Treatment of rhabdomyosarcoma (RMS) with proton therapy led to 5-year local control rates and survival comparable to those observed with photon therapy and achieved a favorable toxicity profile, data from a prospective phase II study suggest.
After a median follow-up of 47 months, 57 patients had a 5-year local control rate of 81%, event-free survival (EFS) of 69%, and overall survival of 78%. Low-risk patients had a 5-year local control rate of 93%, Torunn L. Yock, MD, of Massachusetts General Hospital in Boston, and co-authors reported online in the Journal of Clinical Oncology.
"Proton radiation therapy appears to be a feasible, safe, and effective modality for use in the pediatric RMS population," the authors concluded.
"Additional follow-up and prospectively collected photon toxicity data are needed to determine whether proton radiation therapy truly reduces the incidence and severity of late effects in comparison to patients treated with modern photon techniques," they added.
The rationale for proton-beam therapy has rested heavily on the proposition that the technique would achieve results that are as good or better than those reported with conventional photon-beam therapy but with a lower incidence of radiation-related toxicity. Proton therapy can reduce the radiation dose to normal tissue by a factor of 2 or 3 as compared with photon therapy, making the technology particularly attractive for treatment of pediatric tumors, the authors noted in their introduction.
The medical literature offers a paucity of long-term clinical outcome data to evaluate the safety and efficacy of proton therapy. To increase and improve the evidenced base, Yock and colleagues initiated a phase II trial to evaluate proton therapy in RMS, which affects about 350 children and adolescents in the U.S. each year.
From 2005 to 2012, investigators in Boston and Houston enrolled patients ≤21 years old with localized RMS or age 2 to 10 with metastatic embryonal RMS. All patients received standard chemotherapy plus proton radiation. Surgical resection depended on the tumor site and accessibility.
Half the patients were ≤2 years old, embryonal histology predominated (72%), and two-thirds of the patients had tumors in unfavorable sites for resection.
At data analysis, 41 patients remained alive and disease free, five were alive with recurrent disease, and 11 patients had died of the disease. Eight of 16 recurrences were isolated local treatment failures. No central nervous system failures occurred.
The 15 patients with low-risk characteristics had a 100% survival at 5 years, in addition to 93% EFS. The 42 intermediate-risk patients had 5-year EFS of 61% and overall survival of 70% (P=0.04 for both outcomes versus low-risk patients).
Toxicity included 13 patients who had grade 3 acute toxicity and three who had grade 3 late toxicity. No patient had grade >3 acute or late toxicity.
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