Ans. c. G2M (Ref: Harrison 19/e 103e-4, 18/e p691-692; Schwartz 9/e p264-265)Susceptibility of various phases of cell cycle to radiation: G2MQ >G2>M >G1 >Early S >Late S PhaseQ.Radiation energy is absorbed by tissue causing ionization or excitationQ, which are responsible for various biological effects.Susceptibility of various phases of cell cycle to radiation: G2MQ >G2>M >G1 >Early S >Late S PhaseQ.Phase of Cell CycleCommentG2M>G2Most sensitiveQ to radiationEnd of S phaseMost resistantQ to radiationG1Radiation exposure leads to chromosomal aberrationG2Radiation exposure leads to chromatid aberrationRadiotherapyX-rays and gamma rays are the most common radiations used to treat cancersQ.X-rays are generated by linear acceleratorsQGamma rays are generated from decay of atomic nuclei in radio-isotopes like cobaltQ.RadiotherapyRadiosensitiserRadiation Protectors* OxygenQ (most effective Radiosensitiser)* MetronidazoleQ, misonidazole, tinidazole* 5-FUQ (non-hypoxic cell sensitizer)* HydroxyureaQ (non-hypoxic cell sensitizer)* BUDR and IUDRQ (non-hypoxic cell sensitizer)* CisplatinQ, paclitaxel, gemcitabine* MitomycinQ, topotecan, vinorelbine* Dactinomycin (Actinomycin D)Q* Amifostine* IL-1* GM-CSFIn RadiotherapyMost radiosensitive tissue of body: Bone marrowQLeast radiosensitive tissue of body: Nervous tissue/BrainQMost radiosensitive blood cell: LymphocyteQ (That's why Lymphocytic predominant Hodgkins lymphoma has best prognosis)Least radiosensitive blood cell: PlateletQMost common organ to be affected by radiation: SkinQ (Erythema earliest change, layer most commonly affected stratum basalis)Sebaceous gland function does not recover after radiotherapy.Pinna and axillae are common sites of radionecrosis i.e. for skin doses.Most radio resistant organ: VaginaMost common mucosa to be affected by radiation: Intestinal mucosaQ (Earliest symptom is diarrhea)Most sensitive abdominal organ: KidneyEffect of radiation on Testes and OvariesType B spermatogonia are exquisitely sensitive to the effects of radiationQ.Sertoli cells and Leydig cells are less radiosensitive than spermatogoniaQ.The single dose required for permanent sterilization on human males is 6-10 Gray (Gy)Q.The radiation dose necessary to induce ovarian failure is age dependent.A single dose of 3-4 Gray (300-40(1 rads) can induce amenorrhea in almost all women over 40 years of ageQ.In young women, oogenesis is much less sensitive to radiation than is spermatogenesis in men.Most radiosensitive ovarian tumor* DysgerminomaQMost radiosensitive brain tumor* MedulloblastomaQMost radiosensitive testicular tumor* SeminomaQMost radiosensitive lung tumor* Small cell CAQMost radiosensitive kidney tumor* Wilm's tumorQMost radiosensitive bone tumor* Ewing's sarcomaQ and Multiple myelomaQ