ínr. J Rodimron Onrology Bio/. Phys.. Vol. 6. pp. 785-787 0 Pergamon Pres Ltd.. 1980 Printed in the U.S.A.

0360-3016/80/060785-03102.00/0

??Total Body Irradiation Conference

DISCUSSION: THE RADIOBIOLOGICAL BASES OF TB1

LESTER PETERS, M.D. M.D. Anderson Hospita1 and Tumor Institute, Houston, TX 77030

The aim of TBI, like that of any other radiation therapy, is to maximize the therapeutic ratio, that is, the “kill” of target cells in relation to the damage dorre to critical normal tissues. In the context of bone marrow transplan- tation for leukemia, the target cells are the leukemie stem cells, the normal bone marrow stem cells and the immu- nocytes, which must be eradicated to permit a marrow graft. The prototype of the target cel], the normal hemo- poietic stem cell, can be assayed radiobiologically in two ways: one, the LD50,30, assays the dose of radiation required to cause death from hemopoietic failure in 30 days; the other, the CFU, assay, measures the colony- forming ability in the spleen of cells injected intravenous- lY.

The dose-limiting normal tissues for whole body irra- diation have not yet been clearly defined. For acute reactions, the limiting tissue is the gut, but acute reac- tions are not the centra1 problem. Probably the main critical normal tissue is the lung, especially when graft versus host disease or viral pneumonitis is superimposed upon the radiation: Another possible dose-limiting normal tissue is the heart, particularly at the M.D. Anderson Hospita1 where Rubidazone, a drug closely related to adriamycin, is frequently used in the treatment of leukemia. Many of our patients have had large doses of these anthracycline antibotics, which are cardiotoxic in their own right. prior to TBI; the addition of radiation may precipitate cardiac failure. Another possible dose- limiting tissue is the centra] nervous system, particularly if the patient has received previous craniospinal irradia- tion and intrathecal methotrexate. Other organs, such as the kidneys and liver, could also be dose-limiting if their function is already impaired from some other disease. For the purpose of this discussion we wil] assume that the lung is the principal dose-limiting normal tissue.

Radiation cel] survival curves relate the surviving frac- tion of cells to the dose of radiation received. Typically the survival curve has a shoulder followed by an exponen- tial region. The width of the shoulder region varies considerably between different cel1 lines; the slope, to a lesser extent.

When a dose of radiation is fractionated, the shoulder

on the survival curve repeats itself. In a simple split dose experiment, using large dose fractions and conventional dose rates (> 100 rad/min), the differente in dose required to achieve a certain isoeffect with two fractions vs. one (the D,-D, value) is a measure of the ability of the target cells to recover from sublethal injury between fractions. Usually this ability to repair sublethal radia- tion injury can be correlated with the width of the shouder on the single dose survival curve.

The situation becomes more complex, however, when smal] fractional doses or low dose rates are used because in the low dose region of survival curves, most of the injury is inflicted by single lethal events; very little killing is due to accumulation of sublethal injury. However, when one gets down on the terminal exponuntial region of the survival curves, death of cells due ta accumulated sublethal injury predominates. The slope of that compo- nent of the survival curve representing “single-hit” kill- ing, i.e., the intial slope, determines the limit for the sparing effect of reduced dose rate or dose fractionation.

Eric Hall’ has shown how the effective Do (the effec- tive slope of the survival curve) changes with dose rate for Chinese hamster V79 cells. As the dose rate is reduced, the effective Do increases. In other words, the survival curve becomes shallower. The maximum influence of dose rate is seen between 50 rad/min and 1 rad/min. This is the range in which we are working in TBI, so the specification of dose rate is absolutely critical. For exam- ple, if in Minneapolis they treat at 26 rad/min, and in Seattle at 8 rad/min, there is a tremendous differente in the biological effectiveness of the same number of rad at those two dose rates.

Down to the limit imposed by single-hit killing, frac- tionation or low dose rate maximally spares cells with the largest capacity to accumulate and repair sublethal injury; in other words, the bigger the shoulder on the survival curve (the bigger the value of D, and n, or of D,-D,). the greater the sparing effect of fractionation. Therefore. if we want to improve the therapeutic ratio we need to know the relative values of these parameters for .our target cells and limiting normal tissue cells.

McCullough and Til13 produced the first survival curve

Accepted for publication 8 February 1980.

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786 Radiation Oncology 0 Biolopy 0 Physics June 1980. Volume 6. Number 6

for normal hemopoietic stem cells using the spleen colony assay. Their survival curve, which is typical for this cel1 type. has a very smal1 shoulder, with an extrapolation number of only 1.5 and a D, of about 50 rad. A survey of the literature reporting survival curves for various experi- mental leukemias showed the average extrapolation number to be about 1.6 and the average D, around 65 rad. This suggests that experimental leukemias usually share the characteristic of normal hemopoietic stem cells in having smal1 shoulders on their survival curves. For normal bene marrow stem cells, the D:-D, value for LD (,,,,,, assays is about 100-135 rad; while using the CFU, assay. there is often no split-dose recovery. a finding that reflects technical differences between the two assays. This is unimportant for our purposes, how- ever, since both assays indicate a limited to absent repair capability.

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What about the dose limiting normal tissues? Wara et al4 studied the etfect of dose fractionation on the lethality from late lung fìbrosis 150 days after radiation in mice. With a single dose at high dose rate. the LDSo was 1324 rad. With two doses it was 374 rad more, and as the number of fractions was increased up to 20, the mean dose required to kil1 the mice increased progressively. Thus the lungs have a large capacity to repair sublethal injury between fractions, and fractionation has a large sparing effect.

Fig. 1. Types of survival curves.

One cannot generate direct survival curves for most normal tissues because single cel1 suspensions cannot be prepared, so we must use the D2-D,, or, in the case of the heart. an equivalent value to estimate their repair capac- ity. The important point to note is that al1 of the poten- tially critical normal tissues have D2-D, values that are substantially higher than the equivalent values for the target cells-the leukemie cells and the normal bone marrow stem cells.

given survival leve1 the separation of the fractionated survival curves exceeds that of the single dose (high dose rate) curves; i.e., the therapeutic ratio is improved.

In principle, the same result could be achieved with an optimal low dose rate chosen to permit full recovery of sublethal injury in the critical normal tissue cells. How- ever, for practica1 reasons. we believe that fractionated treatment is preferable to a single dose rate exposure.

What this means is that the therapeutic ratio should improve if we fractionate the dose or if we use a low enough dose rate so that all sublethal injury in the critical normal tissues is repaired during the actual period of the radiation.

Figure 1 is constructed to depict the type of survival curve involved. Curve A represents the target cells-the leukemie cells and the normal bone marrow cells. It has a smal1 shoulder. Curve B represents the critical normal cells, with a larger shoulder on the survival curve. In the shoulder region we do not know anything about the initial slopes of the survival curves; they just have to be guessed at. The optimum therapeutic ratio (the greatest amount of cel1 killing of tissue A with respect to tissue B) is achieved when the radiation dose is fractionated with sizes of dose per fraction that reduce survival just to the end of the initial exponential region of the critical normal cells; for practica1 purposes, that is probably in the vicinity of 200 rad. Inspection of the figure shows that as successive 200-rad increments are given, the fractionated survival curves diverge further and further, and for any

The next question is:If we conclude that fractionated treatment is indicated, what total dose can be given? Irradiation of both lungs has been used clinically for “prophylaxis” or treatment of pulmonary metastases from certain neoplasms. We know that one can give about 2500 rad in 20 daily fractions over 4 weeks to the whole of both lungs without a prohibitive incidence of pneumonitis. In the context of TBI, two major additional factors must be considered: the necessity of changing the fractionation schedule and the effect of the intensive cytoreductive chemotherapy given prior to TBI. Without concurrent drug therapy, we estimate the total dose that could be given to the whole of both lungs would be somewhere in the region of 2000 rad in 10 fractions. The amount of further dose reduction necessary to allow for various chemotherapy regimens is unknown, but on the basis of results of the National Wilms’ Tumor Study, in which concurrent actinomycin D and lung irradiation were given, we believe the absolute lower limit of lung tolerante for TB1 would be 1200 rad in 6 fractions. At present, our policy at M.D. Anderson is to give 1200 rad

Dose

Radiological bases of TB1 0 L. PETERS 787

over 3 days using 200-rad fractions twice a day. Dr. Storb in Seattle tells me that they have given 1400 rad in 200-rad fractions with good tolerante, and 1 believe we may be able to work up to about 1600 without trouble. With fractionated doses of 200 rad, dose rate is of little importante for most tissues, since most cel] killing results from nonrepairable single-hit injury. In practice, we use the maximum dose tate achievable with our machine in the treatment configuration, namely -25 rad/min.

For the sake of comparison, 1 have calculated the amount of hemopoietic stem cel] killing that would be achieved with fractionated treatment, in comparison to existing single dose schedules in use. The calculations were based on McCullough and Till’s survival curve parameters for mouse hemopoietic stem cells and the dose rate effects for those same cells as determined by Feola et al.’ A single dose of 1000 rad at 5 rad/min (as at Seattle) gives a 3.63 log kil]; a single dose of 750 rad at 26 rad/min (as in Minneapolis) gives a 3.52 log kill. Now if we go to 200 rad per fraction and give fractionated doses at 25 rad/min (as at Houston), 1200 rad gives a 4.43 log kill; 1400 rad gives a 5.17 log kil], and 1600 rad gives a 5.91 log kil]. Thus, the fractionated technique has a potential for up to two more logs of killing of the target cells while remaining within normal tissue tolerante.

Single dose TB1 for bone marrow transplantation has

never had any radiobiological foundation. Treatment was logistically difficult; the patients were sick and needed their marrow transplant rapidly. Radiotherapsits sought to minimize the inconvenience of a single dose using low dose rates by using more powerful machines and by increasing the dose rates; however, the higher the dose rate one uses with a single exposure, the worse the therapeutic ratio wil1 be. Fractionated or low dose rate radiation wil] improve the therapeutic ratio, but it becomes a tremendous undertaking to reduce the dose rate in a single exposure below 5 rad/min. The whole procedure takes hours and exceeds the tolerante of the physicists and the radiotherapists, not to mention the patient! The disadvantage of fractionation is that it delays marrow reconstitution and increases the period of risk of infection. However, we believe that if one treats twice a day and completes treatment within 3 days, then this delay is acceptable.

In summary, 1 recommend we move beyond the histori- cal use of a single dose of radiation for TBI, and mount a new program to exploit fractionated treatment, which is used in virtually every other form of radiotherapy; by this method we can signifìcantly improve the therapeutic ratio-that is, get much better target cel1 kil1 for the same normal tissue damage.

REFERENCES 1. Feola, J.M.. Song. C.W.. Khan. F.M., Levitt, S.H.: Lethal

response of C57BL mice to 10 MeV x-rays and to %o y-rays. Int. J. Radiar. Biol. 26: 16 1-165, 1974.

3. Hall, E.J.: Radiation dose-rate: A factor of importante in radiobiology and radiotherapy. Br. J. Radio/. 45: 81-97. 1972.

3. McCullough, E.A., Tik J.E.: The sensitivity of cells from normal mouse bene marrow to y-radiation in vitro and in vivo. Radiat. Res. 16: 822-832. 1962.

4. Wara, W.M., Phillips, T.L., Margolis, L.W.. Smith, V.: Radiation pneumonitis: A new approach to the derivation of time-dose factors. Cancer 32: 547-552, 1973.