Interfractional Variations in the Setup of Pelvic Bony Anatomy and Soft Tissue, and Their Implications on the Delivery of Proton Therapy for Localized Prostate Cancer

Abstract

Purpose-To quantify daily variations in the anatomy of patients undergoing radiation therapy for prostate carcinoma, to estimate their effect on dose distribution, and to evaluate the effectiveness of current standard planning and set-up approaches employed in proton therapy.Methods-We used series of CT data, which included the pre-treatment scan, and between 21 and 43 in-room scans acquired on different treatment days, from 10 patients treated with intensitymodulated radiation therapy at Morristown Memorial Hospital. Variations in femur rotation angles, thickness of subcutaneous adipose tissue, and physical depth to the distal surface of the prostate for lateral beam arrangement were recorded. Proton dose distributions were planned with the standard approach. Daily variations in the location of the prescription iso-dose were evaluated.Results-In all 10 datasets, substantial variation was observed in the lateral tissue thickness (standard deviation of 1.7-3.6 mm for individual patients, variations of over 5 mm from the planning CT observed in all series), and femur rotation angle (standard deviation between 1.3-4.8°, with the maximum excursion exceeding 10° in 6 out of 10 datasets). Shifts in the position of treated volume (98% iso-dose) were correlated with the variations in the lateral tissue thickness.Conclusions-Analysis suggests that, combined with image-guided set-up verification, the range compensator expansion technique prevents loss of dose to target due to femur rotation and soft tissue deformation, in the majority of cases. Anatomic changes coupled with the uncertainties of particle penetration in tissue restrict possibilities for margin reduction in proton therapy of prostate cancer.

Bone marrow sparing in intensity modulated proton therapy for cervical cancer: Efficacy and robustness under range and setup uncertainties

“…Robustness of IMPT for the pelvic region is of significant interest because of the potentially large range uncertainty and setup variation [21–24]. This study investigates the topic of IMPT robustness for pelvic treatment with a new focus on the bone-marrow sparing efficacy and robustness.…”

Investigating CT to CBCT image registration for head and neck proton therapy as a tool for daily dose recalculation

“…6) for evaluation of potential dose distribution degradation. As the positioning of the Bragg peak is sensitive to the patient's three dimensional stopping power ratio distribution as estimated by a planning CT scan, the use of a CT-on-rail inside the treatment room, as investigated in the context of passively scattered proton therapy of the prostate, 7 would offer the optimal data for proton dose recalculation. However, such installations are not common, require a couch movement between imaging and treatment, and increase the complexity of the treatment workflow.…”

Visualization of a variety of possible dosimetric outcomes in radiation therapy using dose-volume histogram bands

“…Various methods of illustrating the uncertainty of delivered dose have been previously used in the literature, including displays of dose distributions and DVH for individual error scenarios [10,16,18], worst-case distributions [17,19], and dose variance distributions [6,8,9]. However, at the moment, their relative utility remains unclear.…”

“…Instead, proton therapy employs other means, such as range compensator expansion, or “smearing”, distal and proximal range margins to ensure that the errors in the estimated range in tissue and patient set-up do not result in a loss of dose to the target [20]. A systematic error in the proton range may arise due to misestimation of tissue stopping powers from CT data, while intrafractional motion and anatomical variations produce range shifts which can be described as random [19]. …”

“…These methods produce plans that are “robust”, i.e., minimally affected by the deviations from the planned set-up or anatomy [15,16,17]. Recognition of the importance of the potential effect of delivery errors required some reconsideration of the way the treatment plans were evaluated, and a number of ideas have been put forward on how to visualize various dosimetric outcomes: display of families of DVH [6,18] and dose distributions for individual error scenarios [10,15], worst-case cases [16,19], and dose variance distributions [8,9]. …”