How would you treat a synchronous oropharyngeal cancer and proximal esophageal cancer?

After Dr. @Dr. First Last has set the tone for definitive chemo-RT that I totally agree, I would like to offer some additional thoughts:

• It is assumed that both synchronous primaries are anatomically separate and have been proven histologically distinct; e.g. the oropharyngeal primary being a SqCC with or without HPV/p16 association, while the esophageal counterpart being either AdenoCa or SqCC without HPV.
• Otherwise, for a tumor bulk that spans contiguously from oropharynx to proximal esophagus, the treatment approach should be geared toward a single cancer entity (H&N, trachea, esophagus, thyroid, etc.) depending on where its epicenter is as well as its specific histology.
• For the two synchronous but distinct primaries each proven to be early stage, definitive treatment is warranted since the prospects for their individual tumor controls remain substantially high. While either could be managed surgically for a medically fit patient, the fact that combined chemo-RT instead of surgery can offer equivalent if not safer outcome speaks for it to be a preferred strategy. This is especially true for SqCC (whether as an HPV-associated H&N primary or as a proximal rather than distal esophageal cancer), whereas cervical esophagectomy can be quite morbid. As for TORS, it might be considered if one were exceedingly confident no adjuvant therapy for the H&N primary would be needed; otherwise, there’s a real risk of delaying the chemo-RT the patient really needs for the esophageal cancer.
• Medical Oncology should be consulted for the appropriate chemo regimen w.r.t. FOLFOX, Carbo-Taxol, etc. But the rule of thumb for multiple primaries remains true: control of the worst actor will determine eventual treatment outcome. Thus, the combined modality approach should be geared mainly toward the esophageal primary, with selected systemic drugs hopefully having some cross-reactivity for the H&N primary. Even if no such drug exists, the control of the H&N SqCC can often be achieved successfully by RT alone (use altered fractionation scheme if needed). Be inventive for the sequencing of the two modalities during multidisciplinary discussions, since few published level-one evidences would have included cases of synchronous primaries in their individual protocols.
• Regarding RT technique, IMRT/SIB comes in very handy especially if delivered via VMAT since the superior-to-inferior (SI) extent of the field coverage can be relatively long while bilateral lung apexes and brachial plexuses can enter as part of OAR constraints in addition to critical H&N structures. The recommended RT dosing scheme for H&N primary is quite standard: Give EQD2 of 70 Gy to gross tumor beds, 60 Gy to high-risk adjacent areas including first-echelon lymphatic drainage, and 50 Gy to subclinical disease. I would personally apply this principle to the esophageal cancer as well, regardless of which practice guidelines others might choose to follow (again, little evidence-based consensus exists, but “absence of evidence may not mean evidence of absence” - as long as common oncologic sense leads the way). If one does wish to stick with the popular notion of lower dosage (say, EQD2 of 50-54 Gy instead of 64-70 Gy) for esophageal cancer due to the concern for chemo-RT toxicity or in view of the apparently ominous second primary, the SIB technique would allow for easy modification by following the teaching of fractionation radiobiology (e.g. by incorporating BED calculation) as long as the number of fractions is kept the same for the treatment course.
• Remember, for synchronous but early-stage primaries, there is perhaps only one chance for cure for both entities treated simultaneously. So, don’t be cute - be aggressive in rendering sufficient tumoricidal dosage as the overriding goal of the treatment scheme, instead of being too conservative simply because the patient has two potentially deadly tumors. They (esp. the bottleneck malignancy) can quickly become truly deadly mostly because of undertreatment.

TL;DR: Synchronous early-stage H&N & esophageal primaries can still be cured with definitive chemo-RT. The esophageal primary is likely the bottleneck to predict patient survival. Leave chemo selection to Med Onc. For RT, use IMRT/SIB via VMAT if feasible.

Definitive RT and chemo.