KEY POINTS
- The study generated single-arc and double-arc VMAT plans for 15 prostate cancer patients, with bilateral titanium hip prostheses simulated by assigning the femoral heads a Hounsfield unit value of 7,227.
- Both strategies used full arcs with Eclipse’s avoidance-structure function to suppress beam entry through the simulated prostheses. The prescription was 78 Gy in 39 fractions, with identical optimization constraints for both approaches.
- Double-arc plans produced statistically better values for several target and rectal metrics. Rectal V70Gy was 6.5% versus 7.7%, V50Gy 21.2% versus 22.4%, and PTV D2% 101.9% versus 102.4% for double- versus single-arc planning.
- Despite statistical significance, the absolute dosimetric differences were small. PTV D95 was essentially identical at 97.0% versus 96.8%, and conformity was almost unchanged (0.798 vs 0.799).
- Robustness was tested by applying simulated 5- and 10-mm shifts in anterior, posterior, superior and inferior directions. Double-arc planning maintained statistically higher CTV D98% after posterior and inferior shifts.
- Even where differences were greatest, they remained small: after a 10-mm posterior shift, CTV D98% was 76.6 Gy with double arc versus 76.0 Gy with single arc, a difference of only 0.6 Gy.
- The more important vulnerability affected both approaches. A 10-mm anterior shift reduced CTV D98% by approximately 1.7% with single arc and 1.6% with double arc, emphasizing the importance of internal-motion management and bladder/rectal preparation.
CLINICAL TAKEAWAY
For prostate radiotherapy with bilateral hip prostheses, adding a second arc improved several dosimetric and robustness metrics, but the differences were probably too small to be clinically important in this simulation. Single-arc VMAT therefore remains a reasonable option and may shorten delivery, although treatment time itself was not measured and no patients actually had hip prostheses.