Academic Courses > DOS 522 > Heterogeneity Correction
DOS 522 - Week 1 Discussion
Writing Prompt
You have read the about homogeneity and heterogeneity corrections. Talk
to your preceptors and/or clinical instructors about the effect each
can have on a treatment plan. Does your site use heterogeneity or
homogeneity? Why or why not?
Initial Post: Calculating Dose With or Without Heterogeneity Correction
In conversations with dosimetrist Stacy Watkins (January 21, 2015) and physicist Mike Mysz (January 21, 2015), I learned about a few scenarios in which heterogeneity corrections might or might not be used. Heterogeneity corrections account for the differences in the interactions that radiation has with different types of tissues in terms of attenuation and scattering. Low density tissue like lung, which is mostly air by volume, does not attenuate or scatter the beam nearly as much more dense tissues like muscle, organs, or fat. The reverse is true of bone. These differences in interaction can not be accounted for in simple models that are based purely on depth in the body, because those models assume the body to be of uniform density equal to water.
Stacy and Mike gave examples of times when heterogeneity correction is valuable, such as in lung or mediastinum treatments. Dose to lung may be overestimated without heterogeneity correction because there is less scatter contribution than the simple model would predict. Dose to the mediastinum may be underestimated without heterogeneity correction because the beam may have passed through a large depth of low-attenuation lung tissue.
Stacy pointed out that some treatments, such as a patient we recently treated for heterotopic ossification around the hip joint, do not need heterogeneity correction because the hip does not have air pockets and the bones have not been found to throw the measurements sufficiently to warrant a more complex plan. For that patient, we did a simple measurement of separation, and then did a simple calculation to deliver the prescribed dose to the prescribed depth.
Mike brought up a good point about CT artifacts caused by high density objects in the field of view such as metal implants. In these cases, the CT's reconstruction algorithm can not correctly determine the density of tissues along rays that include the metallic objects, so high and low density starburst patterns are often superimposed over the image, making other nearby tissues appear more or less dense than they really are. In scenarios such as a pelvic treatment for a patient with a metal hip replacement, the CT density artifacts thrown by the implant will skew the expected dose one way or another in unpredictable ways if heterogeneity corrections are used. Since we usually override bowel gas to water density anyway, and since we would normally override small density artifacts as well, Mike has tested several scenarios and has found that for pelvic treatments of patients with hip protheses, as long as the treatment site is not in close proximity to the implant, we can do our dose calculations without heterogeneity correction and get results that are very close to what they would have been if we had done all of the overrides and used heterogeneity correction.
Stacy and Mike gave examples of times when heterogeneity correction is valuable, such as in lung or mediastinum treatments. Dose to lung may be overestimated without heterogeneity correction because there is less scatter contribution than the simple model would predict. Dose to the mediastinum may be underestimated without heterogeneity correction because the beam may have passed through a large depth of low-attenuation lung tissue.
Stacy pointed out that some treatments, such as a patient we recently treated for heterotopic ossification around the hip joint, do not need heterogeneity correction because the hip does not have air pockets and the bones have not been found to throw the measurements sufficiently to warrant a more complex plan. For that patient, we did a simple measurement of separation, and then did a simple calculation to deliver the prescribed dose to the prescribed depth.
Mike brought up a good point about CT artifacts caused by high density objects in the field of view such as metal implants. In these cases, the CT's reconstruction algorithm can not correctly determine the density of tissues along rays that include the metallic objects, so high and low density starburst patterns are often superimposed over the image, making other nearby tissues appear more or less dense than they really are. In scenarios such as a pelvic treatment for a patient with a metal hip replacement, the CT density artifacts thrown by the implant will skew the expected dose one way or another in unpredictable ways if heterogeneity corrections are used. Since we usually override bowel gas to water density anyway, and since we would normally override small density artifacts as well, Mike has tested several scenarios and has found that for pelvic treatments of patients with hip protheses, as long as the treatment site is not in close proximity to the implant, we can do our dose calculations without heterogeneity correction and get results that are very close to what they would have been if we had done all of the overrides and used heterogeneity correction.
Academic Courses > DOS 522 > Heterogeneity Correction
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Written January 21, 2015
Second Semester, 1 Month into Internship |