Prospective development of an individualised predictive model for treatment coverage using offline cone beam computed tomography surrogate measures in post‐prostatectomy radiotherapy

Abstract

The aim of this study is to prospectively evaluate and model surrogate explanatory variables (SEVs) of target coverage and rectal dose pertaining to soft tissue anatomy visualised on cone beam computed tomography (CBCT) for incorporation into post-prostatectomy treatment coverage verification protocols. Twenty post-prostatectomy patients treated with conformal prostate bed radiotherapy (64-74 Gy) underwent CBCT daily at fractions 1 to 5, and then weekly. Treatment coverage was defined on each CBCT using 'PTV95', percentage of the CBCT PTV covered by original treatment fields, and 'RECTD50', dose delivered to 50% of CBCT rectal volume by original treatment fields. Three candidate SEVs for treatment coverage were defined for each scan: anterior rectal wall movement, change in bladder length and bladder base movement. Both anterior rectal wall movement and increase in bladder length predicted for the decreased PTV95 (P < 0.001 for each). Anterior movement of the anterior rectal wall predicted for increased RECTD50 (P < 0.001). Predictive models for the PTV95 and RECTD50 that accept the significant SEVs as inputs were developed. We developed simple CBCT-acquired soft tissue anatomic surrogate measures that signal changes in target coverage and rectal dose during post-prostatectomy radiotherapy. Conventional bony anatomy patient position verification protocols were inadequate in accounting for soft tissue target and organ variation seen with CBCT.

Prostate Bed Radiation Therapy: The Utility of Ultrasound Volumetric Imaging of the Bladder

“…This would explain the large confidence intervals shown in Figure 2. A further possible explanation is that the variation in rectal filling (not accounted for in the current data analysis) has a compounding effect on the PTV95, as shown in our previous studies [3,11]. We included data from only 15 patients in this part of the study (the largest recruiting site).…”

“…However, for the reasons stated above, we do not believe larger patient numbers would have changed the outcome. Although it is clear from Figure 5 that a number of volumes seem to be receiving a suboptimal dose distribution, we believe use of the bladder scanner before treatment has avoided a significantly larger number of cases based on our previous work showing changes of more than 2 cm in bladder lengths impact on the PTV95 [11]. Based on our clinical practice we believe that a pragmatic selection of a tolerance range of AE100eAE150 ml is reasonable and achievable.…”

“…The motivation for this aim came from our previous work that indicated a bladder length at the time of treatment that was within 2 cm of the length at simulation would most likely have adequate coverage of the PTV with the 95% isodose [11]. For patients with a bladder length <8 cm we were able to achieve a concordance rate of 91% (Figure 4b).…”

“…By definition, the target volume for the prostate bed follows the contour of the anterior rectal wall and encompasses part of the posterior portion of bladder volume [12,13], with both organs subject to considerable interfractional changes in shape and movement relative to one another, and the posterior pubic symphysis that defines the anterior volume [14]. In our previous studies [3,11] we reported the development of a cone beam computed tomography (CBCT) protocol that was designed to limit target volume deformation through surveillance of bladder volume and rectal filling. Briefly, to aid rapid assessment of the CBCT data set, bladder length, distance to anterior and posterior rectal wall were found to be effective surrogates for bladder and rectal volume.…”

Evaluating deviations in prostatectomy patients treated with IMRT

“…15 Some studies show that surgical bed in patients submitted to prostatectomy presents larger mobility compared with patients that were not subjected to surgery. [20][21][22] There are imaging strategies, such as portal image, to correct geometric uncertainties that allow the verification of bony anatomy, although this may be insufficient for the correction of uncertainties. 21 On the other hand, CBCT images allow the visualization of soft tissues leading to the improvement in the correction of uncertainties.…”