Тотальне опромінення тіла (TBI) as a myeloablative conditioning component — whole-body radiation delivered ahead of hematopoietic stem cell transplant, fractionated to balance marrow ablation against normal-tissue toxicity.
Conventional radiation therapy is precision-guided: a beam is shaped to strike a tumor while sparing surrounding tissue. TBI inverts that logic entirely. As a conditioning component before hematopoietic stem cell transplantation, the treatment goal is not to spare tissue but to reach every hematopoietic niche in the body — marrow inside the pelvis, vertebrae, long bones, skull — while also penetrating so-called "sanctuary sites" where systemic chemotherapy achieves only limited drug concentration.
Systemic chemotherapy circulates through the bloodstream, but several anatomical compartments are relatively shielded from adequate drug exposure by physiological barriers — most notably the blood-brain/blood-CSF barrier protecting the central nervous system, and the blood-testis barrier protecting germ cells. Malignant cells that have migrated into these compartments before conditioning can survive a chemotherapy-only regimen even when systemic disease is otherwise well controlled.
Radiation does not rely on vascular drug delivery — a photon beam traversing tissue deposits dose regardless of local blood-barrier physiology. This is the central rationale for including TBI in high-risk conditioning regimens: it treats sanctuary sites with the same reliability as any other tissue plane in the beam path.
Adult hematopoiesis is distributed across the axial skeleton (vertebrae, pelvis, sternum, ribs, skull) and the proximal long bones — not concentrated in one accessible location. A conditioning regimen intended to fully ablate host marrow (myeloablative intent) before donor stem cell infusion needs to reach essentially all of these dispersed niches with a cytotoxic dose.
Because a whole-body beam arrangement irradiates the entire skeleton simultaneously, TBI provides more uniform marrow coverage than is easily achieved by chemotherapy agents alone, whose marrow penetration can vary by drug class, perfusion, and local metabolism.
The body is not a uniform slab — chest is thinner than pelvis, limbs are thinner than trunk, and tissue density varies (lung is far less dense than bone or soft tissue). Achieving a reasonably uniform dose across this irregular target requires compensation: extended treatment distance to reduce beam divergence effects, tissue-equivalent bolus over thin body regions, and beam-spoiling devices to build up surface dose. Planning aims to keep the delivered dose across the whole body within a defined uniformity window of the prescribed value, rather than a single sharply collimated field as in localized radiotherapy.
The defining conceptual shift of TBI is that "coverage" replaces "precision" as the planning objective — every marrow niche and sanctuary compartment must fall inside the treated volume, because anything left outside the beam is a potential site of relapse.
Delivering the full ablative dose in a single exposure would maximize acute toxicity to normal tissue. Instead, TBI is conventionally split into multiple smaller fractions delivered over several days, exploiting the biological principle that normal tissues repair sublethal radiation damage between fractions more efficiently than malignant or rapidly dividing marrow precursor cells — the same rationale that underlies fractionation in radiotherapy generally, adapted to a whole-body target.
Radiobiological damage to a cell can be lethal or sublethal. Sublethal damage — DNA strand breaks that do not by themselves kill the cell — can be enzymatically repaired over hours if the cell is given time before the next insult. Normal tissues generally repair sublethal damage more effectively than many malignant or actively dividing marrow-precursor populations, particularly when fractions are separated by several hours.
By dividing the cumulative target dose into a number of smaller fractions rather than one large exposure, each individual fraction causes proportionally less irreversible injury to normal tissue, while cumulative fractions still add up to a total dose sufficient for the ablative or immunosuppressive intent of conditioning.
For a fixed total dose, fewer fractions mean a higher dose delivered per session — and dose per fraction is one of the strongest predictors of normal-tissue toxicity in radiobiology. A single large fraction achieves the same cumulative dose as several smaller ones, but with substantially higher acute and late toxicity risk because there is no inter-fraction window for repair.
This is the central dial that conditioning-regimen design turns: total dose sets the ablative/immunosuppressive intent, while fraction count sets how gently or aggressively that dose is delivered to normal tissue. A well-fractionated schedule (more, smaller fractions) generally trades a longer treatment course for a more favorable toxicity profile.
A fractionated TBI course is typically compressed into a few consecutive days to avoid delaying stem cell infusion, sometimes using twice-daily fractions with a minimum inter-fraction interval (commonly at least six hours) to preserve a meaningful normal-tissue repair window even within a compressed schedule.
Each fraction requires accurate patient positioning and dose verification, since delivering the wrong fraction size — even once — can meaningfully shift the toxicity profile of the entire course given how sensitive normal tissue response is to per-fraction dose.
Adjust the fraction-count slider while holding total dose fixed to see how the same cumulative target can be delivered "gently" across many small fractions or "aggressively" across few large ones — the dose-per-fraction metric is what actually drives the toxicity-risk estimate.
Not every tissue tolerates the cumulative TBI dose equally well. The lungs are among the most radiosensitive organs in the treatment volume, with interstitial pneumonitis a recognized dose-related complication of TBI. To reduce lung-specific toxicity while still achieving marrow ablation everywhere else, custom partial shielding blocks are commonly positioned over the thorax during select fractions of the course.
Lung tissue is unusually sensitive to radiation-induced injury relative to many other organs in the TBI treatment volume, and the lungs also happen to lie directly in the path of a whole-body beam covering the thorax. Left completely unmodified, cumulative TBI dose to lung parenchyma can approach levels associated with a clinically meaningful risk of interstitial pneumonitis — inflammation and eventual fibrosis of lung tissue that can compromise pulmonary function.
Because the goal of TBI is marrow and sanctuary-site coverage rather than deliberately maximizing lung dose, attenuating the beam specifically over the thorax during part of the course is a targeted way to reduce this risk without abandoning the whole-body treatment intent elsewhere.
Shielding blocks are typically custom-shaped per patient from a low-melting-point attenuating alloy, positioned in the beam path to reduce (not fully eliminate) transmission through the lung volume, and applied during only a subset of the fractions rather than the entire course — preserving full dose to the ribs, sternum, and vertebral marrow that also lie within the thoracic field.
Because the shield is designed to attenuate rather than fully block transmission, and is applied for only part of the treatment course, the marrow embedded in thoracic bone still receives a dose consistent with the overall ablative target — the compensation is organ-specific, not a wholesale exclusion of the thorax from treatment.
Because under-shielding fails to protect the lung and over-shielding risks under-dosing marrow in the shadowed region, shield placement and thickness are verified against the treatment plan before each shielded fraction — confirming block alignment relative to lung contours on the patient's positioning setup, and periodically checking that the compensation achieves the intended organ-specific dose reduction without compromising the ablative target elsewhere in the field.
Lung shielding is a clear example of the broader TBI planning philosophy: the whole body is the target, but a small number of organs receive individualized dose modification where their sensitivity to radiation clearly outweighs the marginal benefit of full-dose coverage at that specific site.
Adding radiation to a conditioning regimen introduces a toxicity profile that chemotherapy-only regimens do not share. Some of these effects are acute and resolve with supportive care; others — particularly in pediatric patients, or in the years following transplant — are late effects that require dedicated long-term surveillance well beyond the immediate transplant period.
Radiation-induced lung injury can present as interstitial pneumonitis in the weeks to months following TBI, reflecting inflammation of lung parenchyma exposed to cumulative dose. Risk correlates with total lung dose, dose rate, and fractionation — which is precisely why partial lung shielding (Stage 3) is used to reduce this specific risk. Even with shielding, lung toxicity remains one of the toxicities most directly attributable to the radiation component of conditioning rather than to chemotherapy agents.
The lens of the eye is among the most radiosensitive tissues in the body, and cataract formation is a recognized late effect of TBI, with risk related to cumulative dose and fractionation schedule — more fractionated delivery is generally associated with a lower cataract risk than the same total dose delivered in very few large fractions. Because cataracts can develop years after treatment, ophthalmologic surveillance is typically part of long-term post-transplant follow-up for patients who received TBI.
Children and adolescents who receive TBI face additional considerations not relevant to adult patients: radiation exposure to growth plates can impair bone growth, and the hypothalamic-pituitary axis and other endocrine organs (thyroid, gonads) can be affected, potentially producing growth hormone deficiency, thyroid dysfunction, or impaired pubertal development. These effects underscore why pediatric conditioning-regimen selection weighs TBI inclusion especially carefully against chemotherapy-only alternatives.
Any therapeutic radiation exposure carries a long-term, dose-related risk of secondary malignancy — new cancers arising years to decades after treatment, distinct from the original disease being treated. Because TBI recipients are, by definition, being conditioned for a serious underlying hematologic condition and are already undergoing intensive combined-modality treatment, secondary malignancy risk from the radiation component is layered onto other treatment-related risk factors, which is why survivors require structured, lifelong oncologic surveillance rather than follow-up limited to the original disease.
None of these four risks is unique to any single fraction — they are cumulative, dose-related consequences of the whole-body radiation component specifically, which is why toxicity-risk estimates in this simulator scale with dose per fraction and total dose rather than with any single treatment stage.
Whether to include TBI in a conditioning regimen at all is a deliberate risk-benefit decision made ahead of any dose or fractionation planning. It weighs the broad sanctuary-site coverage and reliable marrow-ablative efficacy that whole-body radiation provides against a toxicity profile — acute organ effects, pediatric growth and endocrine impact, and long-term secondary malignancy risk — that chemotherapy-only regimens do not carry in the same form.
Including TBI in the conditioning regimen provides two structural advantages that are difficult to replicate with chemotherapy alone: reliable physical coverage of sanctuary sites where drug penetration is limited by biological barriers, and comparatively uniform dose delivery across every hematopoietic niche in the skeleton regardless of local blood flow or drug metabolism differences. For diseases where sanctuary-site relapse is a recognized concern, or where achieving deep, uniform marrow ablation is prioritized, this coverage profile can be a decisive advantage.
A chemotherapy-only conditioning regimen avoids the entire category of radiation-specific toxicity described in Stage 4 — no TBI-attributable pneumonitis risk, no TBI-attributable cataract risk, and no added radiation component to long-term secondary-malignancy or pediatric growth/endocrine risk. For patients where sanctuary-site coverage is less clinically critical, where age or comorbidity make radiation-specific toxicity especially concerning, or where disease biology does not particularly favor whole-body radiation, chemotherapy-only conditioning can achieve comparable myeloablative or immunosuppressive intent through drug selection and dosing alone.
The decision is rarely made on efficacy or toxicity alone — it integrates patient age (pediatric patients carry heavier long-term stakes from radiation-specific growth and secondary-malignancy risk), underlying disease and its known pattern of sanctuary-site involvement, prior treatment exposure (including any earlier radiation), and overall treatment goals (myeloablative versus reduced-intensity conditioning intent).
When TBI is selected, the dose and fractionation choices explored in Stages 1–3 of this simulator become the next layer of the same underlying trade-off: given that radiation is included, how should its total dose and fraction count be tuned to preserve as much of its coverage advantage as possible while minimizing its toxicity cost.
There is no universally "correct" choice between TBI-based and chemotherapy-only conditioning — it is a structured trade-off between breadth of coverage and toxicity profile, resolved individually for each patient and disease context rather than by a fixed rule.