Comparing biological effectiveness guided plan optimization strategies for cranial proton therapy: potential and challenges

Background: To introduce and compare multiple biological effectiveness guided (BG) proton plan optimization strategies minimizing variable relative biological effectiveness (RBE) induced dose burden in organs at risk (OAR) while maintaining plan quality with a constant RBE. Methods: Dose-optimized (DOSEopt) proton pencil beam scanning reference treatment plans were generated for ten cranial patients with prescription doses ≥ 54 Gy(RBE) and ≥ 1 OAR close to the clinical target volume (CTV). For each patient, four additional BG plans were created. BG objectives minimized either proton track-ends, dose-averaged linear energy transfer (LETd), energy depositions from high-LET protons or variable RBE-weighted dose (DRBE) in adjacent serially structured OARs. Plan quality (RBE = 1.1) was assessed by CTV dose coverage and robustness (2 mm setup, 3.5% density), dose homogeneity and conformity in the planning target volumes and adherence to OAR tolerance doses. LETd, DRBE (Wedenberg model, α/βCTV = 10 Gy, α/βOAR = 2 Gy) and resulting normal tissue complication probabilities (NTCPs) for blindness and brainstem necrosis were derived. Differences between DOSEopt and BG optimized plans were assessed and statistically tested (Wilcoxon signed rank, α = 0.05). Results: All plans were clinically acceptable. DOSEopt and BG optimized plans were comparable in target volume coverage, homogeneity and conformity. For recalculated DRBE in all patients, all BG plans significantly reduced near-maximum DRBE to critical OARs with differences up to 8.2 Gy(RBE) (p < 0.05). Direct DRBE optimization primarily reduced absorbed dose in OARs (average ΔDmean = 2.0 Gy; average ΔLETd,mean = 0.1 keV/µm), while the other strategies reduced LETd (average ΔDmean < 0.3 Gy; average ΔLETd,mean = 0.5 keV/µm). LET-optimizing strategies were more robust against range and setup uncertaintes for high-dose CTVs than DRBE optimization. All BG strategies reduced NTCP for brainstem necrosis and blindness on average by 47% with average and maximum reductions of 5.4 and 18.4 percentage points, respectively. Conclusions: All BG strategies reduced variable RBE-induced NTCPs to OARs. Reducing LETd in high-dose voxels may be favourable due to its adherence to current dose reporting and maintenance of clinical plan quality and the availability of reported LETd and dose levels from clinical toxicity reports after cranial proton therapy. These optimization strategies beyond dose may be a first step towards safely translating variable RBE optimization in the clinics

Approach the National Quality Audit System for Radiotherapy in Latvia

It is very important to make National Quality Audit to ensure accurate conformal RT delivery. It is necessary to develop an Audit system to inspect all Conformal RT and IMRT delivery chain including the Quality checks of linear accelerator, Multileaf Collimator (MLC), Computer Tomography (CT) scanner or simulator, target and tissue delineation, plan evaluation, and delivery

Dosimetric Properties of Detectors for Quality Control of Intensity Modulated Radiotherapy

Intensity Modulated Radiotherapy (IMRT) is extensively used for cancer patients treatment in modern Radiotherapy Clinics. Patient-related Quality Control (QC) is essential part of IMRT clinical implementation, as fluencies of Intensity Modulated (IM) fields has to be independently verified prior to delivery of the IMRT plan to the patient. In most cases, combination or ionometric and film dosimetry is in use for patient-related QC. The aim of this study was to evaluate volume-averaging effect of different ionization chambers used for ionometric dosimetry of the IMRT fields. Examination of the response of three ionization chambers of different sensitive volumes was performed, using set of static and IM fields. Chambers were cross-calibrated in static field of size, sufficient to cover with homogeneous dose chamber with the largest sensitive volume. Dynamic IM fields with different dose gradients through chamber sensitive volume were designed. Fluencies of all IM test fields were verified by means of the film dosimetry. Measurements in the penumbra region of the static field were used to separate effects of dynamic dose delivery from the chamber averaging effect. Penumbra profile of the static field was acquired using all detectors under investigation. Differences in the chambers response larger than 8% were found for measurements in high dose gradients both in penumbra of static field, and in gradient region of the dynamic IM field. Chamber of the largest volume had worst results in penumbra region of the static field and largest deviation from analytically reconstructed penumbra of IM fields

Dosimetric Properties of Detectors for Quality Control of Intensity Modulated Radiotherapy

Intensity Modulated Radiotherapy (IMRT) is extensively used for cancer patients treatment in modern Radiotherapy Clinics. Patient-related Quality Control (QC) is essential part of IMRT clinical implementation, as fluencies of Intensity Modulated (IM) fields has to be independently verified prior to delivery of the IMRT plan to the patient. In most cases, combination or ionometric and film dosimetry is in use for patient-related QC. The aim of this study was to evaluate volume-averaging effect of different ionization chambers used for ionometric dosimetry of the IMRT fields

The Impact of the Detector Size Effects in Region of High Dose Gradients on Point Dose Measurement

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