This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Radiotherapy, Subject name: Radiology
Which of the following is used for permanent interstitial implant brachytherapy? 
Select and explain. 
A. Boron, B. Cesium 133, C. Phosphorus, D. Iridium
B: Cesium 133
Answer: B: Cesium 133 
Explanation: Ans. b (Cesium 133). (Ref Introduction to Radiotherapy by Rafla, 7)Following are the methods of treating with radiotherapy:# External radiations is teletherapy# Brachytherapy# Systemic radiotherapyBRACHYTHERAPY (internal radiation therapy/short distance therapy)# Definition : source of emission is within the body/tumor# It is sealed (Platinum coated) sources placed within or near to the tumor for therapy.# It includes:- Interstitial- Intracavitary- Mould therapy# Interstitial radiations- Removable/temporary sources or implants* Iridium-192 (HDR)* Cesium (137 Cs) - LDR* Cobalt (60Co)* Californium* Radium* Tantalum- Permanent sources or implants* 131Cs* Yttrium* Gold* 1125* Radon* Palladium--(Mnemonic: CY GRIP)-# Intracavitary radiations- Removable:* Radium* Cesium 137* Cobalt - 60- Permanent:* Colloidal radioactive gold* Yttrium* Radioactive iodine (131 I)Agents used in Systemic radiation therapy: Iodine (I31 I), Phosphorous (32p), Yttrium, Strontium, Samarium, Rhenium.