DOS 513 - Week 4 Discussion
Initial Post: Contouring for Glioblastomas
In conversation with Chief Dosimetrist Kent McCune (December 10, 2014), I learned that a typical 3D treatment plan would include the following structures:
- Right/Left Eye
- Right/Left Optic Nerve
- Optic Chiasm
- Right/Left Cochlea
- Standard target volumes (GTV, CTV, PTV)
- Brain minus PTV
- Metal and density overrides if necessary
If the patient is involved in a research protocol such as RTOG 0825, several other organ at risk structures will have to be defined,1 including:
- Right/Left Lens
- Right/Left Retina
The reason that these structures need to be contoured is so that radiation tolerances of these sensitive organs can be tracked and so that planned dose can be kept below tolerance limits. If lens tolerance dose is exceeded, there is a risk of developing cataracts, and if retina, optic nerve, or optic chiasm tolerance doses are exceeded, there is a risk of blindness. A contour consisting of the brain minus the PTV is used to try to quantify sparing of the uninvolved portions of the brain.
In protocols such as RTOG 0825, the margins around the visible tumor are very large and sparing of nearby tissues can be quite a challenge, even with advanced techniques such as IMRT or VMAT. At SCCA Proton Therapy, we usually contour a few additional structures, in order to gather additional data about the utility of proton therapy. Additional structures that we usually track include:
- Right/Left Lacrimal Gland
- Brain Stem Core (3mm or deeper from surface)
- Brain Stem Surface (a 3mm rind)
- Right/Left Temporal Lobe
- Right/Left Hippocampus
- Pituitary Gland
In cases where the lesion is primarily on one side of the brain, we can attempt to spare contralateral structures by taking advantage of the proton beam's Bragg peak.
- RTOG 0825 protocol information. RTOG Website. http://www.rtog.org/ClinicalTrials/ProtocolTable/StudyDetails.aspx?study=0825. Updated June 30, 2014. Accessed December 10, 2014.