Hyperbaric Radiation Sensitization of Head and Neck Cancers

The goal of this research is to address the question:

"Does the addition of hyperbaric oxygen to radiation and chemotherapy improve outcomes in locally advanced oropharyngeal or laryngeal squamous cell carcinoma?"

There is reason to believe that hyperbaric oxygen administered immediately prior to radiotherapy will prove beneficial for this cancer type and stage. The basis for this hypothesis is a review of several decades of published work, the conclusion of a recent (2018) Cochrane Review, and results of a Phase I trial. A summary of this body of work follows.

During the 1950's, several reports laid the groundwork for hyperbaric oxygen's potential as an effective radiation sensitizer. Gray and colleagues observed that curability of small animal tumors with radiotherapy was limited by the radio-resistance of the portion of cells that retain their reproductive integrity.(1) Tumor cell sensitivity to irradiation was seen to increase when tumor-bearing mice breathed oxygen under hyperbaric conditions. Gray's group further observed that radiobiological damage demonstrates dependence on the concentration of oxygen in the immediate vicinity of tumor cells at the time of radiation.( 2) It became evident that many solid tumor cell populations exist within a wide range of oxygen tensions.(3) These findings were sufficiently encouraging to warrant a small clinical study to determine if this anticipated radio-sensitization effect could be demonstrated histologically.(4) A small diver recompression chamber was acquired from the Royal Navy and modified to accommodate a recessed acrylic window.(5)

The trial involved eight patients whose breast or lung tumor sites would lie directly below the window, above which a radiation delivery source was mounted. To assess any difference afforded by hyperbaric oxygen, tumors had to be large enough so they could addressed in two aspects. Irradiation of the inferior aspect occurred conventionally, with the superior aspect shielded. Shielding was then reversed and the superior aspect irradiated while patients breathed oxygen to 3.0 atmospheres absolute.(4) Preliminary findings of increased tumor destruction secondary to hyperbaric oxygen exposure promoted investigators to treat another 35 patients in this manner. Despite their uniformly poor prognosis, the hyperbaric effect was again significant and outcomes were deemed "much better than anticipated".(6)

On the strength of this preliminary data there was widespread interest in hyperbaric radiation sensitization.(7,8,9,10) However, frustration at the lack of 'visibility' for other anatomic sites with these chamber types initially limited wider application. Industry responded by manufacturing purpose-built chambers with increasing numbers of windows. By the early 1960's, a completely seamless acrylic hyperbaric chamber had been produced.

It eventually became apparent that hyperbaric oxygen's effectiveness was inconsistent across all tumor types (the concept of varying tumor hypoxic fraction was in its infancy). Quite probably, many of these cancers had already metastasized. Along with suggestions of a higher incidence of new primary tumors and rates of metastasis in hyperbaric oxygen irradiated patients, (11, 12) the testing of alternative sensitizers, and a lack of uniformity in radiation dosing (making comparisons difficult), interest in hyperbaric sensitization eventually began to wane. By the early 1970's, the hyperbaric chamber as a sensitizing agent had largely been abandoned.

Little more was heard of this sensitization technique until 1996, when Japanese neurosurgeons reported the results a small clinical trial investigating malignant gliomas.(13) Due to the evolution of targeted radiation delivery devices it was no longer possible to undertake concurrent hyperbaric oxygen and radiotherapy. This group, therefore, introduced a sequential approach, irradiating patients immediately upon exiting the chamber. They were encouraged enough by their findings to undertake, along with several other Japanese groups, additional brain tumor trials.

In 1997, Machin et al. summarized 30 years of the U.K.'s Medical Research Council sponsored trials of solid tumors, using modern statistical methodology.(14) When the five trials involving hyperbaric sensitization were re-analyzed, a clear survival advantage was evident in each of the two head and neck cancer trials, with mixed results in cancers of the cervix. In 1999, oncologists from Yale reported the results of a head and neck squamous cell carcinoma trial, conducted 20 years earlier.(15) Patients were randomized to receive radiotherapy conventionally or during hyperbaric oxygenation. Significant improvement in local control, and relapse free survival at five years was evident in the hyperbaric group.

In 2000, magnetic resonance imaging demonstrated hyperbaric oxygen's ability to elevate implanted tumor oxygen levels in mice. This effect remained for 20-30 minutes after chamber decompression.(16) Malignant glioma oxygen responses to various conditions were measured via stereotactic CT guided implanted oxygen electrodes in 18 patients.(17) Hyperbaric, but not normobaric, oxygen significantly increased tumor oxygen tension, and this effect likewise remained for more than 20 minutes following patient removal from the chamber. This study had involved pre- and post-hyperbaric recordings. Becker and colleagues took this one step further and measured tumor oxygen response prior to and during hyperbaric oxygen exposure.(18) In seven head and neck squamous cell carcinoma patients, mean baseline tumor oxygen pressure was 17 mmHg, increasing to 550 mmHg in a mean of 17 minutes of hyperbaric oxygen breathing.

Four clinical trials have further evaluated the sensitization potential of hyperbaric oxygen in malignant gliomas. This technique was considered feasible, held promise,(19) and involved minimal toxicity,(20,21) and modestly extended overall survival.(19,20,21,22)

A 2018 Cochrane Review concluded that 'given the findings of improved tumor control and mortality with the use of hyperbaric oxygen for patients with cancers of the head and neck…, there is a case for large randomized trials of high methodological vigor…'.(23)

In contrast to earlier unsystematic reports, a 2003 meta-analysis failed to establish a causal relationship between hyperbaric oxygen therapy and de novo development of a tumor, established tumor growth, or an increase in the degree of metastases.(24)

Key messages from this body of work:

i. Radiation-resistance is largely a function of tumor tissue hypoxia ii. Hyperbaric oxygen elevates squamous cell carcinoma oxygen tension in animals and man.

iii. In humans, squamous cell carcinoma oxygen tensions to peak at a mean of 17 minutes during hyperbaric oxygenation. They remain elevated for more than 15 minutes after exposure.

iv. Provision of hyperbaric oxygen has proven feasible and safe as a radiation sensitizer for both malignant brain tumors and head and neck squamous cell carcinomas.

In preparation for this Phase II trial, a Phase I 'dose escalation' study was undertaken.(25) Its purpose was to verify safety and tolerability of hyperbaric oxygen immediately prior to radiation therapy for oropharyngeal carcinoma. It also assessed the acute toxicity impact of hyperbaric oxygen delivered in different groups twice, three times, and five times weekly. With a mean follow-up of 19 months, five days per week hyperbaric dosing had not increased overall toxicity, and patient compliance was good. (25) Complete clinical response occurred in all patients who completed the protocol. One patient suffered bone and liver metastases. While this study was not designed to assess clinical outcomes, a subsequent report involving a minimum 61 months follow-up confirmed no late toxicities, with overall survival of 100%, zero local recurrence, and an 11% incidence of distant metastases.(26)

Citations are listed in the Reference section