IMRT Virgin no more

[Ursus Martimus]

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I saw my first plans for IMRT today on my first rotation in RadOnc. Very cool! It was for post op radiation to head and neck ca. But I was someone confused by the fact that so much planning goes into the process, I.e contures of the head, and yet no corrections are made for different tissue densities within the fields. Apparently this is even more problematic when there are multiple interfaces such as in Lung tumors, ie bone then air then tissue. How much does this phenom change the actual dose the patient recieves?

 

[stephew]

I saw my first plans for IMRT today on my first rotation in RadOnc. Very cool! It was for post op radiation to head and neck ca. But I was someone confused by the fact that so much planning goes into the process, I.e contures of the head, and yet no corrections are made for different tissue densities within the fields. Apparently this is even more problematic when there are multiple interfaces such as in Lung tumors, ie bone then air then tissue. How much does this phenom change the actual dose the patient recieves?

Mazel tov. countouring is a special thing with IMRT isn't it?

 

[Htowngsp]

Ursus,
I believe some of the manufacturer's attenuation correction software extrapolates tissue densities directly from the Hounsfeld units of the planning CT DICOM files and adjusts accordingly, so the issue of tissue inhomogeneity is reduced, but not fully eliminated.

I am pretty sure that most planning software has a density specification parameter within the code, though most folks just designate dose parameter restrictions for designated voxels via contouring, as Steph said.

 

[JimDiGriz]

I saw my first plans for IMRT today on my first rotation in RadOnc. Very cool! It was for post op radiation to head and neck ca. But I was someone confused by the fact that so much planning goes into the process, I.e contures of the head, and yet no corrections are made for different tissue densities within the fields. Apparently this is even more problematic when there are multiple interfaces such as in Lung tumors, ie bone then air then tissue. How much does this phenom change the actual dose the patient recieves?

IMRT is great! Few places do 'true' IMRT, because most of the time not the radiation intensity is varied (linac's output is fixed), but rather time*intensity product is the variable (sliding window technique, etc). Besides contouring and using HU from CTs, it also depends whether the dose calculation algorithm accounts for the varying attenuation properties of tissues. Most of the dose calculation algorithms a few years ago didn't account for that, but some newer programs use Monte Carlo techniques, which will get you pretty accurate results. I've been out of the field for the last few years (went into imaging/radiology research for my thesis), so my knowledge isn't up-to-date, unfortunately. I was a grad student in a medical physics program, now starting medical school.

 

[stephew]

actually i doubt that the differences in tissue density in the head and neck are as extreme as in the lung; lung corrections historically haven't been made. Remember photons in the compton range are more concerned with electron density that tissue density per se. and the air spaces in the H&N are far less than in the lung.