Why Consider SBRT for Head and Neck Cancer?
Stereotactic body radiotherapy (SBRT) for head and neck cancers has emerged as a powerful tool in the radiation oncologist’s armamentarium. Elderly patients, those with poor performance status, or individuals with life-limiting comorbidities often cannot tolerate lengthy courses of conventional chemoradiation. For these patients, SBRT delivers durable local control through a shortened treatment course with an acceptable side effect profile.
Head and neck SBRT was initially considered primarily in the re-irradiation setting. However, its greatest value may lie in the unirradiated setting, where the extended treatment and recovery time of radical therapy is undesirable or unrealistic for certain patients. High-volume centers demonstrate that SBRT can achieve durable local control, with reported doses of 35 to 50 Gy delivered in 3 to 8 fractions.
The decision against a protracted radiation course typically involves multiple factors: patient preference, tumor factors (expected morbidity of progression versus treatment risk and probability of success), life expectancy (influence of age and comorbid conditions), tolerance of aggressive treatment based on performance status, and non-host factors such as distance from hospital and availability of social, financial, and psychological support.
Multidisciplinary Team and Technical Requirements
Head and neck SBRT demands a highly experienced multidisciplinary team of medical physicists, dosimetrists, and radiation therapists. This is not simply a matter of adapting a lung SBRT protocol to the cervicofacial region — the proximity of critical neurological structures and anatomic complexity impose specific challenges.
Accurate GTV delineation is critical for safe head and neck SBRT. Intraoral photographs documenting clinical exam findings can prove invaluable. Neuroradiology review can clarify tumor extent and localize radiosensitive organs at risk with greater precision.
Simulation and Imaging: Contrast CT and MRI Fusion
Contrast-enhanced computed tomography (CT) simulation is required for precise volume definition. MRI simulation fusion greatly improves gross disease visualization. In practice, MRI fusion is strongly recommended whenever available — the improvement in delineation quality is substantial.
When MRI is not available, double-contrast CT simulation can serve as a viable alternative. An illustrative case describes a 79-year-old woman with T1N1 squamous cell carcinoma of the base of tongue: single-contrast CT simulation (80 mL) did not adequately visualize the GTV, while double-contrast simulation (160 mL) allowed for excellent tumor definition.
Dental fillings that create artifact and impair visualization should be removed prior to SBRT. In one documented case, an 87-year-old frail gentleman with an MRI-incompatible pacemaker had severe artifacts caused by dental fillings. GTV delineation was only possible after tooth extraction. Alternatively, metal fillings can be replaced with non-metal materials.
Immobilization, PTV Margins, and Dose Protocol
A five-point thermoplastic mask combined with daily cone beam CT (CBCT) matching allows for reproducible immobilization and reduction of PTV margins to 3 mm. Toxicity is further minimized by eliminating the traditional comprehensive microscopic volumes.
The standard dose range to the GTV is 40 to 50 Gy, delivered at two fractions per week, with 45 Gy being the most commonly prescribed dose. A critical point: no high-dose or low-dose CTV expansion of the GTV is used. A microscopic nodal CTV can be created for immediately adjacent at-risk lymph node sites. The dose-reduced PTV is created with a 3 mm expansion of the GTV/high-dose CTV.
Hot spots should lie within the GTV and away from organs at risk. A conformity index of 1.1 for the GTV and PTV is desirable. Target coverage must be compromised when in proximity to critical neurological structures — brachial plexus, optic pathways, brain, and brainstem. Dose to the carotid artery, however, should not compromise target coverage, except in re-irradiation.
Target Volumes: Definitions and Expansions
The following table summarizes target volume definitions used in the head and neck SBRT protocol, illustrating the tiered dose approach.
| Volume | Definition and Description |
|---|---|
| GTV40-50 | Primary: all gross disease on physical exam and imaging, including T1-gadolinium, T1 fat-saturated, and T2 MRI sequences. Fusion of contrast-enhanced simulation CT with MRI. If patient factors preclude MRI, GTV visualization on simulation CT can be enhanced using double contrast. Neck lymph nodes: those with necrotic center or PET-avid. |
| CTV40-50 | With precise GTV delineation, this volume equals the GTV40-50. |
| PTV35-40 | CTV40-50 (equivalent to GTV40-50) + 3 mm, with daily CBCT. |
| CTV35-40 | Suspicious nodes (round, enhancing). |
| PTV30-35 | CTV35-40 + 3 mm if this volume is near other high-dose volumes and good cone beam match is expected. If not feasible, CTV35-40 + 5 mm. |
| CTV25 | Includes high-risk lymph node basins immediately adjacent to treatment volumes, where repeat radiation for regional recurrence would be difficult. |
| PTV25 | CTV25 + 3-5 mm. |
Source: Target Volume Delineation and Field Setup, 2nd Edition (Table 3.1)
Quality Assurance and Image Verification
A strong quality assurance (QA) program is essential. The described protocol employs a modified Winston-Lutz isocenter alignment test to ensure tolerance within 2.5 mm. Daily CBCT to match bone and soft tissue is imperative.
An important practical consideration: since the total number of CBCTs is minimal (typically 5 fractions), attempts to decrease CBCT dose are of little value and should not preclude high-quality images. Always prioritize verification image quality over dose savings in this context.
Clinical Cases: SBRT Versatility in Head and Neck
Published cases demonstrate the breadth of head and neck SBRT applications. A 73-year-old woman with unresectable T1N3 squamous cell carcinoma of the piriform sinus compressing the internal jugular vein elected against protracted radiation. She received 50 Gy in five fractions to the nodal GTV and 40 Gy in five fractions to the primary GTV, two fractions per week. Target coverage was not compromised to spare the carotid artery. No evidence of disease at 2 years.
Another striking case: a 65-year-old woman with extensive oral cavity squamous cell carcinoma measuring 6.9 by 4.0 cm, extending from the skull base along the infratemporal fossa into the masticator space and right mandible, causing pathologic fracture and trismus with mouth opening of 1.5 cm. She received 45 Gy in five fractions. Four years later, she could open her mouth 4 cm and remained disease-free.
SBRT works even in very elderly patients. A 100-year-old woman with recurrent squamous cell carcinoma of the skin at the parotid and neck nodes received SBRT. The nodal CTV encompassed the high-risk nodal basin at 25 Gy while the GTV received 45 Gy. She remained well for 6 months, then recurred regionally both inside and outside the low-dose field — illustrating the limitation of SBRT when microscopic regional disease is not addressed.
SBRT also applies to patients with concurrent malignancies. A 66-year-old gentleman with superior vena cava obstruction from a non-small cell lung mass and a T2N1 base of tongue cancer crossing midline received 45 Gy in five fractions, two per week. The primary GTV was delineated in red, the larger nodal GTV in orange (45 Gy), and the smaller in green (40 Gy). After cervical SBRT, he started second-line systemic therapy for his lung tumor. No evidence of cervical disease at 18 months, tolerating all food textures without pain.
Primary parotid tumors represent another relevant indication. A 91-year-old gentleman with facial nerve palsy from poorly-differentiated carcinoma of the left parotid and two retropharyngeal nodes received 50 Gy in five fractions. He achieved complete clinical response with return of facial nerve function. No evidence of disease at 6 months.
Oligometastatic disease adjacent to critical neural structures can also be treated. A 55-year-old woman with an unresectable solitary colorectal metastasis in the supraclavicular fossa — a 6-cm mass — had MRI simulation used to differentiate the GTV from the brachial plexus, enabling safe treatment with 45 Gy in five fractions, two per week. The mass recurred 3 years later in the left neck.
Post-Treatment Assessment and Delayed Response
The rate of regression post-SBRT is variable, and maximal response is often achieved beyond traditional timelines — typically beyond 3 months. Early response assessments can be misleading and generate unnecessary concern.
A case of discordant post-treatment imaging illustrates this well: an 83-year-old woman treated surgically 3 years prior for squamous cell carcinoma of the right tongue presented with a painful right level II nodal mass deep to the parotid, extending into the parapharyngeal space and carotid sheath. She received 45 Gy in five fractions, with GTV delineation aided by MRI fusion. Despite pain improvement, MRI at 4 months showed possible progression on T1 but response on T2. At 9 months, MRI demonstrated disease stability and the patient was pain-free. This case reinforces the need for patience and clinical correlation in post-SBRT evaluations.
For a comprehensive overview of target volume delineation across multiple oncologic sites, see our complete guide on target volume delineation and field setup. Also check out our article on Marie vertical radiotherapy by Leo Cancer Care, another innovative approach in modern radiation therapy.
