**1. Introduction**

Prior to the advent of volumetric radiation oncology treatment planning, physician, physics, and dosimetry teams would construct and calculate radiation therapy treatment plans at the center of the target referred to as the isocenter. Calculations would be derived based on depth measured at isocenter. Beam shaping devices which shaped dose at the isocenter were applied to the sloping surface of the target at a single level. Plans would be calculated to isodose lines which would unintentionally not define the volume and location of areas of radiation dose asymmetry. In breast cancer patients, the areas of asymmetry would be at the medial and lateral regions of the breast in rib/chest wall and extend for the length of the field which by default would include multiple rib segments. In this era injury, when it occurred, was simply ascribed to radiation therapy with limited attention to dose and volume treated. Radiation is not a drug; however, we did not have volumetric computational tools to be more exact in our review of process and convince our medical and surgical colleagues otherwise. We could not determine a specific dose volume effect relative to injury as we did not have tools to validate this point. Our approach to treatment planning changed with the introduction of computer tomography into simulation and volumetric driven radiation therapy treatment planning. With tools for contouring targets with reconstruction algorithms, radiation oncologists and treatment planning teams could visualize targets in three and four-dimensional volumes and review the juxtaposition of target volumes with normal tissue structures. This provided radiation oncologists opportunities to apply therapy in non-coplanar modulated geometries with beam arrangements that were more specific to each patient's target volume.

Radiation therapy treatment planning and dose prescription permanently changed with the introduction of advanced technology. Dose was prescribed relative to volume, not isodose lines, and contouring normal tissues provided the infrastructure to develop strategies for conformal avoidance of normal tissue. Altering fluence profiles by moving multi-leaf collimators across radiation therapy treatment fields provide the opportunity to generate voxel-based dosimetry to further improve dose asymmetry to tumor targets and place sharper dose gradients across normal tissue targets decreasing the risk potential for injury. The weakness in target specificity was therapy reproducibility and image validation of the targets before and during therapy. This was addressed with several manufacturing improvements including the integration of cone beam computer tomography into linear accelerators, use of four-dimensional treatment planning to develop therapy targets, and optical tracking tools to validate the lack of motion during treatment. Volume modulated arc treatment delivery provided the opportunity to decrease the time of treatment delivery with simultaneous multi-leaf and gantry motion. By decreasing treatment time delivery both durability and reproducibility of daily positioning could be confirmed. Motion, including deep inspiration breathing, is now validated with optical tracking systems. Decreasing the time of treatment delivery with volume modulated arc therapy provides confidence that patient care will not be influenced by intra-fractional motion.

These improvements have served to secure the success of radiation oncology moving forward as tumor targets are treated accurately with confidence and normal tissue protection is optimized. The improvements have also served to change the work scope and skill set of colleagues in the radiation therapy physics and treatment planning community. With the advent of volume-based planning, image integration into targeting has become the standard of care. With two-dimensional radiation therapy treatment planning, often the information needed to generate a standard treatment

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

plan was fully available to the dosimetry and physics teams at the end of the simulation hour. Today, most of the work in planning and targeting is performed after the simulation process. The simulation hour is used to create devices for immobilization, perform three- and four-dimensional imaging, and establish target coordinates for planning. Physicians contour targets for treatment after images are acquired and processed, often with diagnostic images fused into radiation therapy planning images. The work of the planning team cannot begin in earnest until the targets for therapy are contoured and constraints for normal tissues are defined for the objectives of the planned therapy. If there are delays in physician contouring, unintentional time constraint can be placed onto the planning and treatment validation process. The planning teams need to be well versed in volumetric therapy language as clinical, motion, and planning targets are applied to the intended areas of therapy by planning teams following protocol and/or institutional guidelines. The plan is developed as best as possible within the confines of the normal tissue volume constraints and validated through the quality assurance process. Image guidance and tracking process for quality assurance is initiated and maintained by the planning teams in collaboration with the therapy teams. Planning teams are essential in all services housed within the department of radiation oncology from the time of simulation to treatment validation. Planning teams are involved in brachytherapy and stereotactic therapy with varied imaging and dose computational algorithms required for modern patient care. The skill set for the modern planning team is diverse requiring knowledge of all aspects of modern planning equipment and tools.

Therefore, the role of the planning team in radiation oncology has expanded to image-based volumetric dosimetry and plan validation. Dosimetrists now have an extended role in defining volumetric anatomy and plan validation. In this chapter we will review skills required by dosimetry and planning teams in each disease and discipline area; the role of imaging and dosimetry in both daily work scope and clinical trials; the skill set for the modern planning team; and define what a modern planning group might resemble soon [1–11].

### **2. Central nervous system**

Patients requiring radiation therapy to the central nervous system (CNS) require a broad range of department services as these patients comprise both pediatric and adult populations including patients with primary and metastatic disease. Patients can require therapy to the entire CNS as well as stereotactic therapy to small targets with sharp dose gradients across critical normal tissues. The objectives for each patient have similarities with the primary goal to successfully treat the tumor target with conformal avoidance to as much central nervous parenchyma and critical structures as feasible and not compromise dose to target. In both pediatric and adult populations, sparing normal tissue now has near equivalent importance in patient management to tumor control and this has become essential to the treatment planning community. The CNS has multiple critical normal tissue structures with limited self-renewal capacity, therefore conformal avoidance when possible is important for optimal clinical outcome. Imaging plays an essential role in defining targets and accurate contouring is fully dependent on image fusion and quality. Most tumor structures are not well visualized on computer tomography. Fusion software is aligned with bony anatomy and the irregular shape of skull landmarks lends itself to accurate integration of multiple image sets for contouring. Investigators have developed protocols in

glioblastoma multiforme using multiple magnetic resonance (MR) sequences involving spectroscopy, fluid-attenuated inversion recovery (FLAIR), and contrast images using dose painting strategies to help limit dose to critical structures. Spectroscopy is helpful when tumor abuts the corpus callosum and can define areas where disease extends to the contralateral hemisphere, FLAIR defines edema which can house disease, and contrast defines regions of blood-brain barrier disruption by disease. Positron tomography imaging with amino acids can define tumor in deoxyribonucleic acid (DNA) synthesis often not well visualized with gadolinium. The datasets help create multiple target volumes which can be treated as a single plan with individual fractionation and total dose to each target. For patients with metastatic disease, treatments are delivered with radiation treatment planning including radiosurgery to subtotal volume CNS targets and hippocampal sparing for improved neurocognition. The growth of MR imaging has facilitated the development of subtotal volume therapy. Pediatric radiation therapy on selected germ cell protocols is delivered to spinal fluid pathways with temporal lobe sparing and standard risk medulloblastoma therapy boosts are now planned to image targets and not the entire posterior fossa in order to spare normal tissue. The plans require creativity with a balance of constraints between multiple normal tissue targets with dose limitations applied to the CNS tissue volume in the treatment field.

Often tumor targets come in close approximation to normal tissue and planning teams need to be fluent in multiple aspects of field geometry including table motion, off-axis fields, and six-degree couch motion and place dose gradients across structures including optic nerves, chiasm, and the cochlea when needed. Artificial intelligence (AI) will have influence in this aspect of care as field design can be optimized to constraints through an iterative process once the contours have been drawn and processed. The plan, once approved, is validated through a quality assurance process

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

#### **Figure 2.**

*Dose delivery to multiple lesions with a mono-isocenter in the central nervous system using a single plan to treat all lesions. (A) Pre-SRS MRI; and (B) Isodose lines (Rx = 18Gy to 11 tumors in single fraction treatment) on post treatment MRI 8 months later.*

and treatment can begin once the patient's therapy portal images are generated and approved. **Figure 1** is the treatment plan of a patient with neurofibromatosis with an astrocytoma in the posterior fossa occupying the fourth ventricle showing high and intermediate risk volumes defined on MR with conformal avoidance of the cochlea.

Planning for diseases in the CNS is clinically important as normal tissues of the CNS have limited self-renewal potential, therefore conformal avoidance to as many structures as possible with radiosurgery (SRS) and stereotactic radiation therapy is essential for outcome. Few diseases alter the well-being of the patient more than injury to the CNS imposed by disease and treatment. Limiting sequelae of therapy is an essential goal for the dosimetry team [12–23]. Further improvements in small field dosimetry permit multiple lesions to be treated in a segmental manner with a single plan with one isocenter. **Figure 2** is an example of a single isocenter plan simultaneously treating multiple lesions in the CNS using the Varian RapidPlan system with volume modulated arcs. The arc permits simultaneous dynamic motion of the treatment gantry and multi-leaf motion to optimize delivery to tumor and limit dose to normal tissue.
