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IntroductionOur department commonly uses a planning target volume (PTV) expansion of 6 mm posterior and 1 cm in all other directions when treating prostate cancer patients with image‐guided radiotherapy (IGRT). This study aimed to test the adequacy of this PTV expansion by assessing geographical miss of the prostate on post‐treatment cone‐beam CT (CBCT) and identify those at risk of geographical miss.MethodsTwenty‐two prostate cancer patients receiving IGRT with implanted fiducial markers underwent daily pre‐treatment orthogonal kV imaging followed by a post‐treatment CBCT for a total of 432 fractions. The prostate was outlined on all CBCTs. For each imaging set, the volume of geographic miss was measured by subtracting the PTV from the planning CT and prostate volume on the post‐treatment CBCT.ResultsThe prostate volume moved outside the PTV by >0.01 cc in 9% of fractions (39/432). This occurred in 13 (59%) of 22 patients. Large prostates >40 cc and >50 cc had significantly more geographical miss events (both P < 0.001). Changes in rectal filling appear to be responsible for prostate motion/deformation in 82% (32/39) of fractions.ConclusionsOur analysis suggests that, despite IGRT, prostate PTV margins are not adequate in some patients, particularly those with large prostates. PTV margins may be reduced in some other patients. Prostate rotation and deformation play an important role in setting margins and may not always be represented accurately by fiducial marker displacements. Individualised and adaptive margins for prostate cancer patients should be a priority for future research.

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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.