Radiation therapy: A "ray" of hope (Proceedings)
Through clinical trials and retrospective studies, we have been able to learn which tumors respond best to what protocols and the body of knowledge continues to expand.
Roentgen first discovered the x-ray in 1895 and radiation therapy for the treatment of disease was implemented shortly thereafter to varying effect. Through clinical trials and retrospective studies, we have been able to learn which tumors respond best to what protocols and the body of knowledge continues to expand.
The treatment of cancer is complex as evidenced by the challenges presented in the decision-making process. What modality do we choose? Do we choose more than one? If radiation therapy is involved, is it pre-operative, or post-operative? Is our goal curative of palliative? Why choose radiation therapy at all? And so on.
Not all tissues are sensitive to radiation. Those most sensitive are lymphocytes, bone marrow, intestine, testis/ovary, skin, lung, kidney, ocular lenses, and lacrimal glands. Those least sensitive are neurons, muscle and bone. These are points for considerations when determining treatment plans for specific tumor types as well as any normal structure that must be included in the treatment field. Radiation is not a modality for gross disease. Surgical debulking is always indicated whenever possible.
Treatment can be divided into two broad categories: definitive and palliative. Definitive therapy can commence either immediately or two weeks post operatively (at presumed surgical healing). Depending upon cell type, total tumor dose may be either 54 Gy or 63 Gy given over 6 to 7 weeks. Palliative therapy is rarely surgically-dependent and is delivered in much larger fractions over shorter treatment schedules. A typical protocol would give a total dose of 24 - 36Gy over 4 weeks. Definitive therapy strives towards a cure while palliative therapy seeks to improve dysfunction and/or relieve tumor related pain. Before embarking on any plan of cancer treatment, it is of paramount importance to consider the owner's wishes and commitment to therapy. It is also critical to assess the patient in their entirety, not just as "a tumor". Thought must be given to prognosis, as well as other concurrent health issues.
While patients' generally tolerate therapy quite well, radiation treatments do produce some side effects that require management to one degree or another. Reactions can be classified as acute or delayed, short or long term. Acute or short-term toxicities most closely mimic tumor toxicities. Signs of delayed or long-term toxicities may have an acute or per acute onset months to years after completion of radiotherapy. Acute radiation toxicity generally appears in the middle or end of treatment and tends to be self-limiting. "Radiation sickness" is rarely seen although nausea/vomiting may occur in cases involving irradiation of the abdomen. Anorexia may also be seen in patients who develop an oral mucositis due to therapy. Cutaneous toxicities may be minor and involve only hair loss and dry skin or they may be temporarily more severe and progress to a phase of moist desquamation. The majorities of these patients have no interruption in therapy from these side effects and are medically managed on an out-patient basis. Delayed or long-term toxicities begin anytime 12 – 18 months post-treatment or thereafter. These side effects are life threatening and require medical intervention. They include bone necrosis, brain and neuronal necrosis, pneumonitis, and tumors induced by radiation.
The more commonly treated tumor types are soft tissue sarcomas, mast cell tumors, nasal and brain tumors, oral melanoma, and thyroid disease. The oral presentation will discuss the individual applications of radiation therapy for each tumor type mentioned as well as others commonly seen in practice.
The future for radiation therapy involves enhanced imagery and greater planning capability that allows us to deliver our dose more precisely. This spares normal surrounding tissue while giving the greatest radiation dose to the tumor. Intensity Modulated Radiation Therapy (IMRT) involves the use of multileaf collimators that are computer driven. Radiation therapy plans of this type are very complex consisting of several, irregularly shaped fields. As treatment of this type becomes more available, more will be learned regarding prognoses as well as projected diminution of certain side effects.
