparison to the other plan, the higher isodose lines on

this plan did seem to make it deeper into the patient before ultimately tapering down near both

beam entrances.

Furthermore, the Hetero_On_MU plan was significantly hotter with an area receiving

over prescription dose as high as 136%. The area with the highest dose on the Hetero_Off plan

was recorded to be 126.4% of the prescription dose. These areas on both plans were located on

the posterior aspect of the patient, lying immediately posterior to the scapula (Figure 3).

There were even more inconsistencies when evaluating the dose volume histogram

(DVH) overlapping both plans, such as in coverage of the PTV as well as dose to OARs (Figure

6). For the first plan, Hetero_Off, 95% of the PTV was only receiving 93.3% of the prescription

dose as compared to receiving 97.3% for the Hetero_On_MU plan. Also, due to the isodose lines

on the Hetero_On_MU plan traveling deeper into the right lung, the right lung received a higher

average dose as well as maximum dose (Figure 3).

Discussion: Comparing these two plans demonstrates the effects heterogeneity corrections have

on treatment planning. As previously mentioned, assuming all tissue within the patient’s body is

equivalent to water is inaccurate, and the Hetero_On_MU displays the actual treatment delivered

if Hetero_Off was to be treated. The effects of inhomogeneities can be explained by how dose

interacts with certain tissues in the body. For instance, the narrowing of the 50%-100% isodose

lines in the central right lung of the plan with corrections is due to the loss of lateral electronic

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equilibrium (Figures 3 and 5). When the beams hit lung tissue, more electrons scatter laterally

outside of the field than are backscattered back within the field, therefore, there is a decrease in

dose in the periphery of the fields.1

Another effect lung tissue caused was the increase of dose to the right lung as seen by the

100% isodose line moving deeper into the body on Hetero_On_MU compared to Hetero_Off

(Figure 3). According to the isodose shift method, isodose curves should be shifted deeper into

the patient when inhomogeneities are present that are less electron dense than water, such as lung

tissue, due to their lack of attenuation.1

The area of maximum dose on both plans having been located in the posterior aspect is

due to the posterior beams delivering more MUs in order to cover the isocenter that was placed

inside the PTV, which is more anterior (Figures 3 and 5). However, as previously mentioned,

Hetero_On_MU had a significantly higher maximum dose. This is due to the corrected plan

recognizing the scapula as being considerably denser, which causes an increase in dose to the

adjacent soft tissue on the entrance side of a photon beam.1 Density of bone results in enhanced

electron backscattering of short range, therefore, the area of maximum dose on Hetero_On_MU

was greater.

Increased coverage of the PTV and increased right lung dose in the Hetero_On_MU plan

is concerning seeing uncorrected treatment plans are delivered in the real world. As Hetero_Off

would be the plan that what seems is being delivered, Hetero_On_MU is what would actually be

delivered, and ultimately would be overdosing the patient. A similar study of 15 patients found

their uncorrected plans had lung tumor doses decreased by 13%, 8%, and 6% in average

minimum, mean, and maximum, respectively.2 “Lung tissue filled with air is significantly less

dense than other body tissues, the failure to use heterogeneity corrections creates plans which

overdose the target because the planning system optimizes beam paths assuming more

attenuation than actually occurs.”2 In another study done by Xiao et al,3 corrected SBRT lung

treatment plans were compared to uncorrected plans once again. These corrected plans, however,

were evaluated with their original MUs when normalized to cover 95% of the treatment volume.

On average, it was found the corrected plans delivered less prescription dose to the target by

10%.3 In concluding their study, the authors determined that prescription doses need to be

altered, possibly hypofractionated, when heterogeneity corrections are utilized in order to

achieve optimal tumor lethal dose.

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Conclusion: In a perfect world, treatment planning could be done using a homogeneous

attenuation coefficient throughout CT scans. Unfortunately, this does not represent what is

actually occurring when photons travel through patients. Multiple studies have shown, just as

this project did, that planning with no inhomogeneity corrections results in overdosing the

patient. It was vital that advanced algorithms came about to correct these inhomogeneities, but

more improvements are sure to come. This demonstrates that patient care is the top priority, and

it is our obligation that the treatment plans delivered to them are precise.

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Figure 1. Treatment plan summary for Hetero_Off plan showing field parameters as well as their monitor units.

Figure 2. Treatment plan summary of Hetero_On_MU plan showing field parameters as well as their monitor units. Note these monitor units were adjusted to match the Hetero_Off plan.

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Figure 3. Axial view of Hetero_Off plan (left) and the Hetero_On_MU plan (right).

Figure 4. Coronal view of Hetero_Off plan (left) and the Hetero_On_MU plan (right).

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Figure 5. Sagittal view of Hetero_Off plan (left) and the Hetero_On_MU plan (right).

Figure 6. DVH including both Hetero_On_MU plan and Hetero_Off plan. The lines with

triangles indicate the Hetero_On_MU plan and the lines with squares indicate the Hetero_Off

plan.

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References

1. Khan FM, Gibbons JP. Khan’s The Physics of Radiation Therapy. 5th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2014.

2. Herman T, Gabrish H, Herman TS, Vlachaki MT, Ahmad S. Impact of tissue heterogeneity corrections in stereotactic body radiation therapy treatment plans for lung cancer. J Med Phys. 2010;35(3):170-173. http://dx.doi.org/10.4103/0971-6203.62133

3. Xiao Y, Papiez L, Paulus R, et al. Dosimetric evaluation of heterogeneity corrections for RTOG 0236: stereotactic body radiotherapy of inoperable stage I-II non-small-cell lung cancer. Int J Radiat Oncol Biol Phys. 2009;73(4):1235-1242. http://dx.doi.org/10.1016/j.ijrobp.2008.11.019