Principles of radiation therapy
Radiation therapy is used in more than half of cancer patients. This method can be applied in two formats:
- As an independent treatment method. Radiation monotherapy demonstrates high clinical efficacy in early-stage lower lip cancer, cervical lesions, and skin cancer.
- As part of combination therapy. Radiation is most frequently combined with surgery (irradiation can be performed in either the preoperative or postoperative period). The method also effectively complements chemotherapy and hormone therapy.
Depending on how the radiation dose is delivered to the pathological focus, three types of radiation exposure are distinguished:
- External beam radiation therapy. Performed using powerful medical units in a $\gamma$-therapy format. Radioactive substances serve as radiation sources, such as cobalt ($^{60}\text{Co}$) or cesium ($^{137}\text{Cs}$) isotopes.
- Intracavitary radiation therapy. Involves introducing a radiation source through natural body orifices. Used when the tumor is localized in the oral cavity, uterus, urinary bladder, or maxillary sinus.
- Interstitial radiation therapy. Radioactive isotopes are implanted directly into the tissue mass as special needles or capsules. This method is often used after the main tumor bulk is removed (e.g., following mastectomy) to exert a prolonged destructive effect on remaining cancer cells and regional lymph node metastases.
Role and place of chemotherapy
Medical treatment of tumors has strict indications, and its efficacy varies depending on the pathology type:
- In solid tumors (which include lung, breast, stomach, and bowel cancer), isolated chemotherapy is significantly inferior in efficacy to surgical and radiation treatment methods.
- In systemic diseases (various types of leukemias, Hodgkin's lymphoma), drug therapy is of decisive importance and forms the basis of the treatment protocol.
- In combination therapy, chemotherapeutic drugs are used in strict combination with other methods (e.g., complementing surgery in ovarian cancer).
Important clinical rule: using chemotherapy exclusively in early-stage tumors that can be radically removed surgically is strictly unacceptable.
Groups of antineoplastic drugs
The pharmacological arsenal of chemotherapy is divided into several main groups based on the mechanism of action and the origin of the active substance:
1. Cytostatic drugs Their mechanism of action consists in inhibiting tumor cell proliferation by directly suppressing their mitotic activity. Representatives: cyclophosphamide, thiotepa, chlorethylaminouracil, vinblastine, vincristine.
2. Antimetabolites This group of drugs severely disrupts the normal metabolism in the cancer cell. Their sites of action vary:
- Suppression of purine synthesis processes (mercaptopurine).
- Toxic effects on enzyme systems (fluorouracil, tegafur).
- Blocking normal folic acid conversion processes (methotrexate).
Cytarabine also belongs to antimetabolites.
3. Antineoplastic antibiotics These are specific biological substances synthesized by certain species of fungi or microorganisms. Representatives: dactinomycin, rufocromomycin.
4. Hormone therapy Indicated exclusively for the treatment of hormone-dependent tumors. The principle of administration is based on the use of antagonist drugs:
- Androgens (male sex hormones: testosterone, methyltestosterone) are prescribed for breast cancer.
- Estrogens (female sex hormones: hexestrol, diethylstilbestrol) are used in the treatment of prostate cancer.
In addition to oral forms, there is the concept of surgical or radiation hormone therapy. These are targeted interventions on endocrine glands for therapeutic purposes (a classic example is surgical or radiation sterilization of women in the comprehensive treatment of breast cancer).
Briefly: answers to topical questions
Which drugs belong to the cytostatic group?
The cytostatic group includes agents that inhibit tumor cell proliferation by affecting their mitotic activity. Depending on the mechanism of action, the following drugs are distinguished:
- Alkylating agents — cyclophosphamide.
- Antimetabolites — azathioprine, mycophenolate mofetil.
- Other cytostatics — thiotepa, chlorethylaminouracil, vinblastine, vincristine.
Which antineoplastic antibiotics are used in clinical practice?
Antineoplastic antibiotics include drugs derived from the metabolic products of fungi or microorganisms. The main groups and representatives include:
- Anthracyclines — doxorubicin.
- Bleomycin — used in lymphomas, germ cell tumors, and squamous cell carcinoma.
- Mitomycin.
- Dactinomycin — historically the first antibiotic in chemotherapy.
- Rufocromomycin.
What specific complications are caused by systemic administration of cytostatic drugs?
Cytostatic drugs can cause the following complications:
- Cardiac complications — pericarditis and myopericarditis; busulfan use may cause pericardial and endomyocardial fibrosis.
- Pulmonary toxicity — interstitial lung disease with busulfan use, as well as severe pulmonary fibrosis ("bleomycin lung") with bleomycin.
- Secondary carcinogenesis — induction of secondary tumors due to the mutagenic activity of cytostatics, especially alkylating agents.
- Other complications — alopecia, oral mucositis, dermatotoxicity, and bladder damage.
For memorizing the sites of action of antimetabolites: "Purines — mercaptopurine, Enzymes — fluorouracil, Folic acid — methotrexate".
Class and exam questions
Can early-stage resectable cancer be treated with chemotherapy alone?
No, this is completely unacceptable. If an early-stage tumor can be removed surgically, surgery should be the priority method.
In which cases is radiation therapy used as an independent treatment method?
As monotherapy, irradiation is highly effective in early-stage lower lip cancer, cervical cancer, and malignant skin lesions.
What is the essence of interstitial irradiation?
This is a method where radioactive isotopes (in the form of needles or capsules) are inserted directly into tissues. This is often done after tumor removal (e.g., in mastectomy) to target remaining cells and lymph node metastases.
Why are estrogens prescribed in oncology?
Estrogens (female sex hormones, e.g., hexestrol) are used as part of hormone therapy for prostate cancer, as it is a hormone-dependent tumor.
What else to study on this topic
- Characteristics of the use of radioactive cobalt and cesium isotopes
- Mechanism of action of cytostatics on cellular mitotic activity
- Principles of surgical and radiation sterilization as a method of hormone therapy
- Combination treatment regimens for ovarian cancer
- Side effects of antimetabolites and antitumor antibiotics
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