CBCT-based online adaptive radiotherapy of the prostate bed: first clinical experience and comparison to nonadaptive conventional IGRT

Abstract

Purpose: Conventional image-guided radiotherapy (IGRT) of the prostate bed is challenged by the varying anatomy due to dynamic changes of surrounding organs such as the bladder and rectum. This leads to changed dose coverage of target and surrounding tissue. The novel online adaptive radiotherapy (oART) aims to improve target coverage as well as reduce dose exposure to surrounding healthy tissues by daily reoptimization of treatment plans. Here we set out to quantify the resulting changes of this adaptation for patients and treatment team.

Methods: A total of 198 fractions of radiotherapy of the prostate bed (6 patients) were treated using oART with the Ethos accelerator (Varian Medical Systems, Palo Alto, CA, USA). For each fraction, volumes and several dose-volume parameters of target volumes and organs at risk were recorded for the scheduled plan (initial plan, recalculated based on daily cone beam computed tomography [CBCT]), the adapted plan, and the verification plan, which is the dose distribution of the applied plan recalculated on the closing CBCT after the adaptation process. Clinical acceptability for all plans was determined using given dose-volume parameters of target volumes. Additionally, the time needed for the adaptation process was registered and compared to the time required for the daily treatment of five conventional IGRT patients.

Results: Volumes of target and organs at risk (OAR) exhibited broad variation from day to day. The differences in dose coverage D_98% of the clinical target volume (CTV) were significant through adaptation (p < 0.0001; median D_98% 97.1-98.0%) and further after verification CBCT (p < 0.001; median D_98% 98.1%). Similarly, differences in D_98% of the planning target volume (PTV) were significant with adaptation (p < 0.0001; median D_98% 91.8-96.5%) and after verification CBCT (p < 0.001; median D_98% 96.4%) with decreasing interquartile ranges (IQR). Dose to OAR varied extensively and did not show a consistent benefit from oART but decreased in IQR. Clinical acceptability increased significantly from 19.2% for scheduled plans to 76.8% for adapted plans and decreased to 70.7% for verification plans. The scheduled plan was never chosen for treatment. The median time needed for oART was 25 min compared to 8 min for IGRT.

Conclusion: Target dose coverage was significantly improved using oART. IQR decreased for target coverage as well as OAR doses indicating higher repeatability of dose delivery using oART. Differences in doses after verification CBCT for targets as well as OAR were significant compared to adapted plans but did not offset the overall dosimetric gain of oART. The median time required is three times higher for oART compared to IGRT.

Keywords: Image-guided radiotherapy; Intensity-modulated radiotherapy; Organs at risk; Prostate cancer; Target coverage.

Fig. 1

The flow chart shows the process of online adaptive radiotherapy (oART) and image-guided radiotherapy (IGRT), respectively, using Ethos, from initial planning computed tomography (CT) to irradiation. AI artificial intelligence

Fig. 2

Patient 3—Delineation of the CTV (blue) and cropped PTV (red) influenced by the proximity of a nearby bowel loop (purple)

Fig. 3

Relative volumes of the bladder (a) and rectum (b), each normalized to their initial volume in the planning computed tomography (CT), encompassing data for every individual patient (Pat.) as well as all patients collectively

Fig. 4

Dose exposure of bladder (a, b) and rectum (c, d) in terms of V_40Gy (a, c) and D_mean (b, d) for scheduled (SCH), adapted (ADP), and verification (VER) plans

Fig. 5

D_0.1 cc of the bowel for scheduled (SCH), adapted (ADP), and verification (VER) plans of patient 3. A dashed line marks the internal clinical standard constraint (60 Gy or 1.82 Gy/fx, respectively)

Fig. 6

Relative volume of CTV, normalized to the initial volume in the planning CT, encompassing data for every individual patient (Pat.) as well as all patients collectively. With the exception of patient 4, all other patients exhibited decreased target volumes throughout the course of treatment

Fig. 7

Scatter plot for CTV and bladder volume, Spearman R = 0.651. Horizontal and vertical lines represent the specific median volume

Fig. 8

Dose coverage (D_98%) for scheduled (SCH), adapted (ADP), and verification (VER) plans, implying significant improvement in differences in D_98% for CTV (left) as well as PTV (right)

Fig. 9

Planning target volume (PTV) near maximum dose (D_2%) of PTV for scheduled (SCH), adapted (ADP), and verification (VER) plans exhibit a significant reduction in dose differences through adaptation. Internal clinical standard constraint (D_2% < 105%) is marked by a dashed line

Fig. 10

Dose coverage (D_98%) of CTV (left in each subplot) and PTV (right in each subplot) for scheduled (SCH), adapted (ADP), and verification (VER) plans from patient 1 (a) to patient 6 (f) demonstrates a significant improvement for all patients except patient 3. The presence of an intestinal loop adjacent to the target volume in patient 3 necessitated adjustments to the target volume, resulting in a notable decrease in dose coverage. Please note the different scaling of the y‑axis in subplot 4 (d) for better readability

Fig. 11

Visualization of the 95% isodose with colorwash towards 105% of the prescribed dose reveals a the scheduled plan (SCH) depicting a planning target volume (PTV, red) barely covered potentially due to fluctuations in bladder volume (yellow), while b the adapted plan (ADP) showcases a newly optimized dose distribution with improved coverage. c Dose reconstructed on the postadaptive verification CBCT