Ref: Adkins DR. DiPersio JF. Total body irradiation before allogeneic stem cell transplantation: is there a magic dose? Curr Opin Hematol. 2008 Nov: 15(6):555- 60. & Nelson Textbook of Pediatrics, 19th edition.Explanation:Total body irradiation (TBI)It is a form of radiotherapy used primarily as part of the preparative regimen for haema-topoietic stem cell (or bone marrow) trans-plantation.As the name implies, TBI involves irradiation of the entire body. though in modern practice the lungs are often partially shielded to lower the risk of radiation - induced lung injury. Causes of Hydrops FetalisAnemiaImmune (Rh, Kell) hemolysisa-ThalassemiaRed blood cell enzyme deiiciencies (G6PD def)Fetomaternal hemorrhageDonor in twin-to-twin transfusionDlamond-Blacktan syndromeCardiac arrhythmiasSupraventricular tachycardiaAtrial flutterCongenital heart bicckStructural cardiac lesionsPremature closure of foramen ovaleTricuspid insufficiencyHypoplastic left heartEndocardial cushion defectCardiomyopathyVascularChcrioangioma of placenta, chorionic vessels, or umbilical vesselsUmbilical artery aneurysmAngiomyxoma oi umbilical cordTrue knot of umbilical cordHepatic hemangiomaCerebral arteriovenous malformation (aneurysm of vem of Galen)Angiosteohype rtrophy (Klippel-Trenaunay syndrome}Thrombosis of renal or umbilical vein or inferior vena cavaRecipient in twin-tc-twin transfusionLymphaticLymphangiectasi aCystic hygromaChylothorax. chylous ascitesNoonan syndromeCentral nervous systemAbsent corpus callosumEnceohaloceleIntracranial hemorrhageHoloprosencephalyRespiratory systemCystic acenomatoid malformation of lungMediastinal teratomaDiaphragmatic herniaSequestered lungTumoursTeratomaChonocarcinomaSacrococcygeal teratomaNeuroblastomaHepatoblastomaStorage DiseasesGaucher diseaseNiemann-Pick diseaseMucd pidosisGM1 gangliosidosisMucopolysaccharidcsisChromosome AbnormalitiesTrisomv 13, 15. 16. 18. 21XX'XY. 45XOPartial duplication of chromosome 11, 15, 17, 18Partial deletion of chromosome 13, 18Triploidy.'TetraploidyBone disordersOsteogenesis imperfectAsphyxiating thoracic dystrophySkeletal dysplasiasCongenital Elections CytomegalovirusParvovirusRubellaToxoplasmosisSyphilisLeptospirosisChagas diseaseMiscellaneous conditionsBowel obstruction 'with perforationHepatic fibrosisBeckwith-Wiedemann syndromePrune-belly syndromeCongenital nephrosisInfant of a diabetic motherMyotonic dystrophyMaternal therapy with indomethacinTotal body irradiation in the setting of bone marrow transplantation serves to destroy or suppress the recipient's immune system. preventing immunologic rejection of transplanted donor bone marrow or blood stem cells.Additionally, high doses of total body irradiation can eradicate residual cancer cells in the transplant recipient, increasing the likelihood that the transplant will be successful.Doses of total body irradiation used in bone marrow transplantation typically range from 10 to >12 Gv.For reference, a dose of 4.5 Gy is fatal in 50% of exposed individuals without aggressive medical care.Such high total body doses are made possible by spreading the total dose out between several sessions, or fractions," with an interval of time in between allowing other normal tissues some time to repair some of the damage caused.However, at these doses, total body irradiation both destroys the patient's bone marrow (allowing donor marrow to engraft) and kills residual cancer cells.Non-myeloablative bone marrow transplantation uses lower doses of total body irradiation, typically about 2 Gy, which do not destroy the host bone marrow but do suppress the host immune system sufficiently to promote donor engraftment.Protocols for allogeneic HSCT consist of 2 parts:- The preparative regimen- Transplantation itself.During the preparative conditioning regimen, chemotherapy, often associated with irradiation. is administered to destroy the patient's hematopoietic system and to suppress the immune system, especially T cells, so that graft rejection is prevented.In patients with malignancies, the preparative regimen also serves to significantly reduce the tumor burden.