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Julie Puzzonia

April 23, 2018

Treatment Planning Project: Inhomogeneities in Lung Planning

Objective: To determine the difference between heterogeneous and homogeneous calculations

for an isolated lung tumor.

Purpose: As radiation traverses through different tissues within the body, it is attenuated

depending on the density of the medium. A radiation beam travelling through a very dense

material such as bone will have a higher attenuation than a beam travelling through lung.

According to Khan1, the effect of tissue inhomogeneities are classified based off absorption of

the primary beam and scatter patterns or changes in secondary electron fluence.

When considering the lung, electron fluence can be of importance when assessing dose

near the lung-tissue interface. The fluence of electrons helps with the build-up and build-down

effect, or interface effect, of the radiation beam passing through the varying tissue types.2 In

general, a build-up region is necessary for the radiation to reach electronic equilibrium before

dose is deposited within the tissue. By introducing a large air cavity, such as the lung, in the

pathway of the beam, the radiation loses some of its electronic equilibrium. Thus, radiation doses

tend to be higher within the lung, and then decrease within the first few layers of soft tissue

beyond the lung.1

This realization can be seen in the comparison of a lung plan with a homogeneous versus

heterogeneous calculation method. The homogeneous plan will consider the patient to be

uniformly made of tissue-equivalent material and ignore the inhomogeneity effects of the various

tissue types found in the body, such as the lung. Meanwhile, the heterogeneous plan will account

for all densities, including the lower density of the lung tissue and more closely portray the effect

of tissue inhomogeneities in treatment planning.

Methods and Materials: This planning lab was completed using Philips Pinnacle planning

system (version 14.0). The patient selected had a lung tumor that does not touch the mediastinum

or chest wall to provide an appreciable tissue-lung interface. The gross tumor volume (GTV) and

planning target volume (PTV) were contoured by the physician for treatment planning. The

normal structures including the heart, right lung, left lung, cord, and skin were contoured by the

medical dosimetrist. The isocenter was placed centrally inside the PTV and a plan was then

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generated using an AP/PA field arrangement with equally weighted fields. The blocks created for

each beam had a 2 cm margin surrounding the PTV. (Figure 1) For simplicity, a palliative

prescription was chosen to display a total dose of 3000cGy prescribed in 10 fractions,

normalizing to the isocenter. The default for this planning system is to perform a heterogeneous

calculation which accounts for the various tissue densities. After calculating the beams, this trial

was then copied, keeping the same field arrangement, prescription, and normalization method to

perform a homogeneous calculation. There is a setting in the treatment planning system within

the dose computation parameters to change the calculation method and make primary and scatter

doses homogeneous. After changing these parameters, an analysis of the isodose distribution to

the tumor volume was performed for each plan.

Results: The initial heterogeneous plan shows that 39.1% of the PTV is covered by 100% of the

dose. At 2850cGy or 95% of the prescription dose, only 97.1% of the PTV is covered. There are

hot regions on both anterior and posterior surfaces of the patient at beam entries, which is

expected with an AP/PA field arrangement. Although the traditional hourglass shape of the

isodose lines for the AP/PA treatment is apparent, there is a noticeable indentation of the lateral

isodose lines in the area near the mediastinum and lateral chest wall. (Figure 2) The maximum

dose for the heterogenous plan is located on the anterior surface of the patient with a dose of

3291cGy or 109.7% of the prescription. The total monitor units (MU’s) calculated for this plan

are 174.6 MU for the AP beam and 183.1 MU for the PA beam. (Figure 3)

The homogeneous plan differs in that the coverage of the PTV receiving 100% of the

dose has decreased to 37.5%. However, the percentage of the PTV receiving 95% of the

prescription dose has increased to 99.7% from the heterogeneous plan. The isodose lines appear

to be very hot anterior within the patient. (Figure 4) This is expected due to the posterior location

of the isocenter and tumor; the AP separation of the patient is greater than in the PA direction.

Thus, if this calculation method were to be used for treatment, adjustment of beam weights

would be necessary to draw the dose more posterior with a more heavily weighted PA field.

However, for comparison purposes, beam weightings were left constant between plans, leaving

the hot region in the anterior portion of the patient. This maximum dose region had a reading of

3392cGy, which is 117% of the prescription dose. (Figure 5) The homogeneous plan also

showed an increase in the number of MU’s required for the AP beam, with 200.8 MU for the AP

beam and 182 MU for the PA beam. (Figure 6)

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A comparison was also completed for the normal structures in the treatment area by

analyzing the dose-volume histogram (DVH) associated with each plan. (Figure 7-8) The

heterogeneous plan showed lowered max doses to organs at risk (OAR’s) such as the right and

left lung, and cord. (Table 1) This analysis is justified when comparing the isodose lines of each

plan. In general, the heterogenous plan was less hot than the homogeneous plan. This follows

suit with the other observations made with the heterogeneous plan showing less total MU’s and a

decreased maximum dose point.

Discussion: As previously mentioned, the two effects of tissue inhomogeneities involve photon

beam attenuation and secondary electrons. When a radiation beam encounters the low density of

lung tissue, there is a greater loss of laterally scattered electrons, which causes a reduction of

dose on the central axis.1 This is seen in the heterogenous plan where the isodose lines display an

indentation toward the central axis. Due to this lateral scatter, there is the potential for a loss of

coverage to the PTV.

When considering the homogeneous plan, the determination of under or over-estimating

target dose depends on which of the two effects of tissue inhomogeneity has a larger impact on

the plan. In this scenario, the homogenous calculation method sets the physical and effective

depths equal. The result is apparent in the isodose lines of the homogenous plan showing a hot

region near the anterior surface. Since the isocenter is located posterior, there is an

overestimation of photon beam attenuation and therefore an underestimation of dose at the depth

of the tumor.3 This explains the decreased coverage of the 100% isodose line when comparing

the two plans. The effects of ignoring lateral scatter loss can also be seen within the isodose

distribution in the homogenous plan by showing an overestimation of dose laterally within the

treatment field. This correlates with the increased coverage seen by the 95% isodose line in the

plan comparisons. (Figure 9)

With this specific tumor location and the results from the plan comparisons, the effect of

primary beam attenuation in the homogenous plan seems to be the more prominent factor in dose

calculation. This effect accounts for the increased MU’s of the AP beam, the increased hot spot,

and lack of target coverage at depth of the prescription dose in the homogeneous plan.

Ultimately, this can lead to a fair amount of dosimetric uncertainty in the representation of the

actual dose delivered.

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Clinical Application: Currently, my clinical site plans lung cases by accounting for tissue

inhomogeneities. Therefore, there are no corrections to densities within the lung during planning.

This is done to try to get the most accurate representation of dose delivered and avoid potential

situations of over or under-dosing the target and surrounding normal tissues. My site is also

conscious when selecting beam energies for lung treatments due to the inhomogeneity effects

within the lung. According to the American Association of Physicists in Medicine (AAPM), 4 a

higher energy beam has a longer range for electrons to travel within a low density medium

resulting in increased lateral electron disequilibrium and decreased target coverage. Therefore,

my clinical site tries to maintain the lowest energy when planning lung treatments.

Some instances that an air cavity might be corrected for during treatment planning

involves structures that fluctuate with daily treatment. Since the low density of the lung tissue is

constant, the dose calculations to lung masses are performed reflecting tissue inhomogeneities.

However, with other air cavities that vary in density with daily treatment, a density correction is

performed. One application of overriding the density to an air cavity would be to correct for the

tissue inhomogeneity within the bowel. It is uncertain whether or not the air spaces within the

bowel will be in the same position or occur daily with treatment, therefore the dosimetrists will

override the structure to a density of 1 g/cm3, equating it to the value of soft tissue.

Conclusion: Overall, tissue inhomogeneities are a common encounter with treatment planning

and can occur across multiple anatomical areas of the body. It is important to understand the

effects of how radiation interacts with different tissues to understand how best to deliver dose to

a target area. As a dosimetrist, it is important to understand which calculation algorithm will

most accurately depict the treatment being delivered to the patient. Any misrepresentations could

lead to scenarios of under-dosing the target or over-dosing surrounding normal tissues. Having a

solid understanding of these principles will aid in determining the optimal treatment plan for a

patient.

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References:

1. Khan FM. The Physics of Radiation Therapy. 5th ed. Philadelphia, PA: Lippincott

Williams & Wilkins; 2014.

2. McDermott PN, Orton CG. The Physics & Technology of Radiation Therapy. Madison,

WI: Medical Physics Publishing; 2010.

3. Altunbas C, Kavanagh B, Dzingle W, Stuhr K, Gaspar L, Miften M. Dosimetric errors

during treatment of centrally located lung tumors with stereotactic body radiation

therapy: Monte Carlo evaluation of tissue inhomogeneity corrections. Med Dosim.

2013;38(4):436-441. http://dx.doi.org/10.1016/j.meddos.2013.06.002.

4. Papanikolaou N, Battista JJ, Boyer AL, Kappas C, et al. Tissue inhomogeneity

corrections for megavoltage photon beams. AAPM.

https://www.aapm.org/pubs/reports/RPT_85.pdf. Published August 2004. Accessed April

22, 2018.

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Figures

Figure 1. Anterior and posterior treatment fields used for planning with a block margin of 2 cm surrounding the PTV (blue outline).

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Figure 2. View of isodose distribution for heterogenous AP/PA plan. Note: lateral indentations of isodose lines near mediastinum and lateral chest wall.

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Figure 3. Monitor unit depiction for AP/PA fields of heterogenous plan.

Figure 4. Isodose distribution for homogenous AP/PA plan. Note 110% isodose lines (red) near anterior surface of patient.

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Figure 5. Side-by-side comparison of maximum dose point for heterogeneous plan (left) and homogeneous plan (right).

Figure 6. Monitor unit depiction for AP/PA fields of homogeneous plan. Note increase in monitor units for AP beam.

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Figure 7. Dose-volume histogram representing the heterogeneous lung plan.

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Figure 8. Dose-volume histogram representing the homogenous lung plan.

OAR Heterogeneous Max Homogeneous Max Percent

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Dose (cGy) Dose (cGy) Difference

Cord 159.4 190.1 +19.3%

Heart 187.1 176.5 -5.7%

Rt_Lung 3233.9 3362.3 +4.0%

Lt_Lung 104.6 123.0 +17.6%

Table 1. Comparison of max doses for organs as risk between heterogenous and homogenous calculated plans with percent difference.

Figure 9. Side-by-side comparison in plane of isocenter for heterogenous (left) versus homogenous plan (right). Note changes in coverage of the 95% isodose line (dark blue) between plans.