Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| Round | Race Name | Circuit | City/Location | Date |
| --- | --- | --- | --- | --- |
| 1.0 | 2006 Imperial Capital Bank Atlantic Race | Streets of Long Beach | Long Beach, California | April 9 |
| 2.0 | 2006 Fast Trac 100 | Streets of Houston | Houston, Texas | May 13 |
| 3.0 | 2006 Monterrey | Fundidora Park | Monterrey , Mexico | May 21 |
| 4.0 | 2006 Trinity Carpet 100K | Portland International Raceway | Portland, Oregon | June 18 |
| 5.0 | 2006 Gehl Atlantic Championship of Cleveland 1 | Cleveland Burke Lakefront Airport | Cleveland, Ohio | June 24 |
| 6.0 | 2006 Gehl Atlantic Championship of Cleveland 2 | Cleveland Burke Lakefront Airport | Cleveland, Ohio | June 25 |
| 7.0 | 2006 Toronto | Exhibition Place | Toronto, Ontario | July 9 |
| 8.0 | 2006 Edmonton | Edmonton City Centre Airport | Edmonton, Alberta | July 23 |
| 9.0 | 2006 San José | Streets of San Jose | San Jose, California | July 30 |
| 10.0 | 2006 Denver | Streets of Denver | Denver, Colorado | August 13 |
| 11.0 | 2006 Montréal | Circuit Gilles Villeneuve | Montreal, Quebec | August 27 |
| 12.0 | 2006 Gehl Championship Finale | Road America | Elkhart Lake, Wisconsin | September 24 |
Question:
When was the circuit portland international raceway?
Answer:
June 18