Radiation Therapy for Malignant Salivary Gland Tumors

Introduction

Primary cancers arising from the parotid gland represent a diverse biologic spectrum. Surgery remains the primary local curative modality with high rates of local regional control (LRC) for the majority of early stage parotid contained neoplasms. For more advanced, aggressive neoplasms, or in the setting of resectable recurrent cancers, adjuvant radiotherapy (RT) reduces the risk of local regional failure. Radiation sterilizes areas of residual microscopic disease, areas of tumor spillage, and areas of peripheral nerve involvement (named or unnamed) while sparing the patient the additional morbidity and disfigurement of a more comprehensive surgical resection. This chapter reviews the indications for adjuvant and primary RT, clinical outcomes, the role of particle therapy, dosing regimens, and current clinical trials.

Indications for Radiotherapy in the Treatment of Salivary Gland Tumors

Radiotherapy in the Adjuvant Setting

Given the rarity of these tumors, there are few prospective randomized trials that have been conducted for primary salivary gland cancers. The use of adjuvant RT is based on available single institutional retrospective studies. These retrospective studies collectively show that in the setting of identified high-risk (HR) factors, significant improvement in 10-year local control (LC) can be achieved. Adjuvant RT is typically offered in the setting of locally advanced tumors (T3–T4 primaries), incomplete resection, high-grade histology, nodal extracapsular extension, perineural invasion, lymphovascular space invasion, and adenoid cystic histology. Patients harboring one or more of these HR pathologic features treated with surgery alone can expect 10-year LC rates ranging from 17–78%, whereas those that underwent RT could expect control rates of 51–95%. Treatment is usually initiated within 6 weeks of resection to allow for adequate postoperative wound healing. Table 52.1 illustrates the improvement in LC with the addition of adjuvant RT in selected retrospective studies. Adenoid cystic carcinomas are more locally aggressive and may have a lower rate of LC when treated with surgery alone ( Table 52.2 ).

TABLE 52.1

Local Control (LC) Rates of Malignant Salivary Gland Tumors From Selected Retrospective Studies Treated With Surgery With or Without Adjuvant Radiotherapy (RT)

Author	Year	Cases ( n )	Surgery (Local Control Rate)	Surgery + RT (Local Control Rate)
Noh et al.	2010	94	78.2% – 5-year LC	94.7% – 5-year LC
Chen et a1.	2007	63	49% – 5-year LC	75% – 5-year LC
Terhaard et al.	2005	538	76% – 10-year LC	91% – 10-year LC
Armstrong et al.	1990	46	16.8% – 5-year LC	51.3% – 5-year LC

TABLE 52.2

Local Control Rates of Adenoid Cystic Carcinomas From Various Retrospective Studies Treated With Surgery With or Without Adjuvant Radiotherapy (RT)

Author	Year	Cases ( n )	Surgery (Local Control Rate)	Surgery + RT (Local Control Rate)
Chen et al.	2006	140	61% – 10-year LC	84% – 10-year LC
Mendenhall et al.	2005	101	N/A	91% – 10-year LC
Silverman et al.	2004	129	79% – 10-year LRC	71.6% – 10-year LRC
Garden et al.	1995	198	N/A	79% – 10-year LC

Radiotherapy in the Definitive Setting

Definitive RT is utilized in cases where patients are medically unable to tolerate resection or are surgically inoperable. Single institution experiences with definitive RT for a variety of malignant salivary gland tumors suggest the 5- and 10-year LC rates to be 70% and 57%, respectively. There may be a dose–response relationship for these tumors with doses ≤66 Gy yielding inferior LC. Patients diagnosed with adenoid cystic carcinomas treated with definitive RT had 5- and 10-year LC rates of 56% and 43%, respectively.

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