Rectal complication probability from composite volumes derived from daily cone beam computed tomography in prostate cancer radiotherapy.

Aim: The aim of this study is to investigate the rectal complication probabilities for various rectum volumes with intensity-modulated radiation therapy (IMRT) and three-dimensional conformal radiotherapy (3D-CRT) in patients undergoing prostate cancer radiotherapy.

Materials And Methods: Thirteen patients undergoing prostate cancer radiotherapy were consecutively selected for this study. All patients were treated with IMRT to a dose of 78 Gy in 39 fractions. Three different rectum volumes: (i) planned rectum (plan-rectum) (ii) Boolean sum of rectum volume based on the cone-beam computed tomography (CBCT) for first five fractions (planning organ at risk volumes [PRV]-CBCT-5), (iii) Boolean sum of rectum volume from all the CBCTs (PRV-CBCT-All) in addition to an average rectal complication (PRV-CBCT-AV) were used for computing the probabilities of rectal complications. To assess the rectal complications with 3D-CRT, a five-field plan was generated for comparison with IMRT. The Lyman-Kutcher-Burman (LKB) normal tissue complication probability (NTCP) model was used to assess the rectal complications for all of the defined rectal volumes.

Results: The NTCPs for rectum as assessed from plan-rectum, PRV-CBCT-5, PRV-CBCT-All, and PRV-CBCT-AV with IMRT were 9.71% ±4.69%, 16.34% ±9.51%, 19.39% ±9.71%, and 12.81% ±7.22%, respectively. Similarly, with 3D-CRT, the NTCPs were 17.41% ±10.44%, 19.61% ±11.08%, 21.03% ±11.06%, and 17.72% ±10.29%, respectively.

Conclusion: Our results showed that the rectal complications are reduced significantly with IMRT as compared to 3D-CRT. As such, the analyses of NTCP with various defined composite rectum volumes indicate that IMRT requires image-guided adaptive radiotherapy as opposed to 3D-CRT.