**6. Breast**

Patients with breast cancer will continue to require significant attention to detail by physics and dosimetry in the development of their care path for radiation therapy treatment planning. Because of irregular topography and multiple sloped surfaces

#### *Clinical Considerations for Modern Dosimetry and Future Directions for Treatment Planning DOI: http://dx.doi.org/10.5772/intechopen.105910*

of the breast which are patient specific, breast planning will not easily succumb to exclusive planning through AI. Optimizing radiation therapy treatment plans remains a balance of constraints. With modern treatment planning techniques, most breast geometries can be planned with no more than 5% of the breast target receiving 107% of the prescription dose when treatment is applied to breast tissue. Contouring, however, must be carefully done as breast requires both the radiation oncologist and the planning team to simultaneously think in 2, 3, and 4 dimensions to achieve objectives of management including contouring regional anatomy and establishing goals. If the oncologist contours from midline to the latissimus muscle in a left-sided breast patient, it is difficult to meet normal tissue constraints for the left ventricle, mean heart dose, and left pulmonary parenchyma as defined by most clinical trials. If arcs are applied, often constraints cannot be met for total lung and contralateral breast if tumor target dose homogeneity constraints are too strict. In these circumstances, the planning engine wants to please the planning team, therefore if constraints to normal tissue are not included in the planning strategy, the engines will work to limit radiation dose asymmetry through the target. However, the more homogeneous the plan becomes through the therapy target/breast, the more dose is "pushed" into normal tissue including heart/lung, and contralateral breast. Therefore, the planning teams and the radiation oncologist must provide balance to constraints to meet both tumor coverage without exaggerated dose to target volumes and limit dose to normal tissue. One of the key elements to planning breast radiation is to contour breast tissue without intentionally contouring volumes to midline or posteriorly to the latissimus. When possible, contour breast tissue including the surgical cavity in the target volume. Because of applying underlying cardiac and pulmonary constraints, dose will naturally be "pushed" to the medial and lateral chest wall parenchyma. If one contours to midline, dose can easily be pushed well into the contralateral breast without intent due to the attempt to provide full coverage to this point and well as the latissimus laterally. As the target becomes more inferior, therefore is less breast tissue in both the medial and lateral planes, providing an opportunity to titrate volumes in these locations and limit cardiac dose for left-sided patients. Therefore, contouring becomes a very important aspect to breast cancer radiation therapy. If the volumes are over-drawn or drawn in a casual manner, there is less physics and dosimetry can do to improve the situation. In a similar manner, strategies for regional care need to be carefully designed to balance dose to target and normal tissue. The internal mammary lymph nodes follow the anatomy of the internal mammary artery and there is direct drainage to level 3 and the sub-pectoral region as well as the axilla, therefore strategy must be applied to extent of regional coverage as seen in **Figure 6** as normal tissue dose is influenced by the contour and dose assigned to the target. Although anatomic guidelines have been established for clinical protocols, recent publications have suggested that regional failures can occur beyond the target volumes defined from anatomical guidelines, therefore we must identify high risk patients and possibly extend our field edges beyond our traditional boundaries. Patients with triple negative disease often now undergo positron emission tomography prior to initiating therapy. In these selected patients, nodal disease is at times identified beyond traditional field boundaries, therefore volume of regional disease now is driven by imaging.

Breast remains a disease requiring thoughtful application of radiation therapy to the breast, surgical cavity, and regional targets. Because outcomes remain excellent, attention to detail including normal tissue constraints is essential to modern planning [73–91].

**Figure 6.**

*MRT plan for breast radiotherapy with intentional (left) and unintentional (R) inclusion of internal mammary structures. Note difference in cardiac dose.*

### **7. Extremities**

For both adults and children, treatment of the extremities requires significant attention to detail. There are multiple tissue compartments that can be affected by therapy and each can have consequence for late effects of management. These can include major blood vessels, bone, joint spaces, nerve, and muscle bundles. Children require additional conformal avoidance to growth plates when possible for optimal outcome for growth and development. This is an area where natural barriers may be helpful in placing dose gradients across structures to limit late effects, especially bone and joint spaces.

Because the extremities resemble cylinders, volume modulated arcs often play an important role in radiation therapy planning. It is important to spare a strip of normal tissue as it extends through the cylinder. Although most oncologists try to generate the strip in a linear manner, is it likewise clinically acceptable to spare tissue in a non-linear serpentine manner if the strip remains non-interrupted. Of equal importance is the concept of immobilization. Historically, radiation oncologists approached this from a perspective that casts and rigid structures provided the optimal security for treatment reproducibility. However, even within a rigid structure, alterations in anatomy can occur either on a pre- or post-operative basis

#### **Figure 7.**

*Near circumferential extremity target with popliteal adenopathy. Note sparing of normal tissue in the posterior compartment.*

### *Clinical Considerations for Modern Dosimetry and Future Directions for Treatment Planning DOI: http://dx.doi.org/10.5772/intechopen.105910*

rendering the utility of rigid structures. Modern alignment and optical tracking tools have altered our approach to this situation. With the generation of these tools, we can build a "virtual cast" and maintain alignment without the rigor of rigid tools. Patients are more comfortable, we are more secure in daily treatment reproducibility, and treatment time is considerably decreased further contributing to security in daily set up. The volumes in these cases lend well to advanced technology therapy and conformal avoidance of key structures can be more readily accomplished. **Figure 7** represents a patient with an extremity lymphoma with popliteal adenopathy using volume modulated arc treatment to treat the circumferential target with sparing oof posterior soft tissue for lymphatic drainage [92–99].
