Abstract
Background and purpose: To analyze deviations of the applied from the planned doses on a voxel-by-voxel basis for definitive prostate cancer radiotherapy depending on anatomic variations and imaging frequency.Materials and methods: Daily in-room CT imaging was performed in treatment position for 10 patients with prostate cancer undergoing intensity-modulated radiotherapy (340 fraction CTs). Applied fraction doses were recalculated on daily images, and voxel-wise dose accumulation was performed using a deformable registration algorithm. For weekly imaging, weekly position correction vectors were derived and used to rigidly register daily scans of that week to the planning CT scan prior to dose accumulation. Applied and prescribed doses were compared in dependence of the imaging frequency, and derived TCP and NTCP values were calculated.Results: Daily CT-based repositioning resulted in non-significant deviations of all analyzed dose-volume, conformity and uniformity parameters to the CTV, bladder and rectum irrespective of anatomic changes. Derived average TCP values were comparable, and NTCP values for the applied doses to the bladder and rectum did not significantly deviate from the planned values. For weekly imaging, the applied D2 to the CTV, rectum and bladder significantly varied from the planned doses, and the CTV conformity index and D98 decreased. While TCP values were comparable, the NTCP for the bladder erroneously appeared reduced for weekly repositioning.Conclusions: Based on daily diagnostic quality CT imaging and voxel-wise dose accumulation, we demonstrated for the first time that daily, but not weekly imaging resulted in only negligible deviations of the applied from the planned doses for prostate intensity-modulated radiotherapy. Therefore, weekly imaging may not be adequately reliable for adaptive treatment delivery techniques for prostate. This work will contribute to devising adaptive re-planning strategies for prostate radiotherapy.
Highlights
Prostate cancer is among the most prevalent malignant diseases in men with between 100 and 170 newly diagnosed patients per 100,000 people annually [1]
More recent work using elastic imaging registration reported no clinically relevant aberrations of the applied doses from the doses prescribed during a prospective study; this analysis relied on weekly imaging and cone-beam CT (CBCT)-based dose accumulation [12]
To assess potential implications of the described anatomic changes on the position of the clinical target volume (CTV), its geometric center was compared between the planning scan and the daily scans after individual rigid pre-treatment realignments based on adjacent bony structures
Summary
Prostate cancer is among the most prevalent malignant diseases in men with between 100 and 170 newly diagnosed patients per 100,000 people annually [1]. The use of intensity-modulated radiotherapy (IMRT) has been shown to reduce late radiation-induced genitourinary and gastrointestinal toxicities, thereby enabling the safe application of higher treatment doses [5, 6]. The advent of high-precision radiotherapy for prostate cancer has made the treatment application more susceptible to inaccuracies due to changes in the pelvic anatomy, and the occurrence of intraand especially interfractional variations of the prostate and the surrounding organs-at risk have been well studied [7,8,9]. Previous work using weekly CT scans and rigid registration demonstrated significant deviations from the prescribed dose for both IMRT and proton radiotherapy [10, 11]. To analyze deviations of the applied from the planned doses on a voxel-by-voxel basis for definitive prostate cancer radiotherapy depending on anatomic variations and imaging frequency
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