Influence of Organ Motion on Conformal vs. Intensity-Modulated Pelvic Radiotherapy for Prostate Cancer

Abstract

Purpose: To compare an intensity-modulated radiotherapy (IMRT) planning approach for prostate pelvic RT with a conformal RT (CRT) approach taking into account the influence of organ-at-risk (OAR) motion. Methods and Materials: A total of 20 male patients, each with one planning computed tomography scan and five to eight treatment computed tomography scans, were used for simulation of IMRT and CRT for delivery of a prescribed dose of 50 Gy to the prostate, seminal vesicles, and pelvic lymph nodes. Planning was done in Eclipse without correcting for OAR motion. Evaluation was performed using the CRT and IMRT dose matrices and the planning and treatment OAR outlines. The generalized equivalent uniform dose (gEUD) was calculated for 894 OAR volumes using a volume-effect parameter of 4, 12, and 8 for bowel, rectum and bladder, respectively. For the bowel, the gEUD was normalized to a reference volume of 200 cm 3 . For each patient and each OAR, an average of the treatment gEUDs (gEUD treat ) was calculated for CRT and IMRT. The paired t test was used to compare IMRT with CRT and gEUD treat with gEUD plan . Results: The mean gEUD treat was reduced from 43 to 40 Gy, 47 to 46 Gy, and 48 to 45 Gy with IMRT for the bowel, rectum, and bladder, respectively (p < 0.001). Differences between the gEUD plan and gEUD treat were not significant (p > 0.05) for any OAR but was >6% for the bowel in 6 of 20 patients. Conclusion: Intensity-modulated RT reduced the bowel, rectum, and bladder gEUDs also under influence of OAR motion. Neither CRT nor IMRT was robust against bowel motion, but IMRT was not less robust than CRT. Ó

The normal tissue sparing obtained with simultaneous treatment of pelvic lymph nodes and bladder using intensity-modulated radiotherapy

“…In a comparable study by Hysing et al on prostate cancer where the target also included the regional lymph nodes, the gEUD using IMRT was 40 Gy and 46 Gy for the bowel and rectum, respectively [13]. The difference in gEUD between our study and the study by Hysing et al reflects the different volumes of normal tissue included in the PTV for prostate and bladder RT, respectively.…”

“…However, it is well known that the ORs, and especially the bowel will change position between treatment fractions [26]. In the study by Hysing et al, the IMRT sparing of the bowel and rectum persisted when the intra-fractional movements were analyzed on repeated CT scans [13]. Because of limited accuracy and definition of the targets and ORs, comparison with other studies is difficult, and the correlation between gEUD and clinical outcome may be problematic.…”

“…Bowel gEUDs were also calculated for CRT plans with a prescribed dose to the PTV1 of 60 Gy and 64 Gy, respectively. For the bowel, we followed the method by Hysing et al, extending the Niemierko EUD equation by introducing an absolute reference volume, Vref, and by replacing the volume fraction with the absolute volume at the i th differential DVH dose bin [12,13].…”

IMRT of prostate cancer

“…Recently it has been pointed out, that over the past decade intensity-modulated radiation therapy (IMRT) has shown its superiority to three-dimensional conformal radiotherapy (3DCRT) in the treatment of prostate cancer, producing more conformal dose distributions for the target volume with minimal longterm toxicities in the organs at risk (OAR) [7,12,15]. Thereby image guidance is recommended to reduce errors of patient setup and internal motion of the prostate [12,19,24,25].Different algorithms for the optimisation have been described.…”

Deformable registration-based segmentation of the bowel on Megavoltage CT during pelvic radiotherapy

“…Large amplitude motion and deformations of the bowel have been observed [7][8][9][10]. It has been reported that only a minor part (i.e.…”