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Proton Therapy Delivery and Its Clinical Application in Select Solid Tumor Malignancies
Published on: February 6, 2019
Patterns of Failure in Pediatric Rhabdomyosarcoma After Proton Therapy
Tamara Z Vern-Gross1, Daniel J Indelicato1, Julie A Bradley1
1Department of Radiation Oncology, University of Florida, Jacksonville, Florida.
Insights
Proton therapy for pediatric rhabdomyosarcoma shows an 88% local control rate. In-field failures occurred in all recurrences, suggesting proton therapy
Area of Science:
- Pediatric Oncology
- Radiation Oncology
- Medical Physics
Background:
- Rhabdomyosarcoma is a common pediatric soft tissue sarcoma.
- Proton therapy offers precise radiation delivery with a sharp dose fall-off.
- Understanding failure patterns is crucial for optimizing treatment in pediatric rhabdomyosarcoma.
Purpose of the Study:
- To evaluate the patterns of treatment failure in children with rhabdomyosarcoma who received proton therapy.
- To assess local control rates and identify sites of recurrence after proton therapy.
Main Methods:
- Retrospective analysis of 66 children with nonmetastatic rhabdomyosarcoma treated with proton therapy (2007-2013).
- Defined clinical target volumes (CTVs) with anatomically constrained margins (1 cm for CTV1, 0.5 cm for CTV2).
- Detailed analysis of failure patterns, including local progression, recurrence sites, and relationship to the radiation field.
Main Results:
- An 88% actuarial 2-year local control rate was observed.
- All 11 failures (100%) occurred within the radiation field (in-field failures).
- One patient experienced a regional nodal recurrence outside the treated field.
Conclusions:
- Proton therapy, with its sharp dosimetric gradient, does not appear to increase the risk of marginal failure in pediatric rhabdomyosarcoma.
- Standard CTV margins (0.5-1 cm) are adequate for maintaining local control with proton therapy in this population.
- These findings support the use of proton therapy for rhabdomyosarcoma, even in high-risk cases.
Purpose:
To report on the patterns of failure in children with rhabdomyosarcoma treated with proton therapy.
Patients And Methods:
Between February 2007 and November 2013, 66 children with a median age of 4.1 years (range, 0.6-15.3 years) diagnosed with nonmetastatic rhabdomyosarcoma were treated with proton therapy. Clinical target volume 1 was defined as the prechemotherapy tumor plus a 1-cm anatomically constrained margin. Clinical target volume 2 was defined as the postchemotherapy tumor (or tumor bed) plus a 0.5-cm anatomically constrained margin, further expanded to encompass potential pathways of spread, including soft tissue infiltrated with tumor at diagnosis.
Results:
Of the 66 children, 11 developed locally progressive disease at a median of 16 months (range, 14-32 months), for an actuarial 2-year local control rate of 88%. Among the children who progressed, median age and tumor size at diagnosis were 6.7 years (range, 0.6-16 years) and 6 cm (range, 2-8 cm), respectively. Of the recurrences, 64% and 36% were embryonal and alveolar, respectively. Disease progression was observed in 7 (64%) parameningeal, 2 (18%) head and neck (other), and 2 (18%) bladder/prostate subsites. At diagnosis, 8 of 11 patients who developed a recurrence were Intergroup Rhabdomyosarcoma Study stage 3, and all 11 were group III. Of the relapses, 100% (11 of 11) were confirmed as in-field within the composite 95% isodose line. One of the 11 patients (9%) developed a new simultaneous regional nodal recurrence outside of the previously treated radiation field.
Conclusion:
Early data suggest that the sharp dosimetric gradient associated with proton therapy is not associated with an increased risk of marginal failure. Routine use of a 0.5- to 1-cm clinical target volume 1/2 margin with highly conformal proton therapy does not compromise local control in children diagnosed with rhabdomyosarcoma with unfavorable risk features.
