Study of Prophylactic Topical Dapsone 5% Gel Versus Moisturizer for Cetuximab-induced Papulopustular (Acneiform) Rash in Patients With mCRC or HNSCC Without Previous or Concurrent RT

Inclusion Criteria: * Patients 18 years or older with underlying diagnosis of metastatic colorectal cancer or head and neck squamous cell carcinoma including newly diagnosed patient * Patients must provide written informed consent to participate in the study * Anticipated initiation of cetuximab treatment with or without additional chemotherapy. * Able to self-administer topical interventions or provide for another person to apply the topical interventions Exclusion Criteria: * Females of childbearing potential who are pregnant or nursing * Patients with allergy, hypersensitivity or other contraindication to dapsone, sulfa antibiotics, or excipients of the dapsone gel product * Patients with pre-existing dermatologic condition affecting the face and chest that would impair assessment of papulopustular rash including dense and/or long facial hair (per investigator discretion) * Patients currently using prescription and/or over-the-counter topical medications to the face and/or chest who are unwilling to discontinue use during the trial intervention period (day 0 ± 2 days through day 28 ± 2 days) * Previous or concurrent radiation therapy to head, neck, and chest (i.e. application sites only) * Previous therapy with cetuximab within 6 months of consent