45 Gy is not sufficient radiotherapy dose for Group III orbital embryonal rhabdomyosarcoma after less than complete response to 12weeks of ARST0331 chemotherapy

45 Gy is not sufficient radiotherapy dose for Group III orbital embryonal rhabdomyosarcoma after less than complete response to 12weeks of ARST0331 chemotherapy
In this paper, the COG retinoblastoma group assessed the efficacy of 45 Gy local radiotherapy in patients with Group III orbital embryonal rhabdomyosarcoma (ERMS) enrolled on the low-risk study ARST0331. More particularly, they assessed whether initial response to chemotherapy predicted outcome for patients with unresected orbital ERMS.
62 patients on ARST0331 had orbital Group III disease. Patients received four cycles of VAC followed by four cycles of VA over 22 weeks and RT starting after 12 weeks of chemotherapy. Complete response (CR) was defined as complete disappearance of the tumor by exam and imaging. Local recurrences, EFS and OS were also reported.
The 5-year EFS and OS for the entire cohort of 62 patients was 87% and 97% respectively. Fifty-three patients were evaluable for the response analysis. Fifteen patients had a CR to VAC chemotherapy. Local control on this study was worse than in the previous IRS-IV study with more intensive therapy.
Although none of the patients with early CR relapsed, this difference did not reach statistical significance.
This study was not the primary outcome of ARST0331 and was apparently underpowered to detect small differences. Also, this study did not explore the utilization of PET CT as a way to assess response after 12 weeks.
For patients with Group III orbital ERMS with CR after induction chemotherapy, the ARST0331 treatment algorithm is associated with lower poorer local control than was achieved on the more intensive IRS-IV therapy and warrants consideration of different local treatment algorithms. Dose escalation to 50.4 Gy for patients with Group III
orbital ERMS on a backbone of ARST0331 chemotherapy could be an option to explore.

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