Mechanism of action:

• Radiation causes DNA damage in both normal cells and cancer cells which leads to apoptotic cell death
• Well oxygenated tissues are more susceptible to radiation, as there is adequate free radical formation.

Types of ionizing radiations:

• Electromagnetic radiations- Photons, gamma rays
• Particulate radiations- Electrons, protons

Sources

• Radioactive elements- Ex- 60Cobalt is source of gamma rays
• Linear accelerator- Source of photon beam

Radiation dose (Measured as energy per unit mass)

• 1 Joule/Kg= 1 Gray
• 1Rad= 1 centi-gray

Fractionated radiotherapy

• Radiation is given in multiple small doses over a given time, instead of single large dose
• It allows normal tissues to repair sublethal damage, while tumor cells resort themselves in the cell cycle
• Standard fractionation- 180-200cGy/day- 5 days a week
• Total dose depends on clinical setting and type of tumor. Ex: Lymphoma- 2000-4000cGy. 
• Different organs have different tolerance doses. They have to be taken into account while planning the therapy.
• Duration of treatment per day- 10-30min.

Methods of radiotherapy

• External beam radiotherapy- X rays from a treatment machine are directed onto patient
• Brachytherapy- Radioactive source is kept in close proximity of patient
• Stereotactic radiotherapy- Using imaging technique radiation is delivered in large doses to a well focused area.

Radiation along with other modalities of therapy

• Neoadjuvant- Radiation is given before definitive therapy
• Adjuvant- Radiation is given after definitive therapy
• Concurrent- Both chemotherapy and radiation are administered together.

Simulation

• Evaluation of patient in radiation oncology department, for localization of tumor and planning of RT.
• This is done using CT or PET/CT
• Takes into account gross tumor volume, clinical target volume and planning target volume.
• This will aid in delivering maximal dose of radiation to tumor tissue while attempting to avoid healthy tissue.
• Often, specialized immobilization devices are created for patient, to reduce inter-treatment variation in patient position.

Types of treatment volumes:

• Wide field radiation
  • Used in olden days, when boundaries were determined by bony landmarks with interspace between 10^th and 11^th thoracic vertebrae forming junction between supradiaphragmatic and infradiaphragmatic fields.
  • Mantle field was used for treating supra-diaphragmatic nodes.
  • Inverted Y field is used for treating infra-diaphragmatic nodes.
  • Present role is only in salvage therapy
• Involved field radiotherapy
  • Radiation delivery is limited to lymph node region involved with macroscopic lymphoma.
  • Even if 1 node is involved entire field is irradiated.
  • Involved fields include
    • Cervical lymph node- Entire neck including supraclavicularfossa
    • Mediastinal lymph node- Entire mediastinum including supraclavicularfossa
    • Paraaortic lymph nodes- Entire paraaortic chain
    • Inguinal nodes- Inguinal and ipsilateral iliac nodes
• Involved site radiotherapy
  • Used in pediatric Hodgkin’s lymphoma protocols.
  • Used for decreasing late effects of radiation
• Involved node RT
  • Same as involved site RT, but no additional margin around the node volume is added.
  • It is potentially hazardous, hence should not be used outside the setting of a clinical trial.

Indication for urgent radiotherapy

• Spinal cord compression
• Uncontrolled bleeding from tumor
• Superior vena cava syndrome