In relation to the treatment of head and neck cancers (HNCs), IMRT has permitted the development of highly conformal radiotherapy plans, which can sculpt the dose in a concave fashion around important organs at risk, such as the spinal cord, oesophagus, trachea and parotid glands (Fig. 10.1). This was not possible previously by using a 3DCRT technique. Randomized controlled trials have shown that IMRT results in superior salivary gland function and quality of life compared with 2-dimensional and 3DCRT techniques in the treatment of HNCs [10, 11].

Studies examining IMRT specifically for the treatment of thyroid cancer are few. However, many of the benefits seen in the treatment of other HNCs would probably apply. In a radiotherapy planning study, Nutting et al. found that IMRT improved planning target volume coverage and reduced the dose to the spinal cord [12]. This would potentially allow dose escalation or minimization of radiation myelopathy after EBRT. Also, in the previously mentioned study of 131 patients treated with EBRT for DTC from MD Anderson, 56 % underwent 3DCRT and 44 % underwent IMRT [5]. The authors found that IMRT was associated with less frequent severe late radiation toxicity compared with 3DCRT techniques (2 % vs. 12 %, respectively).

IMRT is capable of accurately delivering different doses of radiotherapy to separate clinical target volumes (CTVs). The ‘high-risk’ CTV encompasses gross residual disease and positive margins, the tumour bed, central lymph node region, as well as involved nodal regions. The ‘low-risk’ CTV includes uninvolved bilateral cervical lymph nodes (levels II–VI) and superior mediastinal lymph node regions. Supporting evidence suggests that treating larger volumes, including the upper mediastinum, might result in improved disease outcomes [13].

In the author’s practice, the high-risk CTV volume receives 66 Gy in 33 fractions whereas the low-risk CTV volume receives 59.4 Gy in 33 fractions. Other institutions have also used slightly different dose-fractionation schemes with ≤4 dose-levels, depending on the particular high-risk features involved [5].

Using doses of adjuvant EBRT of >50 Gy is supported by clinical evidence. In one study that examined 41 patients with DTC treated between 1988 and 2001 at two UK cancer centres, indications for EBRT included: (i) macroscopic residual disease (56 %), (ii) microscopic residual disease (24 %), (iii) Hürthle cell variant (7 %), (iv) multiple lymph nodes (7 %), and (v) focus of poor differentiation (5 %) [14]. The target volume was from the mastoid to sternal notch, and laterally the junction of the outer and middle-third of the clavicles. Radiotherapy techniques and doses were variable. Doses ranged from 37.5 to 66 Gy over 3–6.5 weeks, and 35 patients (85 %) received at least one dose of RAI. The 5-year local recurrence rates for patients who received <50 Gy, 50–54 Gy and >54 Gy were 63 %, 15 % and 18 %, respectively (p = 0.02 for trend). The authors concluded that doses of at least 50 Gy were required to improve local control [14].

EBRT for thyroid cancer is associated with acute toxicity, including radiation dermatitis, mucositis, oesophagitis, dysgeusia, dysphagia and laryngitis. Late toxicity can include soft-tissue fibrosis, xerostomia, tracheal stenosis, dysphagia and oesophageal stricture. Also, a second malignancy caused by EBRT is a risk. The use of advanced radiotherapy techniques, such as IMRT, can minimize severe toxicity, as mentioned above.

In the Memorial Sloan-Kettering experience delivering a median 63 Gy, acute grade 3 mucositis was present in 18 % of patients and dysphagia in 32 % [4]. IMRT was used in 63 % of these patients. Late, severe (grade 3+) toxicities were generally rare and present in <5 % of patients (grade 3 xerostomia 1 %, grade 3 dysphagia 4 %, grade 4 laryngeal oedema 2 %) [4]. This rate of late toxicity is comparable to that found in the MD Anderson experience in which patients treated with IMRT had a 2 % incidence of late severe toxicities [5].