Clinical target volumes in anal cancer: calculating what dose was likely to have been delivered in the UK ACT II trial protocol

Abstract

Purpose: Preliminary results of the UK Anal Cancer Trial (ACT) II trial in squamous cell carcinoma of the anus (SCCA) are promising, but 2-D planning with parallel-opposed fields provoked significant toxicity. We calculated likely doses delivered in the ACT II protocol to the planning target volume (PTV), nodal clinical target volumes (n-CTV) and organs at risk (OARs).

Methods and materials: Original planning CT datasets of 33 consecutive patients with SCCA, included in the ACT II trial or treated to an identical protocol, enabled dose to the primary tumour, involved nodal PTV's, uninvolved nodal CTVs (inguino-femoral and pelvic lymph nodes) and femoral heads to be retrospectively calculated.

Results: The mean dose delivered to primary PTV was 51.37±1.68 Gy (95% CI), with a maximum dose (D(max)) of 54.63±2.68 Gy (95% CI). Involved inguinal nodes received a mean 51.41±3.08 Gy, D(max) 54.17±2.84 Gy (95% CI). Clinically uninvolved nCTVs received a mean 36.53±3.38 Gy (inguinal nodes) and 34.15±5.59 Gy (external/internal iliac nodes). Femoral heads received a D(max) of 47.32±3.45 (95% CI).

Conclusion: Calculating the likely doses delivered in ACT II from chemoradiation to PTV, n-CTV and OARs facilitates specification of nodal doses and constraints for 3D-conformal/IMRT planning and allows rational dose-escalation for T3/T4 tumours, and potential dose-reduction for T1/T2 tumours.