To establish if the cardiac radiation dose assesment is well aproximated with routine 3D CT scan compared to 4D CT experimental scan with respiratory gating (breath motion monitoring). The study population relates to left side breast cancers female patients that require a radiation therapy treatment.

As a standard of care, the postoperative breast cancers radiation therapy is generally based on a 3 dimensions CT scan that does not incorporate the breathing motion by definition.

Meanwhile, the patients must commonly receive the treatment in free motion breathing condition.

More of that, the Cardiac dose, especially the LAD (left anterior descending artery) dose has been established as the main cause of radiation induced ischemic heart disease (RIHD) and should be consider in the first place.

In more concrete terms, the higher the LAD dose is, the greater the RIHD relates: arise the LAD dose by 1 Gy means a 7.4% higher risk to cause a RIHD during the next 5 years.

That being said, to determine if the cardiac dosimetry and the dose-volume histograms (specifically for the left side breast cancer treatments including or not the internal mammary artery) obtained from a 3D CT scan reflect well or not the reality (which is widely subject to the breathing motion).

Finally, because it has been established that a 4D CT scan can monitor the breathing motion, it seems definitely interesting to compare it with the average 3D CT scan to address this concern.

Inclusion Criteria:

1. Age ≥ 18 yo
2. Sex=female
3. Patients diagnoses with a left side breast cancer for which a radiation therapy is indicated and confirmed in a multidisciplinary consultation meeting.
4. Be able to understand and give her personal free consent, no judicial protection measure.
5. Written or oral consent, in compliance with the clinical investigation rules and regulation.
6. Patient affiliated with social security system
7. Treatment expected to be realized in Saint Quentin Hospital

Exclusion Criteria:

1. Patient < 18 yo
2. Pregnant women.
3. Breastfeeding women.
4. Consent not given
5. Claustrophobia
6. Incapacited subject or judicial protection measure
7. Other research with exclusion period time ongoing
8. All the inclusion criteria not met