Note that this is somewhat related to this question but here I am specifically asking about the suitability of using existing shielding for Mars travel.
This is about this article and video titled "Scientists overlooked a major problem with going to Mars — and they fear it could be a suicide mission" here at Business Insider.
The article says that:
Going to Mars may be more dangerous than we thought. The major problem is high-energy space radiation. Scientists know that cosmic rays can damage DNA. They had just overlooked how bad it could get.
A team re-examined how damaged DNA can cause cancer. They then estimated levels of radiation exposure in space and on Mars. Their results are devastating.
The risk of cancer on Mars is twice as high as previously thought.
It comes down to how damaged DNA spreads throughout the body. A detailed study in mice reveals a sinister side to radiation. Damaged DNA doesn't just keep to itself.
It sends signals to nearby healthy cells, which triggers the healthy cells to mutate, which could cause more cancer.
The protection provided by Earth's magnetic field, referred to in the article, is through the Van Allen belts (held in place by Earth's magnetic field) that trap cosmic rays and high energy particles from the Sun.
Considering that the Moon and Mars are both completely outside of the Van Allen belts (as opposed to ISS which is below the belts), we must have had some (relatively lightweight) radiation shielding when traveling to the moon.
Thus I was thinking that this shielding could for example be sent to Mars in advance (e.g. on some kind of supply ship manned by robots).
Admittedly this would require that the astronauts on Mars would always have to stay inside structures built with such shielding (except for relatively brief moments as was done during the moon landing). This does not sound very unusual to me however.
So the question I would like to ask:
Why is radiation a problem when traveling to or living on Mars when we can use the same shielding as has been used before? I.e. what makes the existing shield technologies unsuitable?