**3. Head/neck**

With the advent and recognition of human papilloma virus (HPV), the incidence of head and neck carcinoma has significantly increased including patients who are lifelong non-smokers. These are challenging patients to plan as there are a multitude of normal tissues to provide both conformal avoidance and titration of dose asymmetry. Our knowledge of dose constraints continues to mature and we are applying strategy to as many normal tissue volumes as the primary target will permit. Spinal cord dose is limited to 50 Gy to 1% when feasible. Dose volume constraints are applied to the mandible, retropharyngeal muscles, carotid vessels, larynx, brachial plexus, and thyroid when possible. When tumors involve the paranasal sinus and skull base, constraints need to be applied to orbital structures, pituitary gland, optic chiasm, cranial nerves and temporal lobe including the hippocampus when possible. These constraints are balanced with tumor location and normal tissue anatomy coupled with knowledge of tissue molecular biomarkers.

In these patients, gross tumor volume (GTV) is often well defined on anatomic and metabolic imaging coupled with findings on physical examination. This permits more expert targeting of disease location as well as clinical target volumes (CTV) thought to be at risk for disease, often considered one lymph node station beyond gross tumor. Planning target volumes (PTV) are designed for daily set up variability, however with the advent of optical tracking tools and improved image guidance, the traditional need for a 5 mm PTV can be re-visited as arbitrary application of expanded targets can often extend target dose into normal tissue structures and outside of the patient if not carefully applied. Vertebral bodies can often be natural barriers especially if volume expansion places the spinal cord at risk for additional radiation dose. Modern image guidance has likewise improved daily reproducibility of radiation therapy treatment. This allows departments to re-visit the concept of a PTV

**Figure 3.** *IMRT plan for ethmoid sinus esthesioneuroblastoma. Note conformal avoidance of the globes.*

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

since daily patient treatment is more secure and consistent. Titrating the volume will decrease radiation dose asymmetry and improve clinical outcome.

In applying constraints, it will be important for therapy teams to track outcome through pathways previously less well studied. For example, although it is likely that the dose/volume relationships of retropharyngeal tissue treated influences outcome, recognizing this is driven in large part by the location of primary disease. We need strategies including speech/swallowing colleagues to study this effect and learn where to dose/volume reduce when feasible. Audiologists will help radiation oncologists apply metrics to outcome for alterations in hearing imposed by therapy. Building a portfolio for outcome analysis will support process improvements in radiation therapy planning and support the identification of patients who would potentially benefit from supportive intervention prior to the appearance of visible sequelae of management. **Figure 3** represents the plan of a patient with recurrent paranasal sinus esthesioneuroblastoma with conformal avoidance of the optic chiasm.

This is a growing population of patients who will benefit from the attention to detail required for optimal tumor control and normal tissue function [21, 24–30].

### **4. Thorax**

Lung cancer has evolved over the past two decades from a disease of habit to a disease of molecular biology. Lesions are now treated with multiple techniques including discontinuous planning such as dose painting/altered fractionation to peripheral nodules and more traditional fractionation to regions of central mediastinal disease where tumor abuts critical central structures. With the advent of immunotherapy, the situation has become more complex as toxicity can occur in both high and low dose regions making planning and dose constraints challenging to meet. Lung tumors can be large and often are in less favorable thoracic locations to meet cardiac and pulmonary constraints. Accordingly, modern lung cancer protocols that include immune-radiotherapy place strict constraints on V20 and V5 with a dose ceiling of 60 Gy. To meet constraints, nearly all modern industry and National Cancer Institute's (NCI) National Clinical Trials Network (NCTN) clinical trials treat limited to no elective at risk volumes. Only gross tumor as defined on anatomic and metabolic imaging is often contoured with dose constraints placed on cardiopulmonary volumes, soft tissue, chest wall, and spinal cord.

Thoracic planning will remain an enigma with imperfections applying dose to structures. Most thoracic plans are now performed with intensity modulation. If left to its own device, intensity modulation will titrate dose asymmetry (hot spots) but unfortunately push dose to vulnerable pulmonary parenchyma if a strict low dose (V5) and moderate dose (V20) constraint is applied. When these constraints are applied, dose will be driven back to high dose segments which in turn will create hot spots, largely in anterior/posterior soft tissue intentionally lateral to the spinal cord. This has the potential of increasing dose to the chest wall and soft tissue structures. Often this is viewed as acceptable in order to prioritize more limited dose to pulmonary parenchyma. This results in the need for balance of constraints.

As the number of cancer survivors grow [31], the modern cancer patient is asking different questions concerning outcome beyond the question of tumor control. Thoracic malignancies including primary lung, esophagus, and lymphoma can generate dose to vascular structures and therefore cardiac dosimetry is an important element to thoracic therapy. Historical therapy directed to the heart as an unintentional target is associated

with coronary artery disease, myocardial dyskinesis, valvular disease, electrical conduction changes, and pericardial disease. It is now also recognized that as blood migrates through chambers during treatment, radiation can decrease white cells and other blood elements. Once thought to be exclusively related to marrow dose, the heart becomes a vehicle for immune suppression through therapy. Therefore, for all epithelial and liquid diseases of the thorax, attention to detail to the heart and cardiac subsegments need to be assigned constraints when feasible. This information becomes invaluable to both the cardiologist and primary care provider teams in evaluating the patient and creating a survivorship plan including cardiac prevention strategies. We cannot evaluate radiation as a "drug" and to optimize survivorship programs, defining dose to subsegments and vessels will provide meaningful information to patients and care providers. Future AI tools will help optimize consistent application of contours to subsegments which will optimize strategies for conformal avoidance by the planning team. The esophagus abuts the left atrium; therefore, radiation oncologists can provide information on dose to the electrical conduction system as a cardiac subsegment as part of survivorship planning as tumor target will abut the posterior wall of the atrium. Improvements such as this instill confidence in providers and patients recognizing that we place value on outcome. The same approach can be adopted to other normal tissue volumes including pulmonary parenchyma. Often thoracic lung cancer patients have compromised baseline function with limited pulmonary capacity, therefore conformal avoidance of parenchyma is important.

Future strategies for application of tools for planning will include functional coefficients for cardiopulmonary volumes. Currently we can only assign anatomical coefficients without recognizing that function may be an important component to future planning paradigms. Lung will remain a balance of constraints including low/ intermediate, and high dose parenchymal segments as well as cardiac, esophageal, spinal cord, the chest wall volumes. Integrated databases with images, radiation therapy plans, and outcome with help us further refine planning strategies for thoracic oncology patients (**Figure 4**) [32–63].

#### **Figure 4.**

*Esophagus patient with a pre-operative radiation therapy treatment plan demonstrating conformal avoidance of cardiac and pulmonary volumes.*

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