EWS and RMS can occur in essentially any location of the body. EWS is most commonly located in the extremity followed by the pelvis, while RMS is most commonly seen in the head and neck region followed by the extremity and genitourinary sites. As such, a general description of relevant normal anatomy is beyond the scope of this chapter.

There are some general guidelines regarding patterns of spread for these two tumors. Uninvolved bone and intraosseous membranes typically provide boundaries for microscopic spread. Also, the margin to create the clinical target volume (CTV) is less compared to the typical adult NRSTS, secondary to the inherent sensitivity of most of these tumors to the combination of chemotherapy and radiotherapy.

Volume reduction is performed for “pushing” rather than invasive such as tumors that displace the lung, intestine, or bladder that show clear return to a more normal anatomic position after chemotherapy.

Nodal regions are not electively treated for EWS or RMS. While the nodal metastasis is uncommon in EWS, it is seen in approximately 15–50 % of RMS arising in the extremity and genitourinary sites. It is believed that subclinical involvement of nodal regions is appropriately addressed by chemotherapy, and only gross or microscopic nodal disease receives additional treatment in the form of radiotherapy.

Both CT scans and MRI scans are helpful for definition of the gross tumor volume (GTV) and the CTV.

CT scan provides better definition of bony involvement, while MRI provides better definition of soft tissue involvement.

The role of positron emission tomography (PET) scans in pediatric sarcomas is still unclear, but increasingly is used to help delineate the gross tumor volume.

For EWS, target volumes include an initial target volume defined at presentation prior to chemotherapy or surgery (GTV1, CTV1) and a volume reduction defined after chemotherapy and surgery (GTV2, CTV2). An additional margin is added to the CTV to account for treatment setup uncertainty to create the PTV (PTV1 for the initial volume and PTV2 for the boost volume). See Table 1 for additional target definitions. Suggested doses (based on the Children’s Oncology Group protocol AEWS1031) are given in Table 2.