UPDATE 4-Venezuela's Chavez heads to Cuba for medical treatment

* Treatment follows cancer operations on communist-run

island

* May receive therapy for side effects of radiation

treatment

* President has made no public appearance in nearly two

weeks

CARACAS, Nov 27 (Reuters) - Venezuelan President Hugo Chavez

will travel to Cuba on Tuesday for medical treatment, following

a nearly two-week absence from the public eye, months after

undergoing cancer surgery on the communist-run island.

Chavez, 58, underwent what appeared to be a remarkable

comeback from an undisclosed type of cancer diagnosed in June

2011. In October, he won re-election despite running a campaign

that was much more subdued than his previous bids.

In a letter to Congress, Chavez said he would receive a form

of therapy known as hyperbaric oxygenation, which is often used

for the prevention and treatment of bone decay caused by

radiation therapy, according to the American Cancer Society.

Chavez has not appeared in public in 12 days. The absence is

unusual for a leader who routinely chats for hours during live

broadcasts, and suggests his health has weakened since the

campaign.

"Six months after I completed the last radiation therapy

treatment, it has been recommended that I begin a special

treatment consisting of various sessions of hyperbaric

oxygenation," Chavez wrote in the letter, which was read by

congressional leader Diosdado Cabello.

"Together with physical therapy, (this) will consolidate the

process of strengthening my health."

The letter did not mention cancer.

Hyperbaric oxygenation therapy, also known as HBOT, involves

breathing pure oxygen while in a pressurized chamber.

Chavez's centralization of power and enormous control over

the country's oil revenue have made him the center of the OPEC

nation that provides about 10 percent of U.S. crude imports.

If his health took a turn for the worse, his unwieldy

coalition of military leaders and leftist social activists could

fall apart. Investors hoping for a more market-friendly

government tend to buy Venezuela's widely-traded bonds on

reports his health is worsening.

The country's benchmark Global 27 bond

extended gains in the wake of the announcement.

It was not clear how long Chavez would stay in Havana. An

official at Brazil's foreign ministry told Reuters that

Venezuela's embassy in Brazil said he was still expected in

Brasilia for a regional summit on Dec. 7.

CANCER STILL SECRET

Chavez's refusal to disclose his actual condition has made

his health the subject of constant speculation, particularly

among some opposition sympathizers who quietly hope he will take

a turn for the worse. He defeated opposition challenger Henrique

Capriles by 10 percentage points in the election last month.

Late last year Chavez declared himself completely cured of

cancer, but within months he had to return to Cuba for surgery

to remove another tumor.

Some opponents of the president took to Twitter on Tuesday

with jabs at the former soldier's followers, repeating the

message: "#FooledChavistas, you voted for Chavez! They made you

believe he had been cured!"

But the usually feverish speculation has been muted by the

approach of the holiday season and a general weariness with

politics after a year of campaigning.

Doctors say a couple of years must pass without a recurrence

before a patient is said to be cured of cancer.

In addition to the bone-weakening side effects of radiation

therapy on cancer sufferers, experts say HBOT is used to treat

conditions including infections, abscesses and decompression

sickness which can afflict deep sea divers.

"It is a palliative treatment that is very common for cancer

patients, and often lasts for several months," said a Venezuelan

medical source with knowledge of Chavez's treatment.

The medical source said HBOT is normally given at a rate of

two or three sessions per week, and could also be used to treat

damage done by radiation therapy to patients' digestive systems.

Many Venezuelan doctors suspect Chavez used steroids and

other treatments to look and feel fit during the election

campaign, which could have caused him other health problems.

Chavez spent several months traveling back and forth to Cuba

to receive treatment. The typically hermetic atmosphere there,

and his friendship with former leader Fidel Castro, helped

prevent details of his condition from leaking to the press.

Venezuela's constitution says that if an incumbent leaves

office in the first four years of a six-year term, a new

election must be held.

Chavez's treatment could make it difficult for him to help

allies campaign ahead of Dec. 16 elections for state governors,

which will be key to expanding his control over the provinces.

His allies, who run 15 of the country's 22 states, have

considerably less popular appeal than he does and can get a leg

up at the elections if Chavez joins them at rallies.

A prolonged absence could also slow the pace of key economic

policy decisions, which often depend on his direct approval.

Most economists expect Venezuela will devalue its bolivar

currency after heavy spending in the run-up to the October vote.

The currency has weakened sharply in the country's black

market for U.S. dollars in recent months - the dollar now trades

at almost four times the official rate of 4.3 bolivars.

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