Intratumoral Chlorine Dioxide Injection in Advanced Solid Tumors

The purpose of this study is to evaluate the safety of injecting a 2% chlorine dioxide solution directly into tumors in adults with advanced solid tumors, and to determine whether the injected solution stays within the tumor under real-time imaging guidance. Chlorine dioxide reacts with and destroys the tissue it contacts and is consumed locally within about a minute. Up to about 50 participants will receive up to 4 treatment sessions per lesion, about one week apart, and will be followed for 12 months.

Inclusion Criteria: * Age \>= 18 years. * Histologically confirmed extracranial solid tumor (primary, locally recurrent or metastatic) that is advanced or unresectable (technically unresectable, medically inoperable, without a remaining standard curative surgical option, or with curative treatment at disproportionate cost such as loss of an entire organ or major function, as documented by a multidisciplinary team), or that has active disease after standard therapy and for which standard local treatment is assessed as unsuitable. * At least 1 and no more than 10 target lesions at entry. A target lesion is a non-intracranial lesion, visible on imaging, that can be safely reached by ultrasound- or CT-guided percutaneous puncture or injected under direct vision. Not every visible lesion has to be injectable. Non-target lesions may be present and are recorded. Where safe and feasible, all safely reachable visible lesions are listed as target lesions. * Contrast-enhanced CT (with ultrasound or MRI where needed) completed before entry, establishing the inventory of all visible lesions and of target lesions with their volumes. PET-CT is encouraged but not required. * Every target lesion \= 50 x 10\^9/L and INR \= 30 mL/min/1.73 m². * Able to understand the study and give written informed consent. * Subjects of reproductive potential, and male subjects with partners of reproductive potential, agree to use effective contraception from consent until 3 months after the last session. Exclusion Criteria: * No lesion that can be safely punctured or injected under direct vision, or the lesion(s) to be treated encase major vessels or nerves such that the risk of access is unacceptable. * Severe coagulopathy or high bleeding risk. * Uncontrolled serious infection, systemic decompensation, severe cachexia or multi-organ failure. * Pregnancy or lactation. * Psychiatric or cognitive impairment preventing cooperation. * Intracranial tumor or intracranial metastasis. * Rapidly progressive, widespread disease for which local treatment of target lesions is assessed by the MDT as having no clinical value. * Contraindication to the procedures or medications required by this protocol without an alternative (e.g., inability to tolerate puncture under local anesthesia with intravenous analgesia). * Candidate for curative treatment that can be delivered with low morbidity. * Hypersensitivity to iodinated contrast or uncontrolled hyperthyroidism, where a lesion requires CT guidance. Such a lesion may be treated under ultrasound guidance only. * Any other condition rendering the subject unsuitable in the investigator's judgment.