From: Inez Petersen
To: jw@jhu.edu
Sent: Saturday, February 26, 2000 1:28 AM
Subject: acoustic-neuroma
Dear Dr. Williams:
Your willingness to answer our never-ending questions is much appreciated.
I was told that the dosage for IMRT at U of Texas was 5400 rads over 6 weeks. I know IMRT is not your specialty, but it just didn't feel right that a small acoustic neuroma such as mine would get the same amount of radiation as a larger one. Without as much mass to address, logically it would seem that the dosage would be less for small ANs verses medium and large size ANs.
I am undecided about which way to go between FSR and IMRT. Dosage and possible permanent nerve damage are of great concern to me.
Can you shed any light on my question of dosage verses tumor size?
Thanks,
Inez Petersen (aka webgirl)
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Subject: Radiation Dosage verses Tumor Size
Dear Inez Petersen,
Thank you for your message and your very important question. The issues include:
1) total dose and fraction size vs. the volume of the treated AN and
2) the method of delivery.
Dose and fraction size: For the acoustic neuromas, the facial nerve drives the equation to determine the safety of the treatment parameters. As the target volume of the facial nerve decreases, the benefit of fractionation (to 30 fractions) is negligible when compared to 5 or even 10 fractions of appropriate doses per each fraction. The benefit of fractionation occurs very early in the schedule and declines exponentially for increasing numbers of fractions. The difference between one and two fraction regimens is huge. The difference between 29 and 30 fraction regimens is negligible.
The biologically equivalent dose (BED) formalism relates total dose, dose per fraction, and the response of tissues (normal nerve vs. AN). For the 500 rad x 5 fraction regimen used for the Hopkins FSR, the BED dose equivalent is 67 Gy (This is not the given dose; rather it is the BED dose, or a means (formalism) for comparing disparate programs of fractionation). For the 5400 rad in 30 fractions, the BED dose is 86.4 Gy, a large increase over the 500 x 5 fraction regimen. Whether this dose is safe may be learned in the future. In its defense, the 5400 rad in 30 fractions is the "default" schedule for any tumor of any size, anywhere in the body, reminiscent of regular radiation therapy (big fields, imprecise delivery).
Method of delivery: The inverse treatment planning system and the Peacock delivery system may be useful for large, convex curved in on itself) tumors. The comparisons of dose homogeneity, dose to surrounding normal structures and conformality have been made. The Peacock system resulted in high homogeneity but higher dose to surrounding normal tissues (OAR: organs at risk; please see below).
The Peacock system uses a multileaf collimator to deliver the dose distribution using arc therapy and segmented fields. This means that an arc is delivered and the patient is moved longitudinally on the table where the next arc is delivered. This moving between arcs requires "registration" so that there is no overlap (resulting in higher dose than desired) or gap (lower dose than desired). Further, if the tumor size is small in relationship to the "vane" of the collimator (the vane opens and closes
to control dose), the dose-shaping ability of this device is reduced. Inverse treatment planning is very conformal for large, convex targets. Its implementation (different from simply planning) has many important variables that determine whether the plan is safely executed.
I hope this is helpful to you. Please respond with any questions that you may have. Thank you.
Jeffery A. Williams, M.D.
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Int J Radiat Oncol Biol Phys 1999 Sep 1;45(2):415-25
Comparison of Intensity-modulated Tomotherapy with Stereotactically Guided Conformal Radiotherapy for Brain Tumors
Khoo VS, Oldham M, Adams EJ, Bedford JL, Webb S, Brada M
Neuro-oncology Unit, The Institute of Cancer Research and the Royal Marsden NHS Trust, Sutton, Surrey, United Kingdom.
Please click HERE to access this research paper (already on my website)
File: DrWilliams.html
Date: Sat, 26 Feb 2000 18:51:49 -0500
From: "Jeffery A. Williams, M.D."
Director, Stereotactic Radiosurgery
Department of Neurosurgery
The Johns Hopkins Hospital
Harvey 811
600 North Wolfe Street
Baltimore, MD 21287-8811
Phone and Voice Mail: 410-614-2886
Fax: 410-614-2982
email: jw@jhu.edu
web: http://www.med.jhu.edu/radiosurgery