**5. Abdomen/pelvis**

Radiation therapy plays an important role in the treatment of abdominal and pelvic malignancies which include the need for stereotactic therapy and brachytherapy. This requires an expanded skill set for the planning team as the team must prepare for additional therapy superimposed on teletherapy.

In the past two decades the liver has become an important focus for radiation therapy. Systemic therapies have improved for metastatic disease and primary hepatocellular carcinoma has significantly increased in incidence; therefore, radiation oncologists are applying advanced technology tools to the management of these patients including stereotactic therapy for definitive management and as a bridge to transplant. Planning teams become fluent in fusion of multiple MR and metabolic image sets into planning computer tomography including motion management techniques for successful treatment planning and delivery. Liver targets are often difficult to visualize in early iterative versions of cone beam computer tomography to validate target positioning for therapy, therefore planning teams have used versions of fiducial tracking to validate positioning for treatment set up. Multiple constraints are applied to liver targets including mean dose and partial volume dose. Constraints are applied to gastric/small tissues in close approximation to the liver as well as cardiac, pulmonary, and renal constraints applied in selected areas relative to target volume location. This is an important area of clinical research as radiation therapy is also being supplied via radio pharmacy with Y-90 and other compounds in development. Additional theranostic therapies have hepatic uptake, therefore this dose will need to be calculated as well for dose volume analysis.

Volumetric dosimetry is needed in this area. Although Y-90 can be applied to the region of disease, tumor vascularity may prevent uniform application of dose and the area of intended therapy may be underdosed with migration of therapy away from the intended target. The strategy for volumetric dosimetry with both diffusion kinetics and evaluation of migration is being developed and will likely include serial single positron computer tomography images obtained frequently (daily) and fused into a planning computer tomography to evaluate areas of disease potentially undertreated which can then be augmented with stereotactic therapy. Likewise, areas of dose migration can be identified as regions of conformal avoidance for the radiation planning team through these processes. Modern planning teams need to be nimble in image fusion and registration in order to optimize patient care in this group of patients. **Figure 5** represents a stereotactic hepatic radiosurgery treatment.

Upper abdominal therapy is often directed to biliary, pancreatic, gastric, and lower esophageal targets. In this cohort of patients often bowel, renal, liver, and occasionally cardio/pulmonary constraints need to be applied with thought. In patients treated in a post-operative setting, tolerance to bowel may need to be titrated, especially if it is devascularized and fixed in position. This is commonly seen after pelvic surgery. Likewise, the regions of the gastrointestinal and biliary anastomoses must be identified for conformal avoidance/dose titration as best as possible. Invariably, these areas can reside in high-risk regions, therefore advanced planning techniques need to be applied to optimize patient care and avoid injury.

For treatment to the pelvis, often bowel, bladder, and rectum are considered targets at risk. Mucosal surfaces of these organs are tissues of self-renewal potential and injury is often related to limitations in cell re-growth along the mucosal surface resulting in bleeding/nerve exposure resulting in pain. Long term effects such as

#### **Figure 5.** *Hepatic stereotactic body radiosurgery plan.*

perforation are unusual appear to center of areas of previous surgical intervention which are inherently devascularized/fixed in position. This is true for all surgical colleagues including gynecological oncologists, urologists, colorectal surgeons, and surgical oncologists. When possible, these areas need to be identified pre- radiation therapy for dose titration when not intentionally included in high-risk tumor volumes. Although we can treat more tissue than a surgeon can remove including extended nodal volumes, outcomes in post-operative patients require careful exchange between the radiation oncologist and surgical colleagues in designing target volumes of interest to optimize patient care and place dose gradients across tissues considered vulnerable to injury.

There is increasing interest in the use of radiation therapy for abdominal and pelvic malignancies and use of stereotactic techniques when feasible to limit sequelae of management. The modern physics team will increasingly use advanced technology in the care of these patients. As we improve our technology, we must be cognizant of knowledge moving forward. For example, most in the radiation oncology community were unaware of insufficiency fractures in the sacrum associated with radiation therapy. However, with symmetric application of extended targets beyond the gross tumor volume defined by pre-sacral lymph nodes, if one is not careful full dose can be applied through the planning target volume (PTV) if the targets are applied in a symmetric manner. With modern MR sequences, we see the fractures on occasion at radiation doses under 6000 cGy, therefore it is important to place dose gradients across the sacrum and try not to place full dose across the entire structure. Investigators have found the pre-therapy exercise programs designed to maintain flexibility supports treatment reproducibility. This will become an important aspect of survivorship programs [64–72].
