8.2.1 After Breast-conserving Surgery
Early breast cancer needs to be irradiated to the whole breast after breast-conserving surgery.The patient is usually supine，the upper limb of the affected side is abducted，and the breast bracket is used for fixation.The whole breast irradiation is most commonly used for two pairs of tangential fields，which are suitable for 6MV-X ray.When irradiated with higher energy X-rays，a low dose area is formed in the shallow region of the breast close to the skin，which may affect the curative effect.Early breast cancer skin infection is unlikely，and it is not necessary to add filler when irradiated with 6MV-X-ray.Otherwise，the skin volume is too high，which may cause radiation reaction of the skin and affect the cosmetic effect.Traditionally，the whole breast has been irradiated with a two-dimensional tangential field.When the mammary gland is positioned under the fluoroscopy of the simulator，the upper boundary is at the lower edge of the clavicle，the lower boundary is 2cm below the breast fold，the posterior boundary of the incision field is in the midline，and the posterior boundary of the excision is in the midline.The front boundary of the irradiation field is 1～2cm beyond the contour of the breast skin，and the thickness of the lung tissue in the field（CLD：the thickness of the lung tissue at the center axis of the field）is generally 2～3cm（Fig.8-04）.Using the body surface contour map，according to the direction，size and isocenter position of the irradiation field during positioning，the radiotherapy plan is formulated by optimizing the dose specific gravity of the inner and outer tangential fields and the degree of the wedge plate.The disadvantage of the two-dimensional plan is that the body surface contour map is only equivalent to a single-level cross-sectional CT image lacking tissue density information.Since the plan is single-level，the dose distribution does not reflect the condition of the whole breast；due to the lack of tissue density information，lung correction cannot be performed，and the selected wedge plate degree will be different from the actual situation.At present，total breast irradiation has rarely been done in two-dimensional plans.
Figure 8-04 Two-dimensional radiotherapy technique，the position of the patient's mammary tangential field positioning A（inside field and outer field boundary line）and the tangential field map B and the two-dimensional plan dose distribution map C（the bright yellow line is 95%isodose line）
The method of supplementation of the breast tumor bed can be selected by electron beam external irradiation，X-ray external irradiation，intraoperative radiotherapy and inter-tissue implantation.Among them，electron beam external irradiation is most commonly used；X-ray external irradiation is mostly used for simultaneous filling of tumor bed；intraoperative radiotherapy requires special intraoperative radiotherapy equipment；inter-tissue interplanting is invasive and requires experienced doctors to operate，currently less used in clinical practice.In the traditional two-dimensional radiotherapy application，when the external beam is applied，the irradiation range and the irradiation depth are determined according to the surgical scar，the CT/B or the B-surgery，and the surrounding metal markers.The energy is selected to be 9～12 MeV.Apatient with a metal marker placed on the surgical tumor bed can be placed 1～1.5cm outside the surgical scar and the metal mark under the perspective of the simulator；the patient without the metal mark is directly drawn on the patient's body surface，and the surgical scar is placed outside.2～3cm（Figure 8-05）.(https://www.daowen.com)
Figure 8-05 Breast tumor bed electron line replenishment field
Note：A and B：the irradiation field formed by the metallization of the tumor bed and the surgical scar outside under the perspective of the simulator；C：the field of radiation drawn directly on the skin of the breast according to the surgical scar.