Brittany Check

Treatment Planning Project

March 4, 2015

Heterogeneity Correction Project

Objective: Compare AP/PA lung plans calculated with and without a heterogeneity correction

factor.

Materials and Methods: A plan was developed using the analytical anisotropic algorithm

(AAA) in the Eclipse treatment planning system. The plan was optimized using a heterogeneity

correction factor for a patient using AP and PA beams. A 2 cm margin was given around the

tumor. Dose was calculated to the center of the tumor volume. Without changing anything else,

the heterogeneity correction factor was turned off and the plan was recalculated for comparison.

Results: Several differences can be noted between the heterogeneous and homogeneous plans.

Isodose lines appear more hourglass-shaped in the heterogeneous plan but do not bow in on the

homogeneous plan (Figures 1 and 2). Tumor coverage is better for the homogenous plan as noted

by the dose volume histograms (DVHs) (Figures 3 and 4). The mean tumor dose for the

heterogeneous plan is 6721.8 cGy and 6904.5 cGy for the homogeneous plan. The MU

calculated for the heterogeneous plan was for the 114 AP beam and for the 164 PA beam. For the

homogeneous plans, the MU calculated was for the 133 AP beam and for the 176 PA beam.

(Figures 5 and 6). Another difference that can be noted is the higher isodose lines superiorly in

the patient in the homogeneous plan as a result of the 15 degree wedge placed.

Discussion: When comparing an AP/PA plan with and without a heterogeneity correction factor,

several differences can be noted. When accounting for heterogeneities and the low electron

density of lung tissue, isodose lines pull in around lung tissue. Changes in dose in lung tissue can

be accounted to changes in attenuation of the primary beam as well as scattering characteristics.1

Compared to water, lung tissue has a lower electron density. This causes a decreased likelihood

of interaction, causing electrons to travel farther between interactions. Therefore, less dose is

deposited in the lung tissue as shown by the bowing of the isodose lines. Furthermore, the dose

distribution may be more spread out if electrons are scattered at a steep angle and travel farther

before interacting.

Another difference that can be noted is the tumor coverage between the heterogeneous and

homogeneous plans. The heterogeneous plan has a lower dose delivered to the tumor volume

than the homogeneous plan. This effect may be attributed to a loss of electronic equilibrium in

the lung tissue.1,2,3 When the beam encounters tissue after loss of electronic equilibrium, a

buildup region will occur similar to that near the surface of the patient. In the case of a lung

tumor, this buildup region will occur as the beam enters the tumor from the low density lung

tissue. It can be important to take this into account by using lower energies. Using lower beam

energies decreases the buildup region and provides better tumor coverage.4 Additionally, as the

beam exits the tumor tissue and enters lung tissue, the backscatter from the lung tissue is less so

dose in the adjacent tumor is less.

Another difference that can be observed from the homogeneous and heterogeneous plans is the

difference in the MU. In the heterogeneous plan, the calculated MU are lower than the calculated

MU for the homogeneous plan. This is because with a heterogeneous calculation, the effective

depth is used rather than only the physical depth.2 Effective depth takes tissue density into

account and assigns the effective depth based off of the depth of water that would cause the same

amount of attenuation. The density of water and soft tissue is near 1.0 g/cm3 while the density of

lung tissue is near 0.2-0.33 g/cm3.1,3 Therefore, the effective depth and attenuation from 10 cm of

lung tissue may be equal to only 3 to 5 cm of water. Therefore, if each beam is traveling through

a portion of lung, the effective depth used in the heterogeneous calculation will be lower than the

depth used for the homogeneous calculation. If the effective depth is lower, fewer MU will be

needed to deliver the same dose.

The used of effective depth can also help to explain the wedge orientation used in the

heterogeneous plan and how this would change if it were to be planned homogeneously. In the

heterogeneous plan, a wedge was placed with the heel inferiorly. On the superiorly aspect of the

field, there is more tissue to travel through before reaching lung tissue than on the inferior aspect.

Therefore, the effective depth is greater on the superior aspect and the wedge evens out the dose.

However, when the heterogeneity factor is turned off, the physical depth is only taken into

account. The superior aspect has a lower separation than the inferior aspect, so it appears as

through the heel of wedge should be placed superiorly.

Conclusion: Overall, planning with or without a heterogeneity correction has a significant effect

on the isodose lines when planning an AP/PA lung treatment. Different electron densities cause

various tissues to attenuate the beam differently. Changes may be seen within low density tissue

as well as near interfaces between tissues with differing densities. Using a lower energy MV

beam can help to avoid underdosing lung tumors. Furthermore, effective depths that are

accounted for with heterogeneity correction factors affect beam weighting and wedges that may

need to be used. Lower tissue density affects attenuation like a lesser depth in water. These

principles are important to consider when planning lung cancer treatments. Whether planning

with or without accounting for heterogeneities, it is important to understand which information

the planning algorithm uses and how heterogeneities affect dose distribution.

References

1. Bentel GC. Radiation Therapy Planning. 2nd ed. New York, NY: McGraw-Hill; 1996.

2. Khan FM. The Physics of Radiation Therapy. 4th ed. Baltimore, MD: Lippincott, Williams,

and Wilkins; 2010.

3. Orton CG, Chungbin S, Klein EE, et al. Study of lung density corrections in clinical trial

(RTOG 88-08). Int J Radiat Oncol Biol Phys. 1998;41(4):787-794.

4. Klein EE, Morrison A, Purdy JA, Graham MV, Matthews J. A volumetric study of

measurements and calculations of lung density corrections for 6 and 18 MV photons. Int J

Radiat Oncol Biol Phys. 1997;37:1163-1170.

Figures

Figure 1. Axial, sagittal, and coronal view of AP/PA lung treatment plan calculated with

heterogeneity correction factor.

Figure 2. Axial, sagittal, and coronal view of AP/PA lung treatment plan calculated without

heterogeneity correction factor.

Figure 3. DVH for AP/PA lung treatment plan calculated with heterogeneity correction factor.

Figure 4. DVH for AP/PA lung treatment plan calculated without heterogeneity correction

factor.

Figure 5. Physics report displaying MU for AP/PA lung treatment plan calculated with

heterogeneity correction factor.

Figure 6. Physics report displaying MU for AP/PA lung treatment plan calculated without

heterogeneity correction factor.