Radiation Therapy Services ( RTSX) was among the biggest small-cap winners Monday as the sector enjoyed a midafternoon surge along with the major indices.

Shares of the Fort Myers, Fla., company skyrocketed 43.3% on late Friday's announcement that Vestar Capital Partners agreed to buy it out for $32.50 a share, or $1.1 billion including debt assumption. The deal should close in the first quarter.

Shares gained $9.34 to $30.90 in support of the Russell 2000, which lately leapt 1.2% to 808.24.

RadiSys ( RSYS) helped fire up both the Russell 2000 and the S&P SmallCap 600 -- the latter recently up 0.9% -- after Cantor Fitzgerald stamped a buy rating on the Oregon-based computer-systems maker. The analyst cited strong earnings at key customer Nokia-Siemens, among other things. RadiSys shares traded 15.8% higher at $13.57.

PetMed Express ( PETS - Get Report), or 1-800-PetMeds, saw climbing fiscal second-quarter income of 18 cents a share on revenue that shot up 17.6% to a slightly better-than-expected $51.5 million. Analysts were looking for 17 cents a share, excluding special items; last year the Florida pet-medications purveyor made 14 cents a share. Petmed stock was up 84 cents, or 6.1%, to $14.68.

Elsewhere, management-and-consulting firm China Direct ( CDS) boosted its full-year earnings guidance to more than $9.5 million from the prior $8.25 million. The Boca Raton, Fla., company also padded its revenue outlook by at least $25 million for a new outlook of more than $175 million. Shares were up 19.1% to $9.93.

And Plug Power ( PLUG - Get Report), of Latham, N.Y., soared 27.3% after retail giant Wal-Mart ( WMT) ordered an undisclosed number of Plug's GenDrive fuel cell power units for what the Canada-based company calls its "largest single GenDrive order to date." Specific terms are confidential. Plug shares were trading at $3.78; Wal-Mart was up 0.8% to $45.33.

On the other hand, Natural Health Trends ( BHIP) slid 16.9% after offering convertible debentures and warrants to institutional investors for proceeds of $3.74 million. The funds are set to provide extra working capital. Shares of the Dallas-based company were down 61 cents to $2.99.

Also on the decline was Mobile Mini ( MINI - Get Report), which dropped 12.3% after cutting at least 3 cents off its third-quarter earnings forecast to between 34 cents and 35 cents a share. Sales were pegged at about $83.5 million. Shares of the Tempe, Ariz., portable-storage-products purveyor lost $2.55 to $18.20.