9.5. CONVENTIONAL RADIOTHERAPY TECHNIQUE FOR TREATING THYROID CANCER

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1 9.5. CONVENTIONAL RADIOTHERAPY TECHNIQUE FOR TREATING THYROID CANCER ROBERT J. AMDUR, MD, SIYONG KIM, PhD, JONATHAN GANG LI, PhD, CHIRAY LIU, PhD, WILLIAM M. MENDENHALL, MD, AND ERNEST L. MAZZAFERRI, MD, MACP The term conventional radiotherapy refers to techniques that do not involve segmental modulation of beam intensity. Intensity Modulation Radiation Therapy and Tomotherapy are examples of delivery systems that are not conventional radiotherapy. The radiation oncology literature contains descriptions of many different conventional radiotherapy techniques for treating patients with thyroid cancer. The use of different techniques is not surprising because thyroid cancer presents the radiation oncology planning team with a difficult technical challenge. The basic problem is that the primary target volume (the thyroid bed) is in the midline of the body, extends both above and below the level of the shoulders, and requires a dose that exceeds spinal cord tolerance. The standard approach to irradiating a midline target in the neck to a dose above spinal cord tolerance is to deliver a portion of the dose with opposed lateral fields that exclude the spinal cord. The problem with this approach for thyroid cancer is that a lateral field that extends above and below the shoulders will produce a high dose gradient across midline structures. Specifically, midline dose above the shoulders is too high when the dose is specified at midline below the plane of the shoulders and too low below the shoulders when the dose is specified in the neck. There is no consensus within the radiation oncology community on how to manage the competing goals of target coverage, spinal cord shielding, and dose homogeneity in patients with thyroid cancer. Almost every textbook chapter and journal article on this subject describes a different radiotherapy technique. We include references at the end of this chapter that describe the range of conventional radiotherapy techniques for thyroid cancer. Every technique has advantages and disadvantages. At the University of Florida,

2 External Beam Radiation Therapy Table 1. Conventional Radiotherapy Dose Guidelines for Thyroid Cancer. Conventional Radiotherapy Dose Guidelines for Thyroid Cancer INITIAL FIELDS: 1.8/treatment (25 treatment days) using 6 MV photons through opposed anterior and posterior fields PHOTON BOOST FIELDS: Standard-risk subclinical disease: 1.8/treatment (10 treatment days) using 20 MV photons through opposed lateral fields that exclude the spinal cord with tissue compensators on the shoulders. Total dose: 1.8/treatment (35 treatments over 7 weeks) High-risk subclinical or gross disease: 1.8/treatment (14 treatment days) using 20 MV photons through opposed lateral fields that exclude the spinal cord with tissue compensators on the shoulders. High-risk subclinical disease usually means a positive surgical margin or nodal metastases with extensive extracapsular extension. Total dose: 1.8/treatment (39 treatments over 8 weeks) ELECTRON BOOST (POSTERIOR NECK STRIP) FIELDS: (treated on the same days as the photon boost fields) The level V nodes and soft tissue of the neck posterior to the plane of the spinal cord are boosted to at 1.8/treatment using 8-12 MeV electrons through lateral fields that match to the posterior border of the photon boost fields above the shoulders. Total dose: We usually boost the posterior strip of a hemineck to 70.2 with pathologically positive nodes in level V, a pathologically positive margin in the neck near the plain of the spinal cord, or gross residual disease. Other situations receive 50.4 or 63. our preference is to use Intensity Modulated Radiation Therapy (IMRT) in patients with thyroid cancer. Our IMRT technique is the subject of the next chapter. When we treat thyroid cancer with conventional radiotherapy, we use a technique that is so old that it predates the era of CT treatment planning. Basically, we build tissue compensators on the shoulders so that we can use standard lateral fields to treat the thyroid bed above spinal cord tolerance. TISSUE COMPENSATORS ON THE SHOULDERS We deliver 45 Gy with 6 MV photons through opposed anterior and posterior fields that cover most of the level II-VII nodes bilaterally (Fig. 1). After 45 Gy, we use opposed

3 9.5. Conventional Radiotherapy Technique for Treating Thyroid Cancer 421 Figure 1. Usual borders of the anterior and posterior 6 MV photon fields that are used to deliver the first 45 Gy (prescribed at midplane) in patients with thyroid cancer. lateral fields that exclude the spinal cord (Fig. 2). The posterior border of these lateral fields is near the middle of the vertebral bodies. The lateral fields are treated with MV photons to the final prescription midline dose. Radiation therapists position the lateral fields each day using laser coordinates and field borders drawn on the skin. Figure 2. Usual borders of the opposed lateral 20 MV photon fields that start following the completion of therapy through opposed anterior and posterior fields. The posterior border of the lateral photon fields is located along the midline of the vertebral bodies.

4 External Beam Radiation Therapy (a) (b) Figure 3. Lateral (A) and superior (B) direction view of the tissue compensators that are used to prevent large dose gradients at midline above and below the level of the shoulders. We use beeswax but commercial products are readily available that work well for this purpose (Radiation Products Design, Inc. at When the patient is in the correct position, the therapists place tissue compensators on the shoulders (Fig. 3). These compensators must be made of a material that is easy to conform to the shape of the patients shoulder and has an electron density that is similar to tissue. We make the compensators out of beeswax that we buy from a local beekeeper. Super Stuff Bolus Material is a commercial product that is equally useful (Radiation

5 9.5. Conventional Radiotherapy Technique for Treating Thyroid Cancer 423 Figure 4. The nodes and soft tissue posterior to the lateral photon fields (and overlying the spinal cord) are boosted with 8-10 MeV electron beams. The photon fields are treated first with the tissue compensators in place. The posterior border of the photon fields is drawn on the skin each day. After removing the tissue compensators, the poster neck strips are treated with electrons. The anterior border of the electron field matches the posterior border of the lateral photon fields on the skin. Products Design, Inc. at It is essential that the compensators extend to the edge of the shoulder laterally and beyond the field border superiorly. The use of tissue compensators eliminates the skin sparing normally associated with high-energy photon beams. When treating patients to a total dose of Gy, we see small areas of moist desquamation of the neck skin. Overall, the acute toxicity and frequency of late complications is similar to what we see with without tissue compensators. It is rare to see skin fibrosis or shoulder problems from radiotherapy. We treat the tissue posterior to the posterior border of the lateral photon fields with 8-12 MeV electrons depending on the estimated depth of nodal tissue (Fig. 4). The therapists remove the compensators when treating electron fields. The anterior border of the electron field matches to the posterior border of the lateral photon field. The posterior border of the electron field is usually just posterior to the spinous process of the second cervical vertebra. The superior border is at the superior border of C-2 and the inferior border is at the junction of the neck and shoulder.