Читать книгу Veterinary Surgical Oncology - Группа авторов - Страница 44

Radiation can be used post‐operatively, pre‐operatively, or intraoperatively, depending on tumor type and location. For example, radiation can be used post‐operatively (adjuvantly) to treat a solid tumor in a location where wide complete margins cannot be achieved without limb amputation to save the limb. In this scenario, the radiation oncologist should be involved prior to surgery so that he or she can see if this approach is feasible; appreciate the size, fixation, and exact location of mass; plan the radiation field size and shape; determine how to spare normal tissue and include a large enough field; meet the owners; discuss complications, costs, expected outcome; etc. The surgeon’s role in this setting is a delicate, minimal surgery with the intent to preserve blood and oxygen supply to the tissue to increase the effectiveness of radiation. A marginal resection to remove all macroscopic tumor and allow primary closure is performed, and radiation therapy is used with surgery to provide long‐term tumor control or cure. This approach differs greatly from a failed curative‐intent surgery and a poorly healed or open, hypoxic, radiation‐resistant wound and a delayed start to radiation therapy: a situation that is avoided by a team approach and good planning. When adjuvant radiation is planned, the surgeon can help the radiation oncologist by decreasing wound complications such as infection, dehiscence, and seroma formation. Preservation of blood supply, gentle tissue handling, aseptic technique, attention to hemostasis, use of fine, non‐irritating (inert) suture material in minimal amounts, obliteration of dead space in the wound, avoidance of tension, post‐operative rest, and use of bandages are all important. Drains should be avoided if possible, and if they are used, drainage entry and exit holes are included in the radiation field. Hemoclips can be placed in the wound intraoperatively to delineate the boundaries of the excised gross tumor burden to assist the radiation oncologist in planning the radiation field (McEntee 2004, 2008).

Figure 2.1 (a) Standard or conventional linear accelerator. Most of the machines used in veterinary medicine produce electrons of varying energies as well as 6–20 MV photons. (b) Cobalt‐60 machine. This type of radiation‐producing machine relies on a Cobalt‐60 radiation source inside the head of the gantry. The half‐life of Cobalt‐60 is 5.27 years, necessitating replacement when the source decays to a negligible level. These types of machines are decreasing in popularity in veterinary medicine. (c) Linear accelerator with on‐board imaging. These linear accelerators are equipped with a cone‐beam CT in order to image a patient immediately prior to a radiation treatment. This is done in order to accurately localize a tumor in relation to surrounding anatomy and ensure precise dose delivery. These machines are often capable of conventional radiotherapy, intensity modulated radiation therapy (IMRT), stereotactic radiosurgery (SRS), and electron therapy.

Photo courtesy of AAPM.org.

(d) CyberKnife Stereotactic Radiosurgical unit. A CyberKnife is a linear accelerator mounted on a robotic arm. This allows delivery of radiation from thousands of angles around a tumor. KV x‐ray sources are positioned at orthogonal angles above the CyberKnife, which allow for accurate tumor localization with sub‐millimeter accuracy.

Source: Photo courtesy of Accuray.com.

Figure 2.2 (a) Conventional (i.e. standard) radiation plan for a nasal tumor (outlined in orange). In order to deliver the prescribed dose of radiation to the tumor, the right eye (outlined in blue) needed to be included in the radiation treatment field.

Source: Image courtesy of Siobhan Haney, DVM, MS, DACVIM (Radiation Oncology); Veterinary Cyberknife Cancer Center, Malvern, PA.

(b) Stereotactic body radiation therapy (SBRT) plan for a nasal tumor, which demonstrates how the dose can be sculpted to treat the tumor with a high dose of radiation (red areas) and the normal tissues receive a significantly lower dose (blue areas). The graph in the top right corner is a dose volume histogram. The tumor (red line: gross tumor volume [GTV], orange line: clinical target volume [CTV], purple line: planning target volume [PTV]) receives a high dose of radiation while the normal tissues (bue lines: eyes, light yellow lines: lenses, pink line: brain, and dark yellow: skin) receive a significantly lower dose.

Source: Image courtesy Bernard Séguin, technical assistance Dr. Erin Trageser.