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Advantages of proton therapy for nasopharyngeal carcinoma

Nasopharyngeal carcinoma (NPC) is a common cancer the head and neck. The incidence rate of nasopharyngeal carcinoma is high, accounting for 47% of the total number of worldwide cases in the world. In Southern China this cancer is 20 times as common as in the rest of the world, thus being called the "Guangdong cancer". Because nasopharyngeal carcinoma originates in tissues adjacent to the skull base, it is relatively easy for it to spread to the brain, neck and distant organs, thus making it unsuitable for surgical management. Pathologically it is usually described as low and moderately differentiated squamous cell carcinoma which is sensitive to radiation. Therefore, radiotherapy is the primary treatment for nasopharyngeal carcinoma.

Disadvantages of photon therapy

In recent years, the technology of intensity modulated radiation therapy (IMRT), targeted therapy, and immunotherapy have been widely implemented, leading to improved 5-year survival rates that exceed 80%. As curative success is being realized for many patients, long-term quality of life for nasopharyngeal patients has become increasingly important. Some of the limitations of traditional photon radiotherapy include late complications such as dry mouth, impairment of mouth opening, dental decay and caries, cranial nerve and brain injury, and pituitary dysfunction.  In addition, low-dose radiation exposure to normal tissues of the head and neck from treatment can lead to subsequent second cancers in 0.5% ~ 3% of patients.

Advantages of proton therapy

Proton therapy is a new treatment technology that takes advantage of its unique "Bragg peak" beam characteristics to treat tumors. At present, it is widely used in Europe, America and other developed countries. When a photon ray enters the human body, the energy of the ray is gradually absorbed as it travels through the depth of the body.  By the time it reaches the tumor target (Fig. 1), its dose is greatly reduced. At the same time, the normal tissues (OAR) around the tumor target such as the brain stem, spinal cord, parotid gland and skin are exposed to higher doses of radiation (Fig. 2), resulting in the side effects of radiotherapy. Due to the special "Bragg peak" characteristic of protons, the dose of proton therapy remains basically stable after entering the body until it rises and reaches the peak within the target tumor.  Here it releases most of its energy, then rapidly decreasing in energy after it passes through the target.  Within a short distance there is little remaining radiation to be absorbed by the surrounding normal tissue.  This potentially reduces adverse reactions from treatment.

Figure 1: Relationship between radiation dose (proton vs. X-ray photon) and depth in vivo

(OAR: organs at risk, including brain stem, spinal cord, parotid gland, skin, etc.)

Fig. 2 Comparison of radiation dose distribution between proton therapy (IMPT) and traditional photon therapy (IMRT) for nasopharyngeal carcinoma

At present, about 10% - 15% of patients with nasopharyngeal carcinoma may still suffer a local recurrence after completing treatment. Most relapses after photon therapy are caused by tumor cells that are relatively insensitive to radiation. If such patients are treated with a repeat course photon radiotherapy, the outcome may not be ideal.  In addition, the toxicity and side effects caused by a repeat course photon radiotherapy are much higher than those caused by the first course radiotherapy. Most patients find such treatment difficult to tolerate and there is risk of life-threatening bleeding. The anatomical structure of the nasopharynx is complex and adjacent to many important tissues and organs, so it is very difficult to provide salvage treatment. Due to the unique advantages of proton therapy from both a physics and biologic perspective, proton therapy is the best treatment choice for patients with recurrent nasopharyngeal carcinoma.

Proton therapy not only improves the curative rate of nasopharyngeal carcinoma patients, it also greatly reduces the toxicity and side effects of radiotherapy and improves patient quality of life. Among patients with recurrent nasopharyngeal carcinoma, proton therapy plays a critical role. Using this technology, Guangzhou Taihe Cancer Hospital aims to further improve the survival rate and quality of life of patients with nasopharyngeal carcinoma.

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