Highlights
- •Gy dose threshold for fetal dose to limit adverse effect of radiation.
- •Estimate fetal dose with Monte Carlo method or phantom measurement before treatment.
- •Shielding and dose reduction techniques to further reduce out-of-field dose.
- •Neutrons contribute most to fetal out-of-field dose in proton therapy.
- •Use of daily CBCT in IGRT for pregnant patient should be carefully considered.
Abstract
Keywords
Purchase one-time access:
Academic & Personal: 24 hour online accessCorporate R&D Professionals: 24 hour online accessOne-time access price info
- For academic or personal research use, select 'Academic and Personal'
- For corporate R&D use, select 'Corporate R&D Professionals'
Subscribe:
Subscribe to Physica Medica: European Journal of Medical PhysicsReferences
- Estimation and reduction of the radiation dose to the fetus from external-beam radiotherapy.Phys Med. 2017; 43: 148-152
- Coexistence of pregnancy and malignancy.Oncologist. 2002; 7: 573https://doi.org/10.1634/theoncologist.7-6-573
- Radiation exposures in pregnancy, health effects and risks to the embryo/foetus—information to inform the medical management of the pregnant patient.J Radiol Prot. 2021; 41: S522-S539
- Fetal dose from radiotherapy with photon beams: report of AAPM Radiation Therapy Committee Task Group No. 36.Med Phys. 1995; 22: 63-82
- AAPM TG 158: measurement and calculation of doses outside the treated volume from external-beam radiation therapy.Med Phys. 2017; 44: e391-e429
- RADIATION THERAPY DURING PREGNANCY.Obstet Gynecol Clin N Am. 1998; 25: 301-321
- Review of radiation therapy in the pregnant cancer patient.Clin Obstet Gynecol. 2011; 54: 591-601
Pregnancy and Medical Radiation; ICRP Publication 84. Annals of the ICRP 30, 1–43 (2000).
- Radiotherapy and pregnancy.Cancer/Radiotherapie. 2022; 26: 417-423
National Council on Radiation Protection and Measurements. & National Council on Radiation Protection and Measurements. Scientific Committee 4-4 on the Risks of Ionizing Radiation to the Developing Embryo, F. Preconception and prenatal radiation exposure: health effects and protective guidance. (2013).
- RETROSPECTIVE AND PROSPECTIVE EPIDEMIOLOGICAL STUDIES OF 1500 KARYOTYPED SPONTANEOUS HUMAN ABORTIONS.Obstet Gynecol Surv. 1976; 31: 617-619
- Genes, chromosomes, and reproductive failure.Fertil Steril. 1980; 33: 107-116
- The effect of x-irradiation on rat embryos at the twelfth day of gestation.University of Rochester, 1955
Konermann G. Postimplantation Defects in Development following Ionizing Irradiation. in 91–167 (1987). doi:10.1016/B978-0-12-035413-9.50007-8.
- Mouse germ development following continuous Co60 gamma irradiation during blastogenesis, organogenesis and fetal period.Strahlentherapie. 1969; 137: 451-466
- The effects of radiation on the preim-plantation stages of the mouse embryo.Anat Res. 1950; 108: 521
- An analysis of the changing radiation response of the developing mouse embryo.J Cell Physiol Suppl. 1954; 43: 103-149
RUSSELL LB, RUSSELL WL. Hazards to the embryo and fetus from ionizing radiation. in Proceedings of the International Conference on Peaceful Uses of Atomic Energy 175–178 (1956).
- Differentiation as a determinant of the reaction of rat embryos to X-irradiation.Exp Biol Med. 1953; 82: 67-70
- X-rays on the ninth day of gestation in the rat.Am J Anat. 1953; 92: 153-187
- X-ray-Induced Teratogenesis in the mouse and its possible significance to man.Radiology. 1971; 99: 433-443
- Differentiation and the reaction of rat embryos to radiation.J Cell Comp Physiol. 1954; 43: 11-37
Wilson JG. Environment and birth defects. (1973).
- Effects of irradiation on embryonic development. I. X-rays on the 10th day of gestation in the rat.Am J Anat. 1951; 88: 1-33
- Neoplasia induced in rat embryos by roentgen irradiation.Cancer Res. 1952; 12: 222-228
- Utilization of developmental basic science principles in the evaluation of reproductive risks from pre- and postconception environmental radiation exposures.Teratology. 1999; 59: 182-204
Konermann G. Consequences of prenatal radiation exposure on perinatal and postnatal development. in Developmental Effects of Prenatal Irradiation 237–250 (1982).
- MORTALITY IN CHILDREN EXPOSED TO THE A-BOMBS WHILE IN UTERO, 1945-19691.Am J Epidemiol. 1971; 93: 435-442
- Threshold for radiation-related severe mental retardation in prenatally exposed A-bomb survivors: a re-analysis.Int J Radiat Biol. 1996; 70: 755-763
- Mental retardation following in utero exposure to the atomic bombs of Hiroshima and Nagasaki.Radiology. 1973; 106: 617-619
- Effects of prenatal exposure to ionizing radiation.Health Phys. 1990; 59: 57-61
- Small head size after atomic irradiation.Teratology. 1976; 14: 355-357
- The outcome of 625 pregnancies in women subjected to pelvic radium or roentgen irradiation.Am J Obstet Gynecol. 1929; 18: 179-187
- Microcephaly following maternal pelvic irradiation for the interruption of pregnancy.Am J Roentgenol. 1942; 48: 356-359
- In utero exposure to A-bomb radiation and mental retardation; a reassessment.Br J Radiol. 1984; 57: 409-414
- Review: radiation-related brain damage and growth retardation among the prenatally exposed atomic bomb survivors.Int J Radiat Biol. 1998; 74: 159-171
- Cognitive function and prenatal exposure to ionizing radiation.Teratology. 1999; 59: 222-226
- The growth and development of children exposed in utero to the atomic bombs in Hiroshima and Nagasaki.Am J Public Health Nation’s Health. 1967; 57: 1374-1380
- Mental retardation in children exposed in utero to the atomic bombs in Hiroshima and Nagasaki.Am J Public Health Nation’s Health. 1967; 57: 1381-1389
- Further observations on abnormal brain development caused by prenatal A-bomb exposure to ionizing radiation.Int J Radiat Biol. 1995; 67: 359-371
- Abnormalities in children exposed to x-radiation during various stages of gestation: tentative timetable of radiation injury to the human fetus. I.J Nucl Med. 1968; 9: 471-477
- A CASE OF MICROCEPHALY FOLLOWING EMBRYONIC ROENTGEN IRRADIATION.Am J Psychiatry. 1930; 86: 871-878
- Amenorrhea during serial roentgen exposures due to intervening pregnancy.Am J Obstet Gynecol. 1929; 18: 696-698
- Microcephalic idiocy following radium therapy for uterine cancer during pregnancy.Am J Obstet Gynecol. 1929; 18: 189-195
- Etiology of ill-health in children born after maternal pelvic irradiation. II. Defective children born after postconception pelvic irradiation.AJR Am J Roentgenol. 1929; 22: 322-331
- Therapeutic abortion by means of X-Ray**Read at a meeting of the New York Obstetrical Society, May 12, 1936.Am J Obstet Gynecol. 1936; 32: 945-957
- Discussion: severe mental retardation and cancer among atomic bomb survivors exposed in utero.Teratology. 1999; 59: 234-235
Miller RW. Delayed radiation effects in atomic-bomb survivors. Major observations by the Atomic Bomb Casualty Commission are evaluated. Science (New York, N.Y.) 166, 569–74 (1969).
- The physical growth and development of children who survived the atomic bombing of Hiroshima or Nagasaki.J Pediatr. 1953; 43: 121-145
- Effect of radiation dose on the height of atomic bomb survivors: a longitudinal study.Radiat Res. 2002; 158: 346-351
Miller RW. EPIDEMIOLOGICAL CONCLUSIONS FROM RADIATION TOXICITY STUDIES. (1970).
- A longitudinal study of growth and development of stature among prenatally exposed atomic bomb survivors.Radiat Res. 1993; 134: 94-101
- Evidence of radiation-induced reduction of height and body weight from repeated measurements of adults exposed in childhood to the atomic bombs.Radiat Res. 1994; 140: 112-122
- Growth and development following prenatal and childhood exposure to atomic radiation.J Radiat Res. 1975; 16: 82-88
- Height reduction among prenatal exposed atomic-bomb survivors.Health Phys. 1995; 68: 766-772
- Persistent stunting following x-irradiation of the fetus.Am J Anat. 1964; 115: 185-197
- Cardiovascular disease risk among atomic bomb survivors exposed in utero, 1978–2003.Radiat Res. 2008; 170: 269-274
- Thyroid diseases in atomic bomb survivors exposed in Utero.J Clin Endocrinol Metab. 2008; 93: 1641-1648
- Infant leukemia in Belarus after the Chernobyl accident.Radiat Environ Biophys. 1998; 37: 53-55
- Cancer mortality among atomic bomb survivors exposed in utero or as young children, October 1950-May 1992.Radiat Res. 1997; 147: 385-395
- Cancer mortality and radioactive fallout in southwestern Utah.Am J Epidemiol. 1987; 125: 44-61
- Childhood leukaemia in Europe after Chernobyl: 5 year follow-up.Br J Cancer. 1996; 73: 1006-1012
- Fallout, radiation doses near Dounreay, and childhood leukaemia.Br Med J (Clin Res Ed). 1987; 294: 603-607
- Trends in childhood Leukaemia in the Nordic countries in relation to fallout from atmospheric nuclear weapons testing.BMJ (Clinical Research Ed). 1992; 304: 1005-1009
- Childhood leukaemias in New Zealand: time trends and ethnic differences.Br J Cancer. 1996; 73: 1141-1147
- Solid cancer incidence in atomic bomb survivors exposed in utero or as young children.JNCI: J Natl Cancer Inst. 2008; 100: 428-436
- Infant mortality among offspring of individuals living in the radioactively contaminated Techa River Area, Southern Urals.Acta Medica Nagasakiensia. 2005; 50: S23-S28
- A screening study of thyroid cancer and other thyroid diseases among individuals exposed in utero to iodine-131 from Chernobyl Fallout.J Clin Endocrinol Metab. 2009; 94: 899-906
- Breast cancer and pregnancy.Obstet Gynecol Surv. 1978; 33: 246-248https://doi.org/10.1097/00006254-197804000-00009
- Breast cancer and pregnancy.Horm Mol Biol Clin Invest. 2017; https://doi.org/10.1515/hmbci-2017-0026
- Radiotherapy for glioma during pregnancy: fetal dose estimates, risk assessment and clinical management.Clin Oncol. 2004; 16: 210-214
- Effect of tertiary multileaf collimator (MLC) on foetal dose during three-dimensional conformal radiation therapy (3DCRT) of a brain tumour during pregnancy.Radiother Oncol. 2004; 70: 49-54
- Fetal radiation doses for model C gamma knife radiosurgery.Neurosurgery. 2003; https://doi.org/10.1227/01.NEU.0000048479.23069.24
- Radiotherapy for a solitary brain metastasis during pregnancy: a method for reducing fetal dose.Br J Radiol. 2001; 74: 638-641
- Fetal dose estimates for radiotherapy of brain tumors during pregnancy.Int J Radiat Oncol Biol Phys. 1995; 32: 823-830
- Radiation dose to the fetus during CyberKnife radiosurgery for a brain tumor in pregnancy.Phys Med. 2016; 32: 237-241
- Brain radiotherapy during pregnancy: an analysis of conceptus dose using anthropomorphic phantoms.BJR. 1999; 72: 274-278
- Technical management of a pregnant patient undergoing radiation therapy to the head and neck.Med Dosim. 1999; 24: 121-128
- Fetal dose during radiotherapy: clinical implementation and review of the literature.Cancer/Radiotherapie. 2002; 6: 352-357
- Tonsil cancer treated with radiotherapy during a pregnancy: a case report.J Radiother Pract. 2020; 19: 197-201
- Foetal radiation dose in radiotherapy for breast cancer.Australas Radiol. 1992; 36: 321-322https://doi.org/10.1111/j.1440-1673.1992.tb03209.x
- Fetal dose evaluation during breast cancer radiotherapy.Int J Radiat Oncol Biol Phys. 1998; 40: 995-999
- Pregnancy and radiation.Cancer Treat Rev. 2001; 27: 1-7https://doi.org/10.1053/ctrv.2000.0193
- Measurement of the peripheral dose for the tangential breast treatment technique with Co-60 gamma radiation and high energy X-rays.Radiother Oncol. 1997; 42: 257-264https://doi.org/10.1016/S0167-8140(96)01884-1
- Breast cancer in pregnancy.Lancet. 2012; 379: 570-579https://doi.org/10.1016/S0140-6736(11)61092-1
- Safety of pregnancy following breast cancer diagnosis: a meta-analysis of 14 studies.Eur J Cancer. 2011; 47: 74-83
- Radiotherapy during pregnancy for clinical stages IA-IIA Hodgkin’s disease.Int J Radiat Oncol Biol Phys. 1992; 23: 407-412https://doi.org/10.1016/0360-3016(92)90761-6
- Management of anaplastic large-cell lymphoma during pregnancy.J Clin Oncol. 2009; 27: e75https://doi.org/10.1200/JCO.2009.22.2042
- Conceptus dose from involved-field radiotherapy for Hodgkin’s lymphoma on a linear accelerator equipped with MLCsDie Strahlendosis im Fetus von Involved-Field-Radiotherapie wegen Hodgkin-Lymphom an einem Linearbeschleuniger mit Multileafkollimatoren (MLC).Strahlenther Onkol. 2009; 185: 355-363
- Incidence of pregnancy-associated cancer in Denmark, 1977–2006.Obstet Gynecol. 2013; 122: 608-617
- Modern radiotherapy in cancer treatment during pregnancy.Crit Rev Oncol Hematol. 2019; 136: 13-19
- Carcinoma of the cervix associated with pregnancy. A study of the radiumhemmet’s series of invasive carcinoma during the period 1932–1956.Obstet Gynecol Surv. 1963; https://doi.org/10.1097/00006254-196304000-00038
- Carcinoma of the cervix associated with pregnancy.Am J Obstet Gynecol. 1966; 95: 99-108https://doi.org/10.1016/0002-9378(66)90634-X
- Cervical cancer in pregnancy.Gynecol Oncol. 1996; 63: 293https://doi.org/10.1006/gyno.1996.0324
- Cervical neoplasia in pregnancy. Part 2: current treatment of invasive disease.Am J Obstet Gynecol. 2008; 199: 10-18
- Assessment of leakage doses around the treatment heads of different linear accelerators.Radiat Prot Dosim. 2012; 152: 304-312
- Peripheral dose from megavolt beams.Med Phys. 1983; 10: 809-818
- Measurements of dose from secondary radiation outside a treatment field.Int J Radiat Oncol Biol Phys. 1983; 9: 1177-1183
- Photon-beam radiotherapy in pregnant patients: can the fetal dose be limited to 10 cGy or less?.Phys Med. 2015; 31: 85-91
- Peripheral doses of two linear accelerators employing universal wedges.Br J Radiol. 1990; 63: 295-298
- Measurements of dose from secondary radiation outside a treatment field: Effects of wedges and blocks.Int J Radiat Oncol Biol Phys. 1985; 11: 2171-2176
- Dosimetric comparison of treatment plans using physical wedge and enhanced dynamic wedge for the planning of breast radiotherapy.J Med Phys. 2018; 43: 46-51
- Neutron spectra in a tissue equivalent phantom during photon radiotherapy treatment by LINACS.Radiat Prot Dosim. 2004; 110: 157-160
- Effects of tertiary MLC configuration on secondary neutron spectra from 18 MV x-ray beams for the Varian 21EX linear accelerator.Med Phys. 2009; 36: 4039-4046
- Secondary neutron spectra from modern Varian, Siemens, and Elekta linacs with multileaf collimators.Med Phys. 2009; 36: 4027-4038
- Out-of-field photon and neutron dose equivalents from step-and-shoot intensity-modulated radiation therapy.Int J Radiat Oncol Biol Phys. 2005; 62: 1204-1216
- Photoneutron intensity variation with field size around radiotherapy linear accelerator 18-MeV X-ray beam.Radiat Meas. 2008; 43: S495-S499
Neutron contamination from medical electron accelerators: recommendations of the National Council on Radiation Protection and Measurements. (The Council, 1984).
- Neutron contamination in radiotherapy: estimation of second cancers based on measurements in 1377 patients.Radiother Oncol. 2013; 107: 234-241
- A review on photoneutrons characteristics in radiation therapy with high-energy photon beams.Rep Pract Oncol Radiother. 2010; 15: 138-144
- Neutron source strength measurements for Varian, Siemens, Elekta, and General Electric linear accelerators.J Appl Clin Med Phys. 2003; 4: 189-194
- Secondary neutron dose during proton therapy using spot scanning.Int J Radiat Oncol Biol Phys. 2002; 53: 244-251
- Beam delivery systems for charged particles.Radiat Environ Biophys. 1992; 31: 219-231
- Secondary dose exposures during 200 MeV proton therapy.Radiat Prot Dosim. 1997; 70: 441-444
- Radiation effective doses to patients undergoing abdominal CT examinations.Radiology. 1999; 210: 645-650
- Scattered neutron dose equivalent to a fetus from proton therapy of the mother.Radiat Phys Chem. 2004; 71: 997-998
- Advances in image-guided radiation therapy.J Clin Oncol. 2007; 25: 938-946
- Comparison of peripheral dose from image-guided radiation therapy (IGRT) using kV cone beam CT to intensity-modulated radiation therapy (IMRT).Radiother Oncol. 2008; 89: 304-310
- Accurate patient dosimetry of kilovoltage cone-beam CT in radiation therapy.Med Phys. 2008; 35: 1135-1144
- Radiation dose from cone beam computed tomography for image-guided radiation therapy.Int J Radiat Oncol Biol Phys. 2008; 70: 272-279
- Measuring foetal dose from tomotherapy treatments.Med Dosim. 2021; 46: 342-346
- Fetal dose estimation for Virtual Tangential-fields Arc Therapy whole breast irradiation by optically stimulated luminescence dosimeters.Physica Medica: Eur J Med Phys. 2022; 101: 44-49
- Normalized data for the estimation of fetal radiation dose from radiotherapy of the breast.BJR. 2006; 79: 818-827
- Dose reconstruction for therapeutic and diagnostic radiation exposures: use in epidemiological studies.Radiat Res. 2006; 166: 141-157
Stratakis J, Papadakis A. Radiation Dose Management of Pregnant Patients, Pregnant Staff and Paediatric Patients. (IOP, 2019).
- An exponential growth of computational phantom research in radiation protection, imaging, and radiotherapy: a review of the fifty-year history.Phys Med Biol. 2014; 59: R233-R302
- Review of computational anthropomorphic anatomical and physiological models.Proc IEEE. 2009; 97: 1938-1953
- Construction of realistic hybrid computational fetal phantoms from radiological images in three gestational ages for radiation dosimetry applications.Phys Med Biol. 2019; 64205003
- Dosimetric impact of a new computational voxel phantom series for the japanese atomic bomb survivors: pregnant females.Radiat Res. 2019; 192: 538
- Assessment of fetal dose and health effect to the fetus from breast cancer radiotherapy during pregnancy.Life. 2022; 12: 1-9
- Development of a pregnant woman phantom using polygonal mesh, for dosimetric evaluations.Braz J Rad Sci. 2015; 3
- Mobile shielding evaluation on the fetal dose during a breast radiotherapy using Monte Carlo simulation.Phys Med. 2021; 84: 24-32
- Fetal dose from proton pencil beam scanning craniospinal irradiation during pregnancy: a Monte Carlo study.Phys Med Biol. 2022; 67035003
- Out-of-field dose calculation by a commercial treatment planning system and comparison by Monte Carlo simulation for Varian TrueBeam®.J Med Phys. 2019; 44: 156
- Accuracy of out-of-field dose calculations by a commercial treatment planning system.Phys Med Biol. 2010; 55: 6999-7008
- Accuracy and sources of error of out-of field dose calculations by a commercial treatment planning system for intensity-modulated radiation therapy treatments.J Appl Clin Med Phys. 2013; 14: 186-197
- A phantom study on fetal dose reducing factors in pregnant patients with breast cancer during radiotherapy treatment.J Med Phys. 2017; 42: 128-132
- Measurement and calculation of ovary and fetus dose in extended field radiotherapy for 10 MV X rays.Int J Radiat Oncol Biol Phys. 1981; 7: 843-846
- Estimation of the radiation dose delivered to any point outside the target volume per patient treated with external beam radiotherapy.Radiother Oncol. 1996; 38: 269-271
- Springer-Verlag, Berlin/Heidelberg1985: 33-55 Lecture Notes in PhysicsMonte-Carlo Methods and Applications in Neutronics, Photonics and Statistical Physics.
- Monte Carlo simulation of radiotherapy for breast cancer in pregnant patients: how to reduce the radiation dose and risks to fetus?.Radiat Prot Dosim. 2017; 175: 10-16
- A feasibility study to calculate unshielded fetal doses to pregnant patients in 6-MV photon treatments using Monte Carlo methods and anatomically realistic phantoms.Med Phys. 2008; 35: 3054-3061
- Fetus dose calculation during proton therapy of pregnant phantoms using MCNPX and MCNP6.2 codes.Radiat Meas. 2021; 149: 106665
- TOPAS: An innovative proton Monte Carlo platform for research and clinical applications.Med Phys. 2012; 39: 6818-6837
- Dose assessment for the fetus considering scattered and secondary radiation from photon and proton therapy when treating a brain tumor of the mother.Phys Med Biol. 2016; 61: 683-695
- Patient dose from kilovoltage cone beam computed tomography imaging in radiation therapy: patient dose from kilovoltage cone beam CT.Med Phys. 2006; 33: 1573-1582
- Absorbed dose measurements for kV-cone beam computed tomography in image-guided radiation therapy.Phys Med Biol. 2014; 59: 7297-7313
- Radiation phantom with humanoid shape and adjustable thickness (RPHAT).Phys Med Biol. 2004; 49: N125-N129
- Evaluation of MVCT imaging dose levels during helical IGRT: comparison between ion chamber, TLD, and EBT3 films.J Appl Clin Med Phys. 2016; 17: 143-157
- Radiation dose from kilovoltage cone beam computed tomography in an image-guided radiotherapy procedure.Int J Radiat Oncol Biol Phys. 2009; 73: 610-617
- Radiotherapy for supradiaphragmatic Hodgkin’s disease: determination of the proper fetal shielding conditions using Monte Carlo methodology.Phys Med. 2011; 27: 181-187
- Fetal dose reduction in head and neck radiotherapy of a pregnant woman.Z Med Phys. 2004; 14: 168-172
- Postoperative radiotherapy for synovial sarcoma of the head and neck during pregnancy: clinical and technical management and fetal dose estimates.Tumori. 2007; 93: 45-52
- IMRT in a pregnant patient: how to reduce the fetal dose?.Med Dosim. 2009; 34: 301-310
- A study of the shielding used to reduce leakage and scattered radiation to the fetus in a pregnant patient treated with a 6-MV external x-ray beam.Health Phys. 2009; 97: 581-589
- Fetus absorbed dose evaluation in head and neck radiotherapy procedures of pregnant patients.Appl Radiat Isot. 2015; 100: 11-15
- Design of a generally applicable abdominal shield for reducing fetal dose during radiotherapy of common malignancies in pregnant patients.Iran J Radiat Res. 2012; 10: 151-156
- Fetal radiation monitoring and dose minimization during intensity modulated radiation therapy for glioblastoma in pregnancy.J Neurooncol. 2014; 120: 405-409
- Evaluation of a new foetal shielding device for pregnant brain tumour patients.Radiat Oncol. 2021; 16: 1-10
- Revisiting fetal dose during radiation therapy: evaluating treatment techniques and a custom shield.J Appl Clin Med Phys. 2016; 17: 34-46
- A special radiation shielding for the radiotherapy of a pregnant patient.Strahlenther Onkol. 2008; 184: 80-85
- Evaluation of fetal dose in breast radiotherapy with shielding and wedges.IEEE Trans Nucl Sci. 2013; 60: 792-796
- Fetal dose measurements and shielding efficiency assessment in a custom setup of 192Ir brachytherapy for a pregnant woman with breast cancer.Phys Med. 2015; 31: 286-292
- Radiotherapy of Hodgkin’s disease in early pregnancy: embryo dose measurements.Radiother Oncol. 2003; 66: 333-339
- Lead versus Bismuth-antimony shield for fetal dose reduction at different gestational ages at CT pulmonary angiography.Radiology. 2011; 260: 560-567
- Reduction of fetal dose in computed tomography using anterior shields.J Comput Assist Tomogr. 2015; 39: 298-300
- Reducing fetal radiation dose in computed tomography for pregnant patients: a literature review.Gazette Med Sci. 2021; 2: 35-43
- Shielding for neutron scattered dose to the fetus in patients treated with 18 MV x-ray beams.Med Phys. 2000; 27: 1800-1803