IMRT versus RapidArc – superiority depends on PTV

MedWire News: For men undergoing radiotherapy of the prostate, conventional intensity-modulated radiotherapy (IMRT) performs better than RapidArc in dose sparing of the bladder, rectum, and small bowel for planning target volumes (PTVs) that include the prostate, seminal vesicles, and lymph nodes, researchers report.

Conversely, for PTVs including just the prostate and seminal vesicles, RapidArc with two arcs provides plans comparable to those for IMRT, while also offering more efficient treatment delivery.

Sua Yoo (Duke University Medical Center, Durham, North Carolina, USA) and colleagues studied 10 patients, comparing dosimetric results and treatment delivery efficiency of RapidArc plans to those of conventional IMRT using the Eclipse treatment planning system for high-risk prostate cancer.

The primary PTV contained prostate, seminal vesicles and pelvic lymph nodes with a margin. The boost PTV contained prostate and seminal vesicles with a margin. The total prescription dose included 46.8 Gy to the primary PTV and 28.8 Gy to the boost PTV, delivered in 1.8 Gy fractions. Three plans were generated for each PTV – multiple field IMRT, one-arc RapidArc and two-arc RapidArc.

For the primary PTV, IMRT delivered average mean doses to the bladder, rectum, and small bowel that were 5.9%, 7.7%, and 4.3% lower, respectively, than those delivered by one-arc RapidArc, and 3.6%, 4.8% and 3.1% lower, respectively, than those delivered by two-arc RapidArc.

For the boost PTV, IMRT delivered mean doses to the bladder and recturm that were 2.6% and 4.8% lower than with one-arc RapidArc and 0.6% and 0.2% higher, respectively, than with two-arc RapidArc. Treatment delivery time was reduced by 2–7 minutes using RapidArc.

Yoo et al comment in the International Journal of Radiation Oncology Biology Physics: “For complicated and large PTVS that included prostate, seminal vesicles, and lymph nodes, conventional IMRT spared the bladder, rectum, and small bowel dose more than did RapidArc.

“For simple and small PTVs that included prostate and seminal vesicles, RapidArc with two arcs provided plans comparable to those achieved with the conventional IMRT technique.”

The researchers conclude: “RapidArc plans should be compared with IMRT plans for individual cases to measure gains and losses before selecting one over the other.”

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