Postprostatectomy target-normal structure overlap volume differences using computed tomography and radioimmunoscintigraphy images for radiotherapy treatment planning

Impact of 18F-Fluciclovine PET on Target Volume Definition for Postprostatectomy Salvage Radiotherapy: Initial Findings from a Randomized Trial

The purpose of this study was to evaluate the role of the synthetic amino acid PET radiotracer ¹⁸F-fluciclovine in modifying the defined clinical and treatment-planning target volumes in postprostatectomy patients undergoing salvage radiotherapy and to evaluate the resulting dosimetric consequences to surrounding organs at risk. Methods: Ninety-six patients were enrolled in a randomized, prospective intention-to-treat clinical trial for potential salvage radiotherapy for recurrent prostate cancer after prostatectomy. The initial treatment plan was based on the results from conventional abdominopelvic CT and MRI. The 45 patients in the experimental arm also underwent abdominopelvic ¹⁸F-fluciclovine PET/CT, and the images were registered with the conventional images to determine whether the results would modify the initial treatment plan. The 51 patients in the control arm did not undergo ¹⁸F-fluciclovine PET/CT. For each patient, the clinical and treatment-planning target volumes that would have been treated before ¹⁸F-fluciclovine registration were compared with those after registration. For organs at risk (rectum, bladder, and penile bulb), the volumes receiving 40 Gy and 65 Gy before registration were compared with those after registration. Statistical comparisons were made using the paired t test. Acute genitourinary and gastrointestinal toxicity as defined by the Radiation Therapy Oncology Group was compared between the control and experimental arms using the χ² test. Results: In 24 cases, radiotherapy was planned to a clinical target volume consisting of the prostate bed alone (CTV) (64.8-66.6 Gy). In 21 cases, radiotherapy was planned to a clinical target volume consisting of the pelvis (CTV1) (45.0 Gy) followed by a boost to the prostate bed (CTV2) (19.8-25.2 Gy). In each case, the respective treatment-planning target volume expansion (PTV, PTV1, or PTV2) was 0.8 cm (0.6 cm posterior). With the exception of PTV2, all postregistration volumes were significantly larger than the corresponding preregistration volumes. Analysis of the rectum, bladder, and penile bulb volumes receiving 40 Gy and 60 Gy demonstrated that only the penile bulb volumes were significantly higher after registration. No significant differences in acute genitourinary or gastrointestinal toxicity were observed. Conclusion: Including information from ¹⁸F-fluciclovine PET in the treatment-planning process led to significant differences in the defined target volume, with higher doses to the penile bulb but no significant differences in rectal or bladder dose or in acute genitourinary or gastrointestinal toxicity. Longer follow-up is needed to determine the impact of ¹⁸F-fluciclovine PET on cancer control and late toxicity endpoints.

Case study of anti-1-amino-3-F-18 fluorocyclobutane-1-carboxylic acid (anti-[F-18] FACBC) to guide prostate cancer radiotherapy target design

PURPOSE OF THE REPORT

Anti-1-amino-3-F-18 fluorocyclobutane-1-carboxylic acid (FACBC) is a novel radiotracer, which has shown some promise for use with positron emission tomography (PET)/computed tomography (CT) for visualizing prostate cancer. Here we describe a case of a prostate cancer patient who underwent radiation treatment and had an FACBC scan obtained as part of a pilot study.

METHODS

We explored the potential impact of FACBC on treatment planning. We registered the FACBC acquisition with the PET/CT, which required a simple translation. Then, we did a deformable image registration of the PET/CT with the planning CT-this process allowed the FACBC-defined gross tumor volume (GTVFACBC) to be projected into the planning CT. An intensity-modulated radiotherapy (IMRT) plan (plan A) not including GTVFACBC (with final dose to 81.0 Gy) was generated, as was an IMRT plan including the GTVFACBC to a final dose of 86.4 Gy (plan B). Target coverage and normal tissue dose volume histogram (DVH) endpoints were tabulated.

RESULTS

In this particular patient, bladder constraints could not be met on any plan due to anatomic limitations. However, the impact on the rectal DVH could be assessed, and inclusion of the GTVFACBC did permit rectal DVH constraints to be met in plan B while maintaining target coverage and inhomogeneity constraints.

CONCLUSION

In our test case, it was feasible to use FACBC to guide IMRT, and highlights the role of deformable image registration of the PET/CT with the planning CT. These findings can guide future studies incorporating FACBC into treatment planning.

The role of indium-111 radioimmunoscintigraphy in post-radical retropubic prostatectomy management of prostate cancer patients

Indium-111 radioimmunoscintigraphy (RIS) has an increasing role in the treatment of prostate cancer and is most commonly performed at this disease site using labeled monoclonal antibody against prostate-specific membrane antigen. There are many limitations of RIS, including low spatial resolution, low diagnostic yield and limited availability. Despite these limitations, the efficacy of RIS has been demonstrated in many clinical studies, including multi-institutional investigations. The highest sensitivity and specificity of RIS appears to be in the post-radical retropubic prostatectomy (post-RRP) setting. RIS has recently been explored for its role in clinical radiotherapy decision-making, and was found to have a significant impact in selecting patients for radiotherapy and for the general radiotherapy treatment volume definition. RIS has also recently been explored for its role in radiotherapy planning and was found to impact clinical target volume design. However, manual editing of the RIS volume is still necessary when projected into the radiotherapy-planning scan, as there is often overlap in the RIS-defined uptake regions with normal structures (rectum, bladder and symphysis bone marrow). The impact of RIS on biochemical control has been explored, with studies in this area yielding conflicting results. It appears that the maximum impact of RIS is possible when areas of labeled antibody uptake regions are co-registered with the radiotherapy-planning computed tomography scan. The larger RIS-guided target volumes do not appear to be prohibitive in increasing radiotherapy-related toxicity. Future directions of the use of RIS for post-RRP prostate cancer are discussed.