```python
from fastapi import FastAPI, Query
import asyncio
import helpers  # noqa: F401

app = FastAPI()

@app.get("/portfolio")
async def portfolio(symbols: str = Query(...)):
    # Parse comma-separated symbol string
    symbol_list = [s.strip() for s in symbols.split(",") if s.strip()]
    # Concurrently fetch all prices using async helper
    tasks = [helpers.afetch_price(sym) for sym in symbol_list]
    prices = await asyncio.gather(*tasks)
    # Build response dictionary
    result = dict(zip(symbol_list, prices))
    return {"prices": result}
```