principles of radiotherapy

PRINCIPLES OF RADIOTHERAPY

E. CHISOR-WABALI

Definition

• It is the therapeutic use of ionising radiation in the treatment of patients with malignant neoplasia (and occasionally benign conditions).

TYPES OF IONISING RADIATION

Classified in terms of:• Their nature e.g. electromagnetic or

particulate • Linear energy transfer (LET)• Mode of production e.g. intra or extra nuclear

production

Linear energy transfer (LET)

-Sparsely ionizing radiation e.g. X-ray, gamma ray, Beta rays

- Intermediate ionizing radiation e.g. Electrons- Densely ionizing radiation e.g. alpha particles,

neutrons, negative pi mesons

Their nature……

• e.g. electromagnetic or particulate radiation-electromagnetic radiation e.g. X-rays and

gamma ray -particulate radiations e.g. electrons, alpha

particle, neutrons, protons, pions (pi mesons)

Mode of production

• e.g. intra or extra nuclear production• intra-nuclear e.g. Gamma rays, Beta rays• extra-nuclear e.g. X –rays, electron, protons,

neutron

Mechanism of cell death by radiation / Radiobiological damage of mammalian

tissue• The specific target of radiation damage in the

mammalian cell is the DNA molecule.• Damage to the DNA can be by:• 1. Direct interaction • 2. Indirect interaction

Direct interaction

• – direct damage to the DNA moleculeThis is true of high LET radiation

Indirect interaction

• more important in radiotherapy and the DNA molecular damage is by radiation induced free radicals.

• When radiation interacts with water, free radical ions are produced

• H20 --- H20+ + e-• H20+ -- H+ + 0H. ( the dot signifies

an unpaired electron )

…….continuation

• Free radicals are uncharged atoms or molecules with unpaired electron in the outer orbit.

• They are very reactive and result in breaks in the chromosomes.

• The biological damage may be repaired or result to:

• cell death• loss of reproductive integrity

Types of Radiation Treatment/ Machine

• Teletherapy (External beam irradiation) –Megavoltage machines-CO- 60, Linear accelerator

• Brachytherapy – Interstitial brachytherapy – radioactive sources are inside the tumour

- Contact brachytherapy or plesiobrachytherapy: radioactive sources are close to the tumour e.g intracavitary,intraluminal, endovascular,

and surface brachytherapy. • Systemic radiotherapy (Under Nuclear Medicine)- in form of

capsules or iv e.g. Iodine- 131(capsule), Strontium- 89(iv), Yttrium- 90

Oxygen Enhancement Ratio

• Is the ratio of the doses necessary to achieve the same biological effect in the presence or absence of oxygen.

• Oxygen enhances the sensitivity of tumor cells to the to the killing effects of ionising radiation.

Radiotherapy Planning

The Aim of Radiation therapy Is to deliver a precisely measured dose of

radiation to a defined tumour volume with as minimal damage as possible to surrounding healthy tissue, resulting in eradication of the tumour , high quality of life and prolongation of survival at reasonable cost.

Treatment Volume

Radiocurability

• Refers to the eradication of tumour at the primary or regional site and reflects the direct effect of radiation.

• Radiosensitivity– Inversely related to cell differentiation

• Type, size, clinical staging at 1st presentation• General condition of the patient• Dose-time relationship

ASSESSMENT BEFORE TREATMENT

A multidisciplinary approach is preferred in cancer management. This enables correct decision to be taken especially when cure is the goal because inappropriate initial treatment may compromise both the quantity and quality of survival.

Advances in Radiotherapy• Fractionation• Hypofractionation• Hyperfractionation• Acclerated Fractionation• CHART• 3D Conformal RT• Intensity Modulated RT• Stereostatic Radiosurgery/RT• Intravascular RT

Clinical Assessment in Radiotherapy

Consultant Radiotherapist and Oncologist is a physician and a technical specialist. Hence clinical and technical factors must be balanced to adequately plan RT.

Joint Clinics

• Collaborative spirit among specialists needed

• Timing of different modalities are properly synchronized

• Patient have access to advise of different specialists

• Patient is more reassured that all treatment options have been considered, before embarking on the chosen course of treatment

..continuation

• Provide basis for audit of treatment• Discussion about treatment is made by

specialists and the primary physician without the patient in attendance to prevent the patient from being intimidated.

RADICAL OR PALLIATIVE TREATMENT?

The choice will depend on

• The tumour• The patient• Resources

THE TUMOUR

• Site• Size• Spread (loco regional/ metastatic spread)• Operability• Radiosensitivity/ chemosensitivity• Histology (including differentiation)• Clearance of surgical resection margins

THE PATIENT

• Age and general condition (physical and mental)

• Morbidity and mortality• Function and cosmetics• Reliability of follow-up after treatment• Preference of patient

RESOURCES

• technical expertise• experience and • equipment

Tumours where RTH is the treatment of choice

• Oral cavity, lip, tongue, cheek• Nasopharynx, Oropharynx, Hypopharynx• Nasal cavity, Larynx, • Skin cancer(except melanoma),• Cervix, Bladder, Testis- seminoma,• Lymphomas –(early), • Meduloblastoma ( ffing surgical debulking),

Astrocytomas (grade 3 and4), Retinoblastoma

Curative Radiotherapy

08/04/23 27

PRE-OPERATIVE RADIOTHERAPY

Pre-operative radiotherapy implies that tumour is irradiated prior to surgery in the same anatomic site.

08/04/23 28

Advantages of Pre-op RT• sterilizing cells at the edges of the resection, • prevent recurrence after surgery, • damage the reproductive capability of the cancer

cells which are likely to be disseminated or implanted in to the wound during surgical manipulation,

• shrink a large mass with doubtful operability or with a high operative mortality risk,

• reducing the tumour volume sufficiently to allow resection,

• alter the lymphatic and vascular channels of the irradiated normal tissue so that the grafted tumour cells will not grow and

• to down stage a tumour.

08/04/23 29

DISADVANTAGES• Delay in wound healing if doses exceed

5000cGy; • The pathology reports are not valuable

because the destruction of tumour prevents ascertainment of the tumour’s biology, prognostic indices, and initial anatomic extent;

• Absence of surgical staging; and • Some patients who would not benefit from

pre-operative radiation are given this treatment.

08/04/23 30

POST-OPERATIVE RADIOTHERAPY

Post – operative Radiotherapy implies that tumour bed is irradiated after surgery.

08/04/23 UDUTH SOKOTO 2005 31

ADVANTAGES• patients who may be helped by radiation can

be defined accurately as a consequence of the surgical exploration and pathologic review and

• unnecessary irradiation to patients who are not likely to benefit can be avoided;

• also the target volumes are tailored to meet what is found at surgery.

• It is indicated where a residual tumour exist or is suspected after surgery and

• its main aim is to sterilize the surgical bed.

08/04/23 UDUTH SOKOTO 2005 32

DISADVANTAGES• It has no effect on seeding at the time of

surgery;• Alteration of the physiology of the tumour left

behind because of reduction of the vascular supply (cells that were well oxygenated may be rendered physiologically hypoxic and more resistant to radiation);

• Adhesions with resultant increased radiation toxicities e.g. radiation enteritis, intestinal perforation.

08/04/23 UDUTH SOKOTO 2005 33

INTRA-OPERATIVE RADIOTHERAPY

• Some radiation centers in developed world are using a single high-dose Intraoperative electron treatment for unresectable abdominal tumours and also to treat the surgical bed in patients not fit for the routine daily fractionations.

08/04/23 34

ADVANTAGES

• The major advantage of this technique is that a single dose is given (once);

• radiation field is well localised and • critical structures are moved out of the

field and thus • toxicity is minimized.

Radiosensitivity of Tumours

Highly SensitiveLymphomaSeminomaMyeloma

Ewing’s SarcomaWilm’s Tumour

• Moderately SensitiveSmall cell lung cancerBreast cancerBasal cell carcinomaMeduloblastomaTeratomaOvarian cancer

Relatively Resistant

Squamous cell Ca of lungHypernephromaRectal CarcinomaSoft tissue SarcomaCervical Cancer

Highly Resistant

MelanomaOsteosarcomaPancreatic carcinoma

SIDE EFFECTS OF RADIOTHERAPY

• Acute side effects radiation (Depend on the area being irradiated)

• But there are some general side effects- fatigue, malaise, anorexia, nausea and vomiting. These may be related to metabolic effects of tumour breakdown, bone marrow depression and the reaction to anxiety and stress.

Acute Effects

• Skin desquamation- dry,wet• Alopecia• Mucositis,dysphagia,xerostomia• Diarrhoea, abdominal cramps• Dysuria • Bone marrow depression- decrease in red

cells, platelets, and leucocytes.

Late Effects Radiation

• -Fibrosis, atrophy- Non –stochastic effect• -Carcinogenesis –damage to somatic cells• -Genetic mutation- damage to germ cells

Advances in radiotherapy