Robustness and dosimetric verification of hippocampal-sparing craniospinal pencil beam scanning proton plans for pediatric medulloblastoma.

Background and Purpose: Hippocampal-sparing (HS) is a method that can potentially reduce late cognitive complications for pediatric medulloblastoma (MB) patients treated with craniospinal proton therapy (PT). The aim of this study was to investigate robustness and dosimetric plan verification of pencil beam scanning HS PT. Materials and Methods: HS and non-HS PT plans for the whole brain part of craniospinal treatment were created for 15 pediatric MB patients. A robust evaluation of the plans was performed. Plans were recalculated in a water phantom and measured field-by-field using an ion chamber detector at depths corresponding to the central part of hippocampi. All HS and non-HS fields were measured with the standard resolution of the detector and in addition 16 HS fields were measured with high resolution. Measured and planned dose distributions were compared using gamma evaluation. Results: The median mean hippocampus dose was reduced from 22.9 Gy (RBE) to 8.9 Gy (RBE), while keeping CTV V95% above 95 % for all nominal HS plans. HS plans were relatively robust regarding hippocampus mean dose, however, less robust regarding target coverage and maximum dose compared to non-HS plans. For standard resolution measurements, median pass rates were 99.7 % for HS and 99.5 % for non-HS plans (p < 0.001). For high-resolution measurements, median pass rates were 100 % in the hippocampus region and 98.2 % in the surrounding region. Conclusions: A substantial reduction of dose in the hippocampus region appeared feasible. Dosimetric accuracy of HS plans was comparable to non-HS plans and agreed well with planned dose distribution in the hippocampus region.

Daily Adaptive Proton Therapy: Is it Appropriate to Use Analytical Dose Calculations for Plan Adaption?

PURPOSE: The accuracy of analytical dose calculations (ADC) and dose uncertainties resulting from anatomical changes are both limiting factors in proton therapy. For the latter, rapid plan adaption is necessary; for the former, Monte Carlo (MC) approaches are increasingly recommended. These, however, are inherently slower than analytical approaches, potentially limiting the ability to rapidly adapt plans. Here, we compare the clinical relevance of uncertainties resulting from both. METHODS AND MATERIALS: Five patients with non-small cell lung cancer with up to 9 computed tomography (CT) scans acquired during treatment and five paranasal (head and neck) patients with 10 simulated anatomical changes (sinus filling) were analyzed. On the initial planning CT scans, treatment plans were optimized and calculated using an ADC and then recalculated with MC. Additionally, all plans were recalculated (non-adapted) and reoptimized (adapted) on each repeated CT using the same ADC as for the initial plan, and the resulting dose distributions were compared. RESULTS: When comparing analytical and MC calculations in the initial treatment plan and averaged over all patients, 94.2% (non-small cell lung cancer) and 98.5% (head and neck) of voxels had differences <±5%, and only minor differences in clinical target volume (CTV) V95 (average <2%) were observed. In contrast, when recalculating nominal plans on the repeat (anatomically changed) CT scans, CTV V95 degraded by up to 34%. Plan adaption, however, restored CTV V95 differences between adapted and nominal plans to <0.5%. Adapted organ-at-risk doses remained the same or improved. CONCLUSIONS: Dose degradations caused by anatomic changes are substantially larger than uncertainties introduced by the use of analytical instead of MC dose calculations. Thus, if the use of analytical calculations can enable more rapid and efficient plan adaption than MC approaches, they can and should be used for plan adaption for these patient groups.

The dosimetric effect of residual breath-hold motion in pencil beam scanned proton therapy - An experimental study.

BACKGROUND AND PURPOSE: Motion management in the treatment of lung cancer is necessary to assure highest quality of the delivered radiation therapy. In this study, the breath-hold technique is experimentally investigated for pencil beam scanned (PBS) proton therapy, with respect to the dosimetric effect of residual breath-hold motion. MATERIAL AND METHODS: Three-dimensional (3D)-printed tumours extracted from CT scans of three patients were inserted into a dynamic anthropomorphic breathing phantom. The target was set up to move with the individual patient's tumour motion during breath-hold as previously assessed on fluoroscopy. Target dose was measured with radio-chromic film, and both single field uniform dose (SFUD) and intensity-modulated proton therapy (IMPT) plans were delivered. Experiments were repeated for each patient without any motion, to compute the relative dose deviation between static and breath-hold cases. RESULTS: SFUD plans showed small dose deviations between static and breath-hold cases, as evidenced by the gamma pass rate (3%, 3 mm) of 85% or higher. Dose deviation was more evident for IMPT plans, with gamma pass rate reduced to 50-70%. CONCLUSIONS: The breath-hold technique is robust to residual intra-breath-hold motion for SFUD treatment plans, based on our experimental study. IMPT was less robust with larger detected dose deviations.

Feasibility of Pencil Beam Scanned Intensity Modulated Proton Therapy in Breath-hold for Locally Advanced Non-Small Cell Lung Cancer.

PURPOSE: We evaluated the feasibility of treating patients with locally advanced non-small cell lung cancer (NSCLC) with pencil beam scanned intensity modulated proton therapy (IMPT) in breath-hold. METHODS AND MATERIALS: Fifteen NSCLC patients who had previously received 66 Gy in 33 fractions with image guided photon radiation therapy were included in the present simulation study. In addition to a planning breath-hold computed tomography (CT) scan before the treatment start, a median of 6 (range 3-9) breath-hold CT scans per patient were acquired prospectively throughout the radiation therapy course. Three-field IMPT plans were constructed using the planning breath-hold CT scan, and the four-dimensional dose distributions were simulated, with consideration of both patient intra- and interfraction motion, in addition to dynamic treatment delivery. RESULTS: The median clinical target volume receiving 95% of the prescribed dose was 99.8% and 99.7% for the planned and simulated dose distributions, respectively. For 3 patients (20%), the dose degradation was >5%, and plan adjustment was needed. Dose degradation correlated significantly with the change in water-equivalent path lengths (P<.01) in terms of the percentage of voxels with 3-mm or more undershoot on repeat CT scans. The dose to the organs at risk was similar for the planned and simulated dose distributions. Three or fewer breath-holds per field would be required for 12 of the 15 patients, which was clinically feasible. CONCLUSIONS: For 9 of 15 NSCLC patients, IMPT in breath-hold was both dosimetrically robust and feasible to deliver regarding the treatment time. Three patients would have required plan adaption to meet the dosimetric criteria. The change in water-equivalent path length is an indicator of plan robustness and should be considered for the selection of patients for whom the plan would require adaptation.

Impact of beam angle choice on pencil beam scanning breath-hold proton therapy for lung lesions.

INTRODUCTION: The breath-hold technique inter alia has been suggested to mitigate the detrimental effect of motion on pencil beam scanned (PBS) proton therapy dose distributions. The aim of this study was to evaluate the robustness of incident proton beam angles to day-to-day anatomical variations in breath-hold. MATERIALS AND METHODS: Single field PBS plans at five degrees increments in the transversal plane were made and water-equivalent path lengths (WEPLs) were derived on the planning breath-hold CT (BHCT) for 30 patients diagnosed with locally-advanced non-small cell lung cancer (NSCLC), early stage NSCLC or lung metastasis. Our treatment planning system was subsequently used to recalculate the plans and derive WEPL on a BHCT scan acquired at the end of the treatment. Changes to the V95%, D95 and mean target dose were evaluated. RESULTS: The difference in WEPL as a function of the beam angle was highly patient specific, with a median of 3.3 mm (range: 0.0-41.1 mm). Slightly larger WEPL differences were located around the lateral or lateral anterior/posterior beam angles. Linear models revealed that changes in dose were associated to the changes in WEPL and the tumor baseline shift (p < 0.05). CONCLUSIONS: WEPL changes and tumor baseline shift can serve as reasonable surrogates for dosimetric uncertainty of the target coverage and are well-suited for routine evaluation of plan robustness. The two lateral beam angles are not recommended to use for PBS proton therapy of lung cancer patients treated in breath-hold, due to the poor robustness for several of the patients evaluated.