Cancer Radiation Therapy
Methods Of Radiation Administration
Radiation therapy can be administered via several methods including external beam radiation, brachytherapy or internal radiation, and radiopharmaceuticals or systemic radiotherapy (ACS 2014a).
External Beam Radiation Therapy
External beam radiation therapy (EBRT) is the most widely used method of radiation therapy. A machine called a linear accelerator (LINAC) is used to deliver the radiation beam to the tumor (ACS 2014a). Techniques to specifically target the tumor and spare healthy tissue have improved over time; however, healthy tissue damage continues to be a problematic side effect of EBRT. Strategies for managing these side effects are discussed at length in the “Preventing Damage to Healthy Tissue” section.
EBRT can be delivered in various ways:
- Three-dimensional conformal radiation therapy (3D-CRT) uses imaging computers to precisely map the location of a tumor in three dimensions (ACS 2014a; Gupta 2012) and is the main form of EBRT used in clinics around the world (Purdy 2008). By targeting the radiation precisely to the tumor, doctors reduce radiation damage to normal tissues (ACS 2014a).
- Intensity-modulated radiation therapy (IMRT) is similar to 3D-CRT. However, IMRT changes the strength of the radiation beam in some areas, increasing it over certain regions of the tumor. This reduces the risk of damage to nearby healthy tissue (ACS 2014a; Purdy 2008; Schild 2017).
- Image-guided radiation therapy (IGRT) combines radiation with an imaging scanner, allowing the doctor to see if minor adjustments are necessary just prior to administering radiation (ACS 2014a; Purdy 2008; Mahase 2015).
- Intraoperative radiation therapy (IORT) is external radiation that is delivered to the tumor during a surgical procedure and is especially applicable to cancers deep inside the body (ACS 2014a; Dutta 2017).
- Stereotactic radiosurgery (SRS) delivers a large dose of radiation to a small tumor, usually in the brain or within the skull, and no surgery is involved (ACS 2014a). Stereotactic body radiation therapy (SBRT) uses this technique in other parts of the body including the spine, liver, and lung (ACS 2014a; Baliga 2017). SRS and SBRT use a machine called a Gamma Knife, which delivers multiple beams from many different angles. Other machines, including CyberKnife or X-Knife, combine advanced cameras to locate the tumor’s position in the body and robotics to deliver highly focused beams of radiation at the tumor. All these technologies aim at avoiding normal tissue (ACS 2014a).
Proton Beam Radiation Therapy
Proton beam radiation therapy, or “proton therapy,” is one of the most precise and sophisticated forms of external beam radiation therapy available (Thariat 2013; Foote 2012; DeLaney 2011; Wang 2015). Protons are subatomic particles that interact with the body’s tissues in unique ways due to their physical characteristics. The physical properties of proton beams allow the technician to better target and deliver high-intensity radiation selectively to tumor tissue compared with standard radiation therapy. Proton beam therapy can minimize damage to healthy tissue outside the tumor (DeLaney 2011; Foote 2012).
Standard external beam radiation therapy penetrates through the body, including healthy tissues. A high dose enters the body and then dissipates as it passes through and exits the body. In contrast, when a beam of protons enters the body, the protons deliver their energy as they approach their target, and then they stop moving. Thus, the radiation dose to healthy tissue is minimized, and side effects are decreased (Foote 2012; DeLaney 2011; Liu, Chang 2011; NAPT 2015; MDACC 2015). Proton therapy, due to its targeted delivery, allows for about 60% lower total radiation dose than other forms of external beam photon radiation (DeLaney 2011).
Proton therapy is appropriate for patients with solid tumors that have defined borders, as it is delivered as a precise beam to a specific area (MDACC 2015; ASCO 2015). According to the American Society of Clinical Oncology, cancers currently treated with proton therapy include (ASCO 2015):
- Certain brain cancers
- Melanoma of the eye
- Head and neck cancers
- Lung cancer
- Liver cancer
- Prostate cancer
- Spinal and pelvic sarcomas
As of 2017, there are over 50 operating proton treatment centers worldwide, with more than 50 additional facilities in building or planning stages (Wikipedia 2017; Trikalinos 2009a).
Brachytherapy or Internal Radiation
Brachytherapy involves the insertion of radioactive materials into the body around or within the tumor. Devices such as small pellets, wires, capsules, or tubes that emit radiation are inserted either temporarily or permanently. Radiation is delivered to the interior of the tumor (ACS 2014a; Tanderup 2017). In some cases, imaging techniques are used to guide placement of a brachytherapy device (Wang, Tang 2017; Tanderup 2017; Yoshida 2017).
Brachytherapy is increasingly used to treat prostate cancer (Heysek 2007), but is also used for cervical, uterine, breast, head and neck, and skin cancers, and several other cancers (Lukens 2014; Lloyd 2017; Wang, Tang 2017; Tanderup 2017; Yoshida 2017). Radioembolization is a technique used for liver cancer in which radioactive beads called microspheres are injected into the artery feeding the tumor (ACS 2014a; Kishore 2017).
Brachytherapy implants may stay in place for minutes to days, or in some cases, permanently. The intensity of the radiation emitted by the implanted device can also vary according to the needs of each patient (ACS 2017b).
Radiopharmaceuticals are drugs that contain radioactive isotopes. They are administered orally or intravenously. Radiopharmaceuticals can be used therapeutically to treat cancer and as tracers for imaging tests to determine the location of a tumor. Strontium 89 (Metastron), samarium 153 (Quadramet), and radium 223 (Xofigo) are used to treat bone metastases, including those from breast and prostate cancers (Heianna 2014; Thapa 2015; Nilsson 2015; Body 2015; Humm 2015; Jong 2016; Florimonte 2016; Choudhury 2012). Iodine 131 is used to treat thyroid cancer (Fard-Esfahani 2014).
Radioimmunotherapy uses drugs called monoclonal antibodies attached to a radioactive isotope. These antibodies target molecules on cancer cell surfaces, delivering their radioactive payload to the malignant cells (Kawashima 2014). Radioimmunotherapy showed promise in treating non-Hodgkin B-cell lymphoma, but has not proven to be as effective for solid tumors (Kawashima 2014; Kraeber-Bodere 2014; Rizzieri 2016). One important side effect observed with some radioimmunotherapy products included a temporary drop in white blood cell and platelet count (Ghobrial 2004).
Approved Radioprotective Medications
Two drugs are approved for protection against radiation damage and minimization of side effects of radiotherapy (Hall 2016; Panjwani 2013).
Amifostine (Ethyol) is a synthetic antioxidant initially developed to protect against radiation in the event of nuclear warfare (Gu, Zhu 2014; Okunieff 2008). This FDA-approved drug reduces the incidence of moderate-to-severe xerostomia (dry mouth) in people with head and neck cancer who are undergoing radiation treatment (MedImmune 2007). Amifostine also reduces the incidence of mouth sores and difficulty swallowing (Gu, Zhu 2014). Amifostine reduces side effects such as dermatitis, lower gastrointestinal mucositis, esophagitis, and pneumonitis in patients treated with radiation therapy (Kouvaris 2007). Importantly, amifostine does not protect the tumor against the toxic effects of radiotherapy and does not reduce survival. Amifostine may cause nausea, vomiting, transient hypotension, or allergic reactions, which limit its use (Gu, Zhu 2014; Kamran 2016).
Palifermin (Kepivance) is an injectable recombinant human growth factor (Lauritano 2014). Palifermin was approved by the FDA in 2004 for certain cancer patients who develop severe oral mucositis as a result of radiation or chemotherapy treatment (Blijlevens 2007; Lucchese 2016; Nooka 2014; Peterson 2010). In clinical trials, the most important adverse effects from palifermin included rashes, itching, and skin-related swelling (NCI 2013; Gold Standard 2016).