A method for evaluation of proton plan robustness towards inter-fractional motion applied to pelvic lymph node irradiation

Abstract

Background. The benefi t of proton therapy may be jeopardized by dose deterioration caused by water equivalent path length (WEPL) variations. In this study we introduced a method to evaluate robustness of proton therapy with respect to inter-fractional motion and applied it to irradiation of the pelvic lymph nodes (LNs) from different beam angles. Patient-versus population-specifi c patterns in dose deterioration were explored. Material and methods. Patient data sets consisting of a planning computed tomography (pCT) as well as multiple repeat CT (rCT) scans for three patients were used, with target volumes and organs at risk (ORs) outlined in all scans. Single beam spot scanning proton plans were optimized for the left and right LN targets separately, across all possible beam angle confi gurations (5 ° angle intervals). Isotropic margins of 0, 3, 5 and 7 mm from the clinical target volume (CTV) to the planning target volume (PTV) were investigated. The optimized fl uence maps for the pCT for each beam were applied onto all rCTs and the dose distributions were re-calculated. WEPL variation for each beam angle was computed by averaging over beams eye view WEPL distributions. Results. Similarity in deterioration patterns were found for the investigated patients, with beam angles delivering less dose to rectum, bladder and overall normal tissue identifi ed around 40 ° and around 150 ° -160 ° for the left LNs, and corresponding angles for the right LNs. These angles were also associated with low values of WEPL variation. Conclusion. We have established and explored a method to quantify the robustness towards inter-fractional motion of single beam proton plans treating the pelvic LNs from different beam confi gurations and with different CTV to PTV margins. For the patients investigated we were able to identify beam orientations that were robust to dose deterioration in the target and ORs.

Automatic selection of lung cancer patients for adaptive radiotherapy using cone-beam CT imaging

“…Only the beam angles that are used for treatment can then be analyzed for differences. Another application of using the WEPL estimations was presented for robust optimization of proton therapy planning [29,30]. For proton therapy the water equivalent path length is of utmost importance to determine the range of the proton beams making such a WEPL based decision criteria an attractive method for patient treatment verification.…”

On-line dose-guidance to account for inter-fractional motion during proton therapy

“…configuration was sensitive towards inter-fractional motion, causing target underdosage [24]. However in another study of pelvic LN irradiation with protons, lateral oblique fields were found robust towards density changes [14].…”

“…Two IMPT plans using multi-field optimisation were created (Eclipse treatment planning system (TPS), Varian Medical Systems, Palo Alto, CA, USA) on the pCT, with one using two lateral opposed beams (gantry angles 90°/270°) and the other using two lateral oblique beams (gantry angles 35°/325°) identified in a previous inter-fractional robustness study [14] (Suppl. Fig.…”

“…The dose distribution may therefore deteriorate, with inferior target coverage and increased doses to the normal tissues [9]. It has been shown that selecting beam angles from variations inside the beam path in tissue density or water equivalent path length may reduce the impact of motion for IMPT [10][11][12][13][14].…”

Prospective MRI-based imaging study to assess feasibility of proton therapy for post-prostatectomy radiation

“…Several prior studies have examined the robustness of proton therapy plans for patients receiving radiotherapy to the prostate, seminal vesicles, and/or the pelvic lymph nodes [16][17][18][19][20]. The results of these studies have identified strategies to optimize proton therapy delivery through selection of beam angles less sensitive to patient setup error, modification of planning optimization techniques, or patient alignment strategies that minimize displacement of internal pelvic anatomy (e.g.…”