Comparative analysis of volumetric modulated arc therapy versus intensity modulated radiation therapy for radiotherapy of anal carcinoma.

PURPOSE

Volumetric modulated arc therapy (VMAT), an evolution of intensity modulated radiation therapy (IMRT), utilizes the dynamic modulation of angular dose rate through changes in gantry speed, linear accelerator dose rate, and multileaf collimator motion to deliver a treatment fraction in rotational fashion with improved efficiency and in shorter time. In general, target coverage relative to adjacent organ-at-risk sparing is highly dependent on the complexity of the treatment site. Therefore, we critically evaluated VMAT compared to IMRT for definitive treatment of anal carcinoma with respect to dosimetry and efficiency.

METHODS AND MATERIALS

Using SmartArc (Philips Healthcare, Andover, MA), VMAT treatment plans were generated for 10 patients treated at our institution for anal carcinoma, and compared to the IMRT plans used for clinical treatment. The patients were all female, had T classification TX/1 (n = 5), T2 (n = 3) or T3 (n = 2), were node-negative (n = 6) or node-positive (n = 4), and were treated to a total dose of 50 to 58 Gy. Pairwise comparisons were made between VMAT and IMRT plans with respect to dose-volume histogram parameters relating the dose received by target volumes, relevant organs at risk, and normal tissues. The plans were machine-delivered, with actual beam delivery times measured.

RESULTS

VMAT plans had superior planning target volume coverage and dose homogeneity, with improved conformality in treatment of the elective nodal volume, in comparison to IMRT. Mean dose to the small bowel, genitalia, and femoral heads were significantly lower with VMAT, and similar with respect to bladder, pelvic bones, and normal tissues. Integral dose was comparable between the 2 techniques. VMAT plans required 36.8% fewer monitor units, and beam delivery time was shorter by 9 minutes.

CONCLUSIONS

Our results indicate that VMAT represents an ideal treatment modality for anal carcinoma, generating plans with excellent target coverage, lower doses to organs at risk, and shorter treatment times, in comparison to IMRT.