What is the role of radiation therapy (RT) in the treatment of carcinoma of the parotid gland?

Evolution of Radiation Therapy in Parotid Carcinoma

Postoperative radiation therapy has evolved from a selective intervention to a standard component of treatment for most parotid carcinomas, with strong evidence supporting its routine use in adenoid cystic carcinoma, high-grade tumors, and any case with adverse pathologic features including positive margins, perineural invasion, lymph node metastases, or T3-4 disease. 1

Historical Evolution: From Surgery Alone to Combined Modality

The role of RT in parotid cancer has fundamentally shifted over the past several decades:

• Early era (pre-1990s): Surgery alone was standard, with RT reserved only for gross residual disease 1
• Modern era (1990s-present): Postoperative RT became standard for high-risk features after retrospective data demonstrated improved local control from 49% to 75% (p=0.005) and improved survival 2, 3, 4
• Current practice: The 2021 ASCO guidelines now mandate postoperative RT for specific indications with strong evidence-based recommendations 1

Current Indications for Postoperative RT (Strong Recommendations)

Mandatory RT (Strong Evidence)

All patients with adenoid cystic carcinoma should receive postoperative RT regardless of stage, as this histology demonstrates infiltrative growth and perineural spread patterns that benefit from adjuvant treatment even in early-stage disease 1

Postoperative RT is mandatory for tumors with any of these features 1:

• High-grade histology
• Positive surgical margins
• Perineural invasion
• Lymph node metastases
• Lymphovascular invasion
• T3-4 tumors

Conditional RT (Weaker Evidence)

RT may be offered for 1:

• Close surgical margins (even <1mm if microscopically negative in low-grade T1-2 tumors) 1, 5
• Intermediate-grade tumors 1

The evidence for these indications is less conclusive, with one series showing only 9.4% local recurrence in 32 low/intermediate-grade parotid cancers with close margins treated by surgery alone 1

Technical Evolution of RT Delivery

Target Volume Definition

The high-dose target must cover the surgical bed and appropriate nodal levels, with minimum dose of 60 Gy in conventional fractionation 1

For perineural invasion, cover the involved nerve to skull base with elective/intermediate dose (46-54 Gy) to prevent retrograde nerve failure 1

Technique Evolution

• Historical approach: Wedged paired cobalt-60 fields resulted in higher complication rates 2
• Modern standard: Ipsilateral electron fields (often mixed with photons) provide excellent control with minimal severe toxicity 2
• Current options: IMRT photon therapy has largely replaced older techniques, with particle therapy (proton, carbon ion) available but showing no proven superiority over modern photon techniques 1

Neutron therapy may be considered only for large-volume residual disease but has no indication over photon/electron therapy in standard cases 1

Definitive RT for Inoperable Disease

For medically or technically inoperable patients, definitive RT to 70 Gy achieves clinical complete response in 64.7% of cases and can eliminate need for surgery or reduce facial nerve sacrifice risk 1, 6

This represents a major evolution, as definitive RT was historically considered palliative but now provides cause-specific survival of approximately 40% at 10 years for unresectable disease 1

Elective Nodal Irradiation

Elective neck RT should be offered for cN0 disease with T3-4 tumors or high-grade histology, as microscopic nodal involvement risk exceeds 12% for parotid tumors with these features 1

Elective doses ≥46 Gy show trend toward improved regional control 1

Role of Concurrent Chemotherapy (Evolving Area)

Concurrent chemotherapy with adjuvant RT should NOT be routinely offered outside clinical trials 1

This represents an important evolution in thinking:

• No randomized trials support routine chemoradiation for salivary gland malignancies 1
• Only 4 of 10 retrospective studies showed benefit, primarily in specific histologies (ACC, squamous cell carcinoma) 1
• Three ongoing randomized trials (NCT01220583, NCT02776163, NCT02998385) will clarify this question 1

Critical Pitfalls to Avoid

Do not withhold RT based on achieving negative margins in high-risk histologies - even early-stage ACC benefits from adjuvant RT 1

Do not perform more extensive parotidectomy solely to avoid RT - for T1-2 low-grade tumors, partial superficial parotidectomy with negative margins provides excellent control without RT 1, 7, 5

Do not use wedged paired fields when ipsilateral electron technique is available - complication rates are significantly higher with the former 2

Do not assume particle therapy is superior - no prospective data support neutron/proton/carbon ion over modern IMRT photon therapy 1