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A Study On Rectal Complication Probability From Composite Volumes Derived From Daily CBCT in Prostate Cancer Radiotherapy

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P Ramachandran

P Ramachandran*, R Oates, J Chang, D Jones, S Gill, J Cramb, Peter MacCallum Cancer Centre, Melbourne, Victoria

SU-E-J-20 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Dose escalation improves local control in prostate cancer radiotherapy. However, dose escalation is limited due to late rectal toxicity. Bladder and rectal filling / rectal gas during the course of treatment tend to alter the dose to the target volume and critical structures. In this study, an effort has been made to assess the late rectal complication probability from a serial of cone beam computed tomography (CBCT) scans for prostate cancer with IMRT and 3D conformal radiotherapy.

Methods:Twelve patients were selected for this study. All patients were treated with IMRT to a dose of 78 Gy with pre-treatment kV/kV orthogonal imaging matched to gold seed fiducials. CBCT scans were acquired at the end of treatment on fractions 1-5 and then every alternate fraction. Alternatively, five field 3D-CRT plans were generated for the above patients. Three different rectum volumes: (i) Plan rectum (ii) Boolean sum of rectum volume based on the CBCT for first 5 fractions (Rectum-5) and (iii) Boolean sum of rectum volume from all the CBCTs (Rectum-all) were used for computing the rectal complications. The Lyman-Kutcher-Burman normal tissue complication probability (NTCP) model has been used in this study to assess grade-2 rectal toxicity.

Results:The NTCP for rectum with IMRT as assessed from Plan-Rectum, Rectum-5, Rectum-all and average rectum were 11.03 ± 4.91%, 17.45 ± 9.07%, 21.89 ± 7.66% and 13.83 ± 6.82% respectively whereas for 3D-CRT it is 20.22 ± 9.45%, 21.61 ± 9.56%, 23.81 ± 8.62% and 19.78 ± 8.97% respectively. Statistically significant differences were observed between Plan-Rectum, Rectum-5 and Rectum-All with IMRT.

Conclusion:Our results show that large variation in NTCP for IMRT between Plan-rectum, Rectum-5 and rectum-all indicates a potential role for adaptive radiotherapy for prostate cancer. This could in turn lead to dose escalation with IMRT for prostate cancer.

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