Po-Poster - 03: A comparison of intensity-modulated radiotherapy versus 3D conformal radiotherapy for prostate cancer patients whose rectal reaction to prior conventional radiotherapy was known

Abstract

Background and purpose: To evaluate the difference in biological and physical endpoints between intensity-modulated radiotherapy (IMRT) plans and conformal plans for thirteen prostate cancer patients whose rectal reaction to prior radiation therapy was known (five patients experienced Grade 0 late rectal complications, six Grade 1, and two Grade 2). Materials and methods: Conformal plans were generated for each patient with Eclipse version 7.1.59 while the Helios inverse optimization component within Eclipse was applied for computation of the IMRT plans. Physical endpoints applied to compare the IMRT and conformal plans included the target dose statistics (minimum, maximum, and mean dose) as well as dose-volume histogram results for the normal tissues (rectum, bladder, and femoral heads). Biological endpoints used for comparison of the IMRT and conformal plans were tumour control probability (TCP), normal tissue complication probability (NTCP), and probability of uncomplicated local control (P+). Rectal NTCP was computed for both RTOG Grade 3 or lower complications and severe rectal complications (severe proctitis, necrosis, stenosis, and fistula). Results: On average, IMRT increased the TCP by 2.8% and increased the TDI by 7.7%. The average NTCPs for the Grade 3 or lower rectal complications were 15.1% and 4.4% for the conformal and IMRT plans, respectively. For severe rectal complications the average NTCP decreased from 2.1% (conformal) to 1.2% (IMRT). The average P+ increased from 59.4% (conformal) to 69.9% (IMRT). Conclusions: IMRT was found to reduce the rectal NTCP for RTOG Grade 3 or lower complications to ∼0.36 times the NTCP for the conformal plan.