Radiation oncology-a primer for veterinarians (Proceedings)

The process of radiation oncology from the time of patient identification to completion of radiation therapy is undoubtedly a complex one and requires significant communication between all services from the moment a potential radiation patient is seen and certainly prior to any surgical intervention. Listed below is an outline of the radiation process and treatment considerations at each step.

Identification of potential radiation patients

It is anticipated that most radiation patients will be identified by referral through the medical oncology service. Patients may additionally be identified by internal medicine, surgery and other specialty services. The referral veterinary community may also make direct referral for radiation therapy. Patients with unresectable disease which may include nasal, oral or brain/spinal cord etc. should be identified early in the process and a consultation with radiation oncologist scheduled. Building a radiation oncology programs means that many patients will be screened for treatment even if only a small percentage are eventually treated. This is part of the process especially in the palliative setting where therapy may not be given at the time of initial consult.

Determination of goals

This is the most vital decision node during the entire process as it determines not only expectations from the owner but also lays the foundation for additional decisions such as completeness of staging, histopathologic confirmation, need for advanced imaging such as CT, MRI. Determination of treatment goals (palliation vs. definitive) is complex and requires owners to understand not only the basic biology of the specific tumor type, but also an understanding balance between likelihood of success vs cost, side effects, hospitalization stay etc. Palliative cases can rarely be converted to curative intent patients without adding even more uncertainty to the clinical situation.

Determination of imaging/planning needs

Depending on the case details (tumor type, location) and treatment goals, additional diagnostic tests may be required for treatment planning. Tumors of the distal limbs generally do not require computerized treatment planning, although proximal lesions can be complicated by changes in the anatomy at the body wall level. Tumors treated with a palliative course of radiation therapy may or may not require computer-based planning depending on size, location and other factors. Patients with tumors in more complex anatomic locations (head, neck, body wall, etc.) may require CT imaging for computer-based planning. dose prescription. If computer-based planning is required, input is necessary regarding imaging and patient positioning. In many cases (eg. brain tumors, nasal tumors) patient positioning can be standardized during the early diagnostic phases and can be routinely performed by internal medicine or surgery services In other cases, patient positioning will be determined by tumor size and location in addition to patient conformation and radiation oncology input is needed. Determination of appropriate patient positioning can be made based upon telephone conversation, digital imaging and, if necessary, video conferencing.

For patients requiring computerized treatment planning, the first step would be to identify the type of imaging needed. Most computer planning is based on CT imaging but fusion with MRI is possible and helpful in the case of spinal cord and brain tumors. Unfortunately, treatment planning systems based solely on MRI imaging are rare and dose issues involving tissue heterogeneity often necessitate both forms of imaging, adding anesthesia and cost. Field of view is extremely important with imaging as the data will be transferred to the treatment planning system (TPS) so the entire outline of the patient should be included. While the diagnostic value of the scan may be limited to the 4th lumbar vertebrae, the radiation oncologist needs several adjacent vertebrae in order to generate an accurate plan. More is always better. Slice thickness should be a minimum of 5 mm, 3 mm if possible and 1 mm if subtle brain lesions are suspected such as cribiform plate involvement of nasal tumors. Scars if visible should be identified via wire or barium paste, recognizing the potential impact of this on image quality. Draining lymph nodes should be included in the scan if they may be included in the field. Caudal thigh or perineal tumors often will have an iliac node treated which is distant from the primary site. It is also important to place hemoclips to delineate the tumor bed at the time of surgery as the incision no longer has relevance and will overestimate the region to be irradiated. Hemoclips aid in region of interest contouring in delineating the tumor bed in radiation treatment planning, can be visualized on beam's eye views as well as sometimes on portal radiographs

Patient positioning during therapy will attempt to be an exact duplication of the patient position at the time of CT. All movements of the treatment table during the setup will be determined by the instructions from the treatment planning computer around a specific reference point (fiducial mark) determined at the time of the CT scan. Similarly to patient positioning for CT scans, determination the specified reference point can be standardized for most patients. Patient positioning and repeatability is crucial. Cage paper tracing, Vac-lock bags, Aquaplast headframes and bite blocks are all strategies to keep body position the same for the planning CT as for the daily treatments. Use of lasers in the CT room and treatment room will allow the patient to consistently straight and aligned.

Determination of treatment volume

The radiation oncologist or technician will outline the gross tumor volume/surgical scar and target volumes. Once this task has been performed, the surgeon and/or medical oncologist will confirm that the tumor volume has been adequately identified. This step is essential to ensure that target volumes are being adequately identified and treated and may be done in consultation with board certified radiologist as indicated. The Gross tumor volume (GTV) is the visible tumor. The clinical target volume (CTV) is the GTV plus an appropriate margin for the tumor type (usually 2-3 cm) The margin for the CTV is based on the tumor type and its biological behavior. The Planning Target Volume is the CTV plus an appropriate margin that takes into account patient/organ movement, set-up variation and machine limitations.( probably another 0.5-1.0 cm) While the GTV and CTV are set by the tumor itself, limitations in technology help determine the PTV. Advanced imaging, more modern equipment and patient positioning devices can all allow us to shrink the PTV. Dose Volume Histograms are graphs that let you see what volume of tissue is receiving what dose of radiation. This is useful in ensuring that the tumor is receiving the desired dose and that normal structures (such as lung, spinal cord or eyes) are getting the minimal dose possible. Individual plans can then be compared. Many of the decisions involved in this process are subjective such as where to place the center of the beam, what to block out, how large of a margin of normal tissue is needed.

Modifiers of the radiation dose can be accomplished using wedges, multiple beams and custom blocking to avoid critical structures. If the tumor is superficial electron beam therapy is very useful as the dose falloff is steep, limiting dose in the deeper structures such as thoracic cavity in the case of a body wall sarcoma. Blocking can be done for hand or computer plans but is more accurate with computer planning of course. If computer planning is not done, the plan is done manually with simple measurement of margin and establishment of a treatment field encompassing the margin. Manual planning is a estimation of dose in most cases and inferior to computer planning in all but the most simple cases.

Patient setup using computer-based planning

Once the patient has been positioned and the field set up as per computer instructions, portal radiographs will be taken to determine the adequacy of patient positioning. The port films will be compared to digitally reconstructed radiographs (DRRs) generated from the treatment planning computer. The radiation oncologist will review the port film to confirm accurate patient positioning and treatment setup; We recommend a Simulation day, where all the above steps are performed, but the dose of radiation therapy may or may not delivered. This will allow a period of time for review of the plan, port films and dose prescription prior to actual delivery of radiation. This is routine in human radiation oncology, but is often not done in veterinary medicine due to relative ease of most treatment fields

Utilitity of newer technologies

3-D conformal radiation therapy is similar to conventional 3-D planning except that multiple beams are used to shape the dose closely to the target volume. This allows the radiation beam to be tightly shaped to the tumor and can allow sparing of normal tissues. Intensity modulated radiation therapy (IMRT) allows the computer controlled collimator to move during treatment allowing the beam to change over the course of a single treatment. Because the ratio of the dose delivered to normal tissues compared to the tumor is reduced to a minimum with the IMRT, potentially higher and more effective radiation doses can safely be delivered with fewer side effects compared with conventional radiotherapy techniques. Long term studies await veterinary medicine

Selected references

Charney SC, Lutz WR, Klein MK, Jones PD. Evaluation of a head-repositioner and Z-plate system for improved accuracy of dose delivery. Vet Radiol Ultrasound. 2009 May-Jun;50(3):323-9.

Farrelly J, McEntee MC. Principles and applications of radiation therapy. Clin Tech Small Anim Pract. 2003 May;18(2):82-7

Lawrence JA, Forrest LJ. Intensity-modulated radiation therapy and helical tomotherapy: its origin, benefits, and potential applications in veterinary medicine. Vet Clin North Am Small Anim Pract. 2007 Nov;37(6):1151-65; vii-iii.

Green EM, Forrest LJ, Adams WM. A vacuum-formable mattress for veterinary radiotherapy positioning: comparison with conventional methods. Vet Radiol Ultrasound. 2003 Jul-Aug;44(4):476-9.

Mori A, Shida T, Maruo T, Fukuyama Y, Imai R, Ito T, Kayanuma H, Suganuma T.Examination of the utility of a bite block-type head immobilization device in dogs and cats. J Vet Med Sci. 2009 Apr;71(4):453-6.

McEntee MC, Steffey M, Dykes NL. Use of surgical hemoclips in radiation treatment planning. Vet Radiol Ultrasound. 2008 Jul-Aug;49(4):395-9.

McEntee MC. Portal radiography in veterinary radiation oncology: options and considerations. Vet Radiol Ultrasound. 2008 Jan-Feb;49(1 Suppl 1):S57-61.

McEntee MC. Veterinary radiation therapy: review and current state of the art.J Am Anim Hosp Assoc. 2006 Mar-Apr;42(2):94-109.