PHYSICAL PRINCIPLES OF RADIOTHERAPY

Dr Sneha George

‘ Radiotherapy ‘ is defined as the treatment of disease with X-rays or other forms of radiation

Basis: Ionizing radiation produces more damage in malignant than in normal tissue

• Radiation necrosis - Normal tissue destruction due to excessive irradiation

• Tumour recurrence - Due to inadequate dosage failing to kill the tumour cells which after a specific period recover from the damage sustained start multiplying again

• General aim of radiotherapy

- To deliver as uniform a dose of radiation as possible to all parts of the tumour bearing zone

Parameters influencing biological effects of radiation - Radiation dose magnitude - Duration of treatment - Pattern of ionization produced by the radiation RBE (Relative biological effectiveness) Absorbed dose of 250 keV Xrays for a given effect = 0.9 Absorbed dose of 4 MV Xrays for the same effect

• Therapeutic ratio Ratio of damage sustained by the tumour

compared with the damage to normal tissue

• Radiation oncologist• Radiation Physicist• Radiotherapy technician

Types of Radiotherapy1) Teletherapy- ‘Tele’ – far- External source of radiation is at a distance from the

part being treated- X rays, Gamma rays, electrons and neutrons are used- Penetrating power of Xrays is dependent on the

generating voltage , filtration being used and the S.S.D

• 3 types of cancer therapy

‘Superficial’ therapy 60 to 140 kV 5 to 15 cm F.S.D 1-3 mm Al filter

‘ Deep’ or kilovoltage therapy 200 to 300 kV 50 cm F.S.D 1-2 mm Cu or 0.5 mm Sn filter

Megavoltage therapy 2 to 10 MV 70 to 100 F.S.D -

• ‘Grenz’ ray therapy – For superficial skin conditions ( 10-30 kV)

• Dosage data - Output of each radiation beam- Percentage depth dose values

2) Plesiotherapy‘ Plesios ‘ – near - Radiation source is close to the tissues being treated- S.S.D few centimetres or sources implanted into the tissues- Egs: Radium, radon, cobalt 60, cesium 137, iridium 192,

gold 198- Dose calculated by inverse square law and gamma ray dose

rate of the isotope- Dose uniformity by arranging the sources according to pre

determined patterns such as those prescribed by the Manchester Dosage system

3) Internal therapy - Radioactive isotopes are incorporated into the body tissues (Internal irradiators) - Radiation involved : Beta particles, Gamma rays - Dosimetry difficult because their distribution is not uniform