In pediatric sarcoma, target volume delineation and field setup are defined before the first contour is drawn. Histology, primary site, and response to induction chemotherapy directly change the final volume, the prescription, and even the simulation strategy. Ewing sarcoma and rhabdomyosarcoma both demand tight integration of CT, MRI, PET, and regional anatomy, because a tumor that merely displaces normal structures is not handled the same way as a tumor that truly invades them. For the broader framework behind this chapter, see our Target Volume Delineation and Field Setup – Complete Clinical Guide.
Background, Anatomy, and Patterns of Spread
The practical starting point is to treat pediatric sarcoma as a family of diseases rather than a single workflow. Management varies by histology, stage, risk grouping, and, as the chapter notes, even by geography. Ewing sarcoma is the second most common pediatric bone tumor; osteosarcoma is more common, but radiotherapy is usually less central there. Among soft tissue sarcomas, rhabdomyosarcoma is the dominant pediatric entity.
For both Ewing sarcoma and RMS, conventional treatment combines systemic chemotherapy with local therapy. That local component may be surgery, radiotherapy, or both. When disease is unresectable, radiotherapy alone is commonly used for definitive local management, and postoperative radiotherapy remains important in selected high-risk settings. Site distribution already hints at how complex local therapy may become. Ewing sarcoma most often arises in the pelvis, about 25% of cases, followed by the femur at 16%. Pelvic tumors are frequently poor surgical candidates and often move toward definitive radiotherapy.
RMS has a broader anatomic distribution: head and neck account for 35% of primaries, the genitourinary tract for 20%, and extremities for another 20%. Head-and-neck RMS is then split into parameningeal, orbital, and other head-and-neck sites. Parameningeal tumors represent 15% of all RMS, orbital primaries 10%, and other head-and-neck sites another 10%. That distinction matters because parameningeal lesions, arising in one of the eight MMNNOOPP sites, have a higher risk of direct central nervous system extension and are classified as unfavorable primary sites.
Local spread still follows anatomy. Uninvolved bone and intraosseous membranes act as barriers to microscopic extension, although bone erosion and frank invasion are not uncommon and should be evaluated carefully on CT. The chapter also makes a useful distinction between pushing and invading tumors. If pretreatment imaging shows a mass displacing lung, bladder, or bowel, then volume reduction after chemotherapy can follow the return of those organs toward a more normal position. True invasion is different. Areas that were infiltrated on pretreatment imaging should retain at least some coverage after induction therapy.
Nodal dissemination is not a dominant feature across most pediatric sarcomas, but it does appear in selected RMS settings. Extremity RMS has higher nodal risk and is often evaluated with sentinel node biopsy. Some genitourinary RMS cases, especially paratesticular disease, may undergo ipsilateral nerve-sparing retroperitoneal nodal dissection, generally in patients older than 10 years. Even so, elective nodal coverage is usually not recommended for most RMS and Ewing patients. Once nodal metastases are documented, the chapter advises against treating only the visible node. At least part of the entire involved nodal basin should be covered.
Diagnostic Imaging for Target Volume Definition
CT and MRI are complementary, not interchangeable, in pediatric sarcoma planning. PET has also taken on a larger role in staging and in documenting sites that were involved before induction chemotherapy changes the radiographic picture.
CT is particularly valuable when the planning question centers on bony erosion or cortical destruction. MRI provides the soft tissue definition needed to map disease extent and is especially useful for intracranial extension. That is why both modalities are routinely used in Ewing sarcoma and RMS. PET is increasingly used at initial staging for both diseases, with supporting data favoring it over older approaches such as bone scanning in several contexts. In practice, PET can preserve the memory of initially involved disease sites before chemotherapy partially sterilizes or obscures them.
This becomes critical when contouring depends on the difference between contact, displacement, and invasion. A parameningeal RMS with skull-base erosion or a thoracic Ewing tumor that previously occupied space now re-expanded by normal lung cannot be understood from the late simulation scan alone. The target has to be built from both the anatomy at presentation and the anatomy after response.
Target Volume Delineation and Treatment Planning
In Ewing sarcoma, the practical rule is to contour two moments of the disease: the initial extent and the post-induction residual. In RMS, treatment may use a single dose level or a two-volume approach, but the same core question still applies: was the tumor simply pushing into adjacent spaces, or was it invading them?
Ewing sarcoma target volumes
The chapter separates Ewing volumes into initial and reduced targets. The key nuance is that GTV2 is not limited to what remains visibly bulky after chemotherapy. Pretreatment osseous involvement is typically retained even when the soft tissue component shrinks dramatically.
| Target volume | Definition and description |
|---|---|
| Initial target volumes (pre-induction treatment) | |
| GTV1 | Pretreatment extent of gross disease, including bone, soft tissue, and unresected enlarged or suspicious nodes. GTV1 may be modified if initial tumors extended into body cavities or spaces, such as the pelvis or thorax, and then regressed with chemotherapy. |
| CTV1 | GTV1 + 1.0-1.5 cm. CTV1 includes involved nodal basins with clinical or pathologic disease. |
| PTV1 | CTV1 + a setup margin specific to institutional practice and image guidance, often 3-5 mm. |
| Reduced target volumes (post-induction treatment) | |
| GTV2 | Residual tumor after induction chemotherapy. Even so, the full pretreatment extent of bony involvement is typically included in GTV2. Postoperatively, GTV2 is defined by residual bone or soft tissue disease and sites of positive margins. |
| CTV2 | GTV2 + 1.0-1.5 cm. |
| PTV2 | CTV2 + a setup margin specific to institutional practice and image guidance, often 3-5 mm. |
Source: Target Volume Delineation and Field Setup: A Practical Guide for Conformal and Intensity-Modulated Radiation Therapy, 2nd Edition (Table 33.1)
Ewing sarcoma dose guidance
The chapter links these target definitions to Children's Oncology Group AEWS1031 dose recommendations, all delivered in 1.8 Gy daily fractions.
| Setting | PTV1 (Gy) | PTV2 (Gy) |
|---|---|---|
| Definitive radiotherapy, all sites except vertebral | 45.0 | 10.8 |
| Definitive radiotherapy, vertebral | 45.0 | 5.4 |
| Extraosseous Ewing sarcoma with complete response to chemotherapy | 50.4 | 0 |
| Postoperative microscopic residual disease (R1 resection) with >90% tumor necrosis on pathology | 0 | 50.4 |
| Postoperative microscopic residual disease (R1 resection) with <90% tumor necrosis on pathology | 50.4 | 0 |
| Postoperative gross residual disease (R2 resection) | 45.0 | 10.8 |
Source: Target Volume Delineation and Field Setup: A Practical Guide for Conformal and Intensity-Modulated Radiation Therapy, 2nd Edition (Table 33.2)
The example cases show how those rules are applied. In the pelvic case, CTVs were contoured as GTV + 1.5 cm; at the most inferior illustrated slice, there was no residual disease, so no GTV2 remained at that level. PTV1 received 45.0 Gy and PTV2 another 10.8 Gy for 55.8 Gy total. In the thoracic case, a four-dimensional CT simulation was performed to capture respiratory motion. There, the CTV expansion was 1 cm, and GTV1 was adapted to spare lung that had re-expanded after induction therapy while still covering all original sites of contact.
For RMS, the chapter highlights an important shift in risk definition. Current COG protocols are moving away from the older embryonal-versus-alveolar split alone and toward molecular fusion status. FOX01-related translocations, typically PAX3-FOX01 in t(2;13) and PAX7-FOX01 in t(1;13), are linked to higher-risk natural history. Fusion-negative alveolar RMS, by contrast, behaves more like embryonal disease.
Rhabdomyosarcoma target volumes
Volume reduction for RMS boost treatment is mainly recommended for tumors that were pushing into the thorax or pelvis. Invasive disease, especially parameningeal lesions, usually requires full respect for the pretreatment extent even when the post-chemotherapy images look smaller.
| Target volume | Definition and description |
|---|---|
| GTV1 | Pretreatment extent of gross disease, including bone, soft tissue, and unresected enlarged or suspicious nodes. |
| CTV1 | GTV1 + 1 cm. CTV1 includes clinically or pathologically involved nodal basins. |
| PTV1 | CTV1 + a setup margin specific to institutional practice and image guidance, often 3-5 mm. |
| GTV2 | Residual tumor after induction chemotherapy, excluding regions where the initial tumor only pushed into surrounding structures such as the thorax or pelvis. Pretreatment invasive disease, particularly parameningeal head-and-neck RMS, should generally remain in GTV2 regardless of chemotherapy response. |
| CTV2 | GTV2 + 1 cm. |
| PTV2 | CTV2 + a setup margin specific to institutional practice and image guidance, often 3-5 mm. |
Source: Target Volume Delineation and Field Setup: A Practical Guide for Conformal and Intensity-Modulated Radiation Therapy, 2nd Edition (Table 33.3)
Rhabdomyosarcoma dose guidance
The dose table below ties RMS prescription to surgical group, nodal status, treatment response, and fusion status. All doses are given in 1.8 Gy daily fractions.
| Group | Fusion status | Dose (Gy) |
|---|---|---|
| I (R0 resection) | Negative (embryonal) | 0 |
| I (R0 resection) | Positive (alveolar) | 36.0 |
| II, node-negative with microscopic residual disease (R1) | Either | 36.0 to pretreatment disease |
| II, node-positive with involved node resected | Either | 41.4 to the pretreatment site and nodal region |
| III, non-orbital and orbital with incomplete response after induction chemotherapy | Either | 50.4* |
| III, orbital with complete response after induction chemotherapy | Either | 45.0** |
| Special considerations | ||
| III, per ARST1431, tumors >5 cm before chemotherapy without complete response after induction | Either | 59.4* |
| III, per ARST1431, radiographic or biopsy-proven complete response at week 9 | Either | 36.0*** |
| Extremity RMS, clinical and pathologic N0, status post amputation | Either, including alveolar or fusion-positive disease | 0 |
Source: Target Volume Delineation and Field Setup: A Practical Guide for Conformal and Intensity-Modulated Radiation Therapy, 2nd Edition (Table 33.4)
* Under ARST1431, a volume reduction can be performed after 36.0 Gy so that PTV1 receives 36.0 Gy and PTV2 receives an additional cone-down dose of 14.4 Gy or 23.4 Gy depending on tumor size and scenario.
** Under ARST1431, group III disease with complete response at week 9 may be treated with a single 36.0 Gy dose level to PTV1 without additional boost. The chapter still lists 45.0 Gy as the general orbital reference.
*** Complete radiographic response by CT or MRI plus complete metabolic response by FDG-PET, or biopsy-proven absence of residual disease at week 9, allows single-dose-level treatment to 36.0 Gy at PTV1.
The figure set shows where this becomes clinically decisive. The infratemporal fossa parameningeal RMS with intracranial extension and bone erosion was treated with a single 50.4 Gy dose level because local therapy had to start with chemotherapy rather than wait for later shrinkage. The orbital RMS case also stayed at a single 50.4 Gy level because response to induction was incomplete; CTV was drawn as GTV + 1 cm and extended beyond the bony orbit in selected slices because bone erosion was suspected. A deliberate rightward eye deviation was used to optimize lens and optic nerve sparing. In the extremity RMS example with axillary nodal metastases, the full axillary basin was contoured as GTV to avoid undercoverage of diffuse nodal disease seen on staging PET. The hand primary and the axillary basin were both treated to 50.4 Gy as single dose levels because response remained limited, and no in-transit disease was seen between the hand and the axilla.
Simulation, Immobilization, Treatment Devices, and Daily Localization
Setup strategy in pediatric sarcoma is site-specific by necessity. The chapter recommends thermoplastic mask immobilization for head-and-neck lesions, arms-up positioning with VacLok or a similar cradle plus wingboard for thoracic targets, pelvic and upper-leg immobilization with VacLok for pelvic disease, and custom cradles for extremity lesions. Some extremity cases may benefit from feet-first or even non-supine positioning.
When respiratory motion is a concern, four-dimensional simulation should be considered to quantify target excursion. That principle is explicit in the thoracic Ewing example and fits well with the motion-management logic discussed in our article on lung cancer target delineation and fields. Pelvic lesions, especially genitourinary tumors, add another reproducibility problem: bladder filling. For prostate and bladder targets, consistent filling may be desirable and can be checked with daily ultrasound. In younger children or in patients who cannot reliably reproduce bladder state, simulation with both full and empty bladder may better capture the range of target motion.
The chapter adds several practical refinements. For male patients with pelvic or proximal leg sarcomas, frog-leg positioning may be used if a testicular shield is planned. The type and frequency of image guidance determine the CTV-to-PTV expansion; many institutions use daily kV imaging and therefore accept 3-5 mm PTV margins. Smaller margins can be considered when image guidance is robust and critical organs sit close to the target, a familiar problem in head-and-neck disease. MRI-based simulation in treatment position may also complement CT simulation. Finally, daily sedation or anesthesia may be required in younger children, typically those younger than 8 years.
Plan Assessment
Plan review starts with a clear coverage threshold: at least 95% of the PTV, or PTVs, should receive the prescription dose. The chapter also recommends minimizing hotspots above 110%, ideally with no more than 10% of the PTV receiving 110% or greater.
Conventional normal tissue constraints
The following table summarizes the conventional constraints cited for Ewing sarcoma and RMS. The text explicitly frames them as a baseline, not a comfort zone, because late effects are a defining issue in pediatric radiotherapy.
| Organ/tissue | Volume (%) | Dose (Gy) |
|---|---|---|
| Brainstem | Point max | 54 |
| Optic chiasm/optic nerve | Point max | 54 |
| Spinal cord | Point max | 45 |
| Lens | Point max | 6 |
| Cochlea | Point max | 35 |
| Heart | 100 | 30 |
| Lungs, bilateral | 20 | 20 |
| Lungs, bilateral | 100 | 15 |
| Liver | 100 | 23.4 |
| Liver | 50 | 30 |
| Kidneys, bilateral | 50 | 24 |
| Kidneys, bilateral | 100 | 14.4 |
| Small bowel | 50 | 45 |
| Bladder | 100 | 45 |
| Rectum | 100 | 45 |
Source: Target Volume Delineation and Field Setup: A Practical Guide for Conformal and Intensity-Modulated Radiation Therapy, 2nd Edition (Table 33.5)
That baseline matters because pediatric patients carry substantial long-term risk. The chapter argues for sparing organs at risk more aggressively whenever possible and notes that proton therapy may be considered, even though proton-specific planning details, range uncertainty, and beam arrangement are outside its scope. Plan assessment should also shape how families are counseled. Head-and-neck treatments raise concerns about dentofacial abnormalities, xerostomia, xerophthalmia, loss of visual acuity, cataract formation, facial asymmetry, endocrinopathies, and neurocognitive dysfunction. Extremity treatment raises the risk of epiphyseal closure and reduced bone growth. Vertebral irradiation may reduce final height and contribute to kyphosis, lordosis, or scoliosis, partially mitigated by covering the full vertebral body in prepubertal children. Thoracic treatment adds pneumonitis, pulmonary fibrosis, and cardiac toxicity. Pelvic treatment carries cystitis, urinary incontinence or stricture, and infertility risk, which must also be interpreted alongside the chemotherapy regimen, especially cyclophosphamide exposure.
In practice, this chapter makes one point repeatedly and well: pediatric sarcoma planning rewards biologic memory and geometric discipline. The best plans are anchored to disease extent at presentation, then refined by response and by a setup strategy the team can actually reproduce every day. For the larger map of this subject, return to the complete clinical guide.
