Here is a exchange of posts with Dr. Ting about External Beam Radiation and when rads are rads.

Question by Don Cooley:

Could we assume that radiation is radiation when it is given over the same time period? For example if I gave 20 EBRT ZAP's is that the same effect as 20 3D-EBRT, or 20 Photons, etc. Of course one would have to assume that the all were radiating basically the same location assuming that is possible. If these ZAP's were given every day for a week (4 weeks), would the effect be the same on the cells and tissue?

Answer by Dr. Ting:

You are correct, these "ZAPs" are identical if they are given, say, 80 cGy per day for 20 fractions in each case. However, 3D-EBRT can < potentially > (this is the most important word in this sentence) reduce the irradiated volumes of healthy organs, such as bladder and rectum. The 3D-EBRT uses very tight treatment margins via tight blocks and many treatment portals to have the radiation doses tightly conformed to the intended target. Since there are less volume of the healthy tissues, or critical organs in the irradiated volume, 3D-EBRT can increase the dose to the intended target by quite a few fractions over the regular EBRT. Thus, 3D-EBRT is better than the regular EBRT. Needless to say, it must be done right and with great care otherwise tumor can be missed or under treated. Nothing is worse than having the original tumor re-grow at the same site.

Question by Don Cooley:

We commonly give ZAP's every day for a 5 days and two days off - is there anything magic about this? Could it be every day for two weeks, or every other day for two weeks, etc.? Is the week schedule just a convenience for the administration of the ZAP's? If we want the tissue to recover and not do as much damage - why not 5 days on and 9 days off - or some such schedule?

Answer by Dr. Ting:

You got it....! There is no magic for 5-day Vs 7-day ZAPs. The reason is human logistics (i.e.: weekends ...). Though there were some studies done long time ago for 4-day treatments Vs 5-day treatments, I do not remember what the outcome was. I do not know of any studies dealing with 7-day treatments. I should say, occasionally, we would treat someone over the week-end if the patient has the first treatment started on Thursday or Friday. It not beneficial to the patient to receive one or two fractions of radiation and have a long weekend treatment break. In general, we will try to avoid situations like that.

About 9 days off: O, boy, this is a tough topic. Again, you are correct, if the treatment course has a long break in between, the BIOLOGICAL dose is lessened. Here is a fine example of "when a ZAP is not a ZAP". 3600cGy is really not 3600cGy. Thus, addition fractions of radiation must be added to compensate for the long treatment break. The concept of TDF (Time, Dose Factor) has been around for quite some time. In (very, very) general kind of hand waving arithmetic's, 1 additional fraction should be added for each 7-day un-planned treatment break in the middle of the treatment. Of course, this totally depends on the clinicians.

Question by Don Cooley

I understand the 3D should be less damaging because it can concentrate toward the prostate without going elsewhere. Brings up another question. If we are giving 180uGy per ZAP does it ZAP the field equally. do we get 180uGy in the prostate and 180 in the surrounding tissue. If we concentrate the area by 3D does this mean that the prostate gets more uGy or still the 180uGy. Can the machines concentrate more to one area had less to another? If so would the 180uGy always be the most any area could get? Would Photon be the same?

Do those question make sense to you. To say it another way - if we concentrated the beam down to a point the size of a pin head - would that pin head just get 180cGy?

One last question. How far around the prostate would be treated with 3D vs EBRT if all things were equal and the operators well trained and equal in skills.

Answer by Dr. Ting

A). 3D EBRT

Yes, it is correct. 3D EBRT < potentially > can concentrate more radiation dose to the target and spare the normal, health organs which are ever so close to the target. For example, in a well executed 3D EBRT, the prostate organ plus suitable margin (to account for possible organ movements and/or day-to-day setup uncertainties) around the prostate could be taken to a minimum dose of 7920 cGy (nationwide clinical protocol dose level in RTOG-9406) and keep the doses to majority of the bladder and the rectum to below 5580cGy. Radiobiology plays a big role here. The center of the target gets daily fractional dose of around 190cGy and the surrounding organs get daily fractional dose of around 120cGy. Therefore, there is a significant benefit to critical organs, which we wish to protect, because of the lower DAILY FRACTIONAL doses.

B). A bridge too far ... ?

There is no clear answer to this question (actually, in most medicine related questions, there is no clear answer). Most of us use a margin of 1.5 cm around the prostate, except around the rectum (we use 1.0 cm) and around the bladder (we also use 1.0 cm). Side effects of treatments are directly related to the volumes and the doses of these organs being irradiated. Obviously, one should try to minimize these parameters. However, knowing that the prostate position could change from day to day and there is a daily setup uncertainties associated to each daily setups, we must add a margin to the prostate and call this enlarged volume (prostate plus margin) the "planning target volume". And, we try to deliver a large dose of radiation to this planning target volume (7920cGy).

Joseph Ting

josepht@mail.uch.org