The never-ending controversy about TBI schedules

Whole body radiotherapy: A TBI-guideline

Total Body Irradiation (TBI) is one main component in the interdisciplinary treatment of widely disseminated malignancies predominantly of haematopoietic diseases. Combined with intensive chemotherapy, TBI enables myeloablative high dose therapy and immuno-ablative conditioning treatment prior to subsequent transplantation of haematopoietic stem cells: bone marrow stem cells or peripheral blood progenitor stem cells. Jointly prepared by DEGRO and DGMP, the German Society of Radio-Oncology, and the German Association of Medical Physicists, this DEGRO/DGMP-Leitlinie Ganzkoerper-Strahlenbehandlung - DEGRO/DGMP Guideline Whole Body Radiotherapy, summarises the concepts, principles, facts and common methods of Total Body Irradiation and poses a set of recommendations for reliable and successful application of high dose large-field radiotherapy as essential part of this interdisciplinary, multi-modality treatment concept. The guideline is geared towards radio-oncologists, medical physicists, haematooncolo-gists, and all contributing to Whole Body Radiotherapy. To guide centres intending to start or actualise TBI criteria are included. The relevant treatment parameters are defined and a sample of a form is given for reporting TBI to international registries.

[Biological basis of total body irradiation]

A comprehensive understanding of the radiobiological bases of total body irradiation (TBI) is made difficult by the large number of normal and malignant tissues that must be taken into account. In addition, tissue responses to irradiation are also sensitive to associated treatments, type of graft and a number of patient characteristics. Experimental studies have yielded a large body of data, the clinical relevance of which still requires definite validation through randomized trials. Fractionated TBI schemes are able to reduce late normal tissue toxicity, but the ultimate consequences of the fractional dose reduction do not appear to be equivocal. Thus, leukemia and lymphoma cells are probably more radiobiologically heterogeneous than previously thought, with several cell lines displaying relatively high radioresistance and repair capability patterns. The most primitive host-type hematopoietic stem cells are likely to be at least partly protected by TBI fractionation and may hamper late engraftment. Similarly, but with possibly conflicting consequences on the probability of engraftment, the persistence of a functional marrow stroma may also be fractionation-sensitive, while higher rejection rates have been reported after T-depletion grafts and fractionated TBI. In clinical practice (as for the performance of relevant clinical trials), the influence of these results are rather limited by the heavy logistic constraints created by a sophisticated and time-consuming procedure. Lastly, clinicians are now facing an increasing incidence of second cancers, at least partly induced by irradiation, which jeopardize the long-term prospects of otherwise cured patients.

MarCell software for modeling bone marrow radiation cell kinetics

Differential equations were used to model cellular injury, repair, and compensatory proliferation in the irradiated bone marrow. Recently, that model was implemented as MarCell, a user-friendly MS-DOS computer program that allows users from a variety of technical disciplines to evaluate complex radiation exposure. The software allows menu selections for different sources of ionizing radiation. Choices for cell lineages include progenitor, stroma, and malignant, and the available species include mouse, rat, dog, sheep, swine, burro, and man. An attractive feature is that any protracted irradiation can be compared with an equivalent prompt dose (EPD) in terms of cell kinetics for either the source used or for a reference such as 250 kVp x rays or 60Co. EPD is used to mean a dose rate for which no meaningful biological recovery occurs during the period of irradiation. For human as species, output from MarCell includes: risk of 30-day mortality; risk of whole-body cancer and leukemia based either on radiation-induced cytopenia or compensatory cell proliferation; cell survival and repopulation plots as functions of time or dose; and 4-week recovery following treatment.

Single dose versus fractionated total body irradiation before bone marrow transplantation: radiobiological and clinical considerations

PURPOSE

This present review is intended to evaluate the specific influence of fractionation of total body irradiation on the outcome of a subsequent bone marrow transplantation.

METHODS AND MATERIALS

Available experimental and clinical data on the influence of fractionation on leukemia cell killing, immunosuppression, and sparing of normal tissues were analyzed.

RESULTS

Review of available data shows: (a) The role of fractionation on leukemia cell killing may vary with the leukemia type. For acute nonlymphoblastic leukemia, a few experimental and several clinical studies show no or little fractionation effect; a 12-13 Gy fractionated scheme could, therefore, be more efficient than a conventional 10 Gy single dose total body irradiation. For chronic myelogenous leukemia, some sensitivity to fractionation is suggested, so that an increase in total or fractional dose may be necessary in fractionated schemes to equate the efficacy of a 10 Gy single dose. For acute lymphoblastic leukemia, a high fractionation sensitivity was observed for some leukemic cell lines in vitro, without undisputable clinical confirmation for the moment. (b) Numerous experimental studies have demonstrated that the immunosuppressive effect of total body irradiation, a major determinant of engraftment, is highly fractionation sensitive. In humans, high rates of graft failures have been reported when T-cell depletion of the graft was associated to fractionated total body irradiation schedules. (c) A large amount of radiobiological and clinical data have demonstrated that late radiation-induced injuries to normal tissues and organs are highly fractionation sensitive. However, in a context of total body irradiation for bone marrow transplantation, the number of other determinants of normal tissue damage makes it difficult to demonstrate a clear-cut advantage of fractionated over single dose scheme, with a possible exception for children.

CONCLUSIONS

In 1994, available data suggest that very cautious attempts could be made to adapt total body irradiation schedules to the potential normal tissue toxicity, T-cell depletion, and to the type of leukemia.