Pencil Beam Scanning Proton Plans Research Articles

Pencil Beam Scanning (PBS) Proton Therapy for Children Requiring Radiotherapy with Anesthesia (GA) for Pelvic Rhabdomyosarcoma (RMS)

<h3>Purpose/Objective(s)</h3> Radiotherapy (RT) for pediatric pelvic RMS presents unique challenges, particularly for young children who require GA during RT, for whom bladder/rectal filling can be particularly variable. PBS may reduce dose to OARs and risk of second malignancy vs. double scatter (DS) protons; however, the precision of PBS requires highly reproducible daily setup. This study was undertaken to describe use of PBS in patients requiring RT for pelvic RMS with daily GA. <h3>Materials/Methods</h3> Pts with RMS of pelvic soft tissues treated with PBS from 1/2013-6/2020 with GA were included in this IRB-approved retrospective analysis. PBS proton plans were developed for all. DS plans were generated for all, and photon (VMAT) backup plans for 9/11 pts. Statistical analyses used 2-sided Student's t-tests. <h3>Results</h3> 11 total pts, median age 2.4 y (R 9 m – 5.9 y), 6 male patients, all had fusion-negative RMS (10 group 3 disease; 1 recurrent bladder/vaginal wall). Primary site included bladder (1), prostate (6), uterus (2), and vagina (1). During simulation/daily treatments under GA, Foley catheter with bladder filling to 75% expected capacity was used for 10. Rectal balloon was used for 2; aggressive management of constipation to promote empty rectum was used for all. All pts had IGRT (daily kV +/- CT [6]). RT dose was 41.4 Gy after delayed primary excision (DPE) (3) and 50.4 Gy without (8). Of 10 pts with FU data, all were without evidence of disease at median FU 1.3 y (range 0.3 – 5.6 y). Acute G3 toxicities included anorexia (3), diarrhea (1), and dermatitis (1), with no G4+ toxicities. One pt developed late RT cystitis. Among pts treated to 50.4 Gy, mean bladder dose (p<0.05) and body V5 (p<0.001) were significantly lower for PBS vs. photons. Max bladder, rectum, and body doses were significantly lower for PBS vs. DS (p<0.05 for each). <h3>Conclusion</h3> PBS proton therapy for very young children with pelvic RMS demonstrates acceptable rates of local control and toxicity when delivered under GA with deliberate management of bladder/rectal filling. Integral dose is reduced with PBS vs. VMAT, and OAR doses are reduced with PBS vs. DS. PBS should be considered in future trials as a standard RT approach for pediatric pelvic RMS.

Measurement-based range evaluation for quality assurance of CBCT-based dose calculations in adaptive proton therapy.

The implementation of volumetric in-room imaging for online adaptive radiotherapy makes extensive testing of this image data for treatment planning necessary. Especially for proton beams the higher sensitivity to stopping power properties of the tissue results in more stringent requirements. Current approaches mainly focus on recalculation of the plans on the new image data, lacking experimental verification, and ignoring the impact on the plan re-optimization process. The aim of this study was to use gel and film dosimetry coupled with a three-dimensional (3D) printed head phantom (based on the planning CT of the patient) for 3D range verification of intensity-corrected cone beam computed tomography (CBCT) image data for adaptive proton therapy. Single field uniform dose pencil beam scanning proton plans were optimized for three different patients on the patients' planning CT (planCT) and the patients' intensity-corrected CBCT (scCBCT) for the same target volume using the same optimization constraints. The CBCTs were corrected on projection level using the planCT as a prior. The dose optimized on planCT and recalculated on scCBCT was compared in terms of proton range differences (80% distal fall-off, recalculation). Moreover, the dose distribution resulting from recalculation of the scCBCT-optimized plan on the planCT and the original planCT dose distribution were compared (simulation). Finally, the two plans of each patient were irradiated on the corresponding patient-specific 3D printed head phantom using gel dosimetry inserts for one patient and film dosimetry for all three patients. Range differences were extracted from the measured dose distributions. The measured and the simulated range differences were corrected for range differences originating from the initial plans and evaluated. The simulation approach showed high agreement with the standard recalculation approach. The median values of the range differences of these two methods agreed within 0.1mm and the interquartile ranges (IQRs) within 0.3mm for all three patients. The range differences of the film measurement were accurately matching with the simulation approach in the film plane. The median values of these range differences deviated less than 0.1mm and the IQRs less than 0.4mm. For the full 3D evaluation of the gel range differences, the median value and IQR matched those of the simulation approach within 0.7 and 0.5mm, respectively. scCBCT- and planCT-based dose distributions were found to have a range agreement better than 3mm (median and IQR) for all considered scenarios (recalculation, simulation, and measurement). The results of this initial study indicate that an online adaptive proton workflow based on scatter-corrected CBCT image data for head irradiations is feasible. The novel presented measurement- and simulation-based method was shown to be equivalent to the standard literature recalculation approach. Additionally, it has the capability to catch effects of image differences on the treatment plan optimization. This makes the measurement-based approach particularly interesting for quality assurance of CBCT-based online adaptive proton therapy. The observed uncertainties could be kept within those of the registration and positioning. The proposed validation could also be applied for other alternative in-room images, e.g. for MR-based pseudoCTs.

A study to investigate the influence of cardiac motion on the robustness of pencil beam scanning proton plans in oesophageal cancer.

While proton therapy offers an excellent dose conformity and sparing of organs at risk, this can be compromised by uncertainties, e.g. organ motion. This study aimed to investigate the influence of cardiac motion on the contoured oesophagus using electrocardiogram-triggered imaging and to assess the impact of this motion on the robustness of proton therapy plans in oesophageal cancer patients. Limited cardiac-induced motion of the oesophagus was observed with a negligible impact on the robustness of proton therapy plans. Therefore, our data suggest that cardiac motion may be safely ignored in the robust optimisation strategy for proton planning in oesophageal cancer.

Evaluation of proton and photon dose distributions recalculated on 2D and 3D Unet-generated pseudoCTs from T1-weighted MR head scans

Introduction: The recent developments of magnetic resonance (MR) based adaptive strategies for photon and, potentially for proton therapy, require a fast and reliable conversion of MR images to X-ray computed tomography (CT) values. CT values are needed for photon and proton dose calculation. The improvement of conversion results employing a 3D deep learning approach is evaluated.Material and methods: A database of 89 T1-weighted MR head scans with about 100 slices each, including rigidly registered CTs, was created. Twenty-eight validation patients were randomly sampled, and four patients were selected for application. The remaining patients were used to train a 2D and a 3D U-shaped convolutional neural network (Unet). A stack size of 32 slices was used for 3D training. For all application cases, volumetric modulated arc therapy photon and single-field uniform dose pencil-beam scanning proton plans at four different gantry angles were optimized for a generic target on the CT and recalculated on 2D and 3D Unet-based pseudoCTs. Mean (absolute) error (MAE/ME) and a gradient sharpness estimate were used to quantify the image quality. Three-dimensional gamma and dose difference analyses were performed for photon (gamma criteria: 1%, 1 mm) and proton dose distributions (gamma criteria: 2%, 2 mm). Range (80% fall off) differences for beam’s eye view profiles were evaluated for protons.Results: Training 36 h for 1000 epochs in 3D (6 h for 200 epochs in 2D) yielded a maximum MAE of 147 HU (135 HU) for the application patients. Except for one patient gamma pass rates for photon and proton dose distributions were above 96% for both Unets. Slice discontinuities were reduced for 3D training at the cost of sharpness.Conclusions: Image analysis revealed a slight advantage of 2D Unets compared to 3D Unets. Similar dose calculation performance was reached for the 2D and 3D network.

PO-0942 Optimization of adaptive aperture margins in robustly optimized pencil beam scanning proton plans

PO-0942 Optimization of adaptive aperture margins in robustly optimized pencil beam scanning proton plans

Effects of anatomical changes on pencil beam scanning proton plans in locally advanced NSCLC patients

Daily anatomical variations can cause considerable differences between delivered and planned dose. This study simulates and evaluates these effects in spot-scanning proton therapy for lung cancer patients. Robust intensity modulated treatment plans were designed on the mid-position CT scan for sixteen locally advanced lung cancer patients. To estimate dosimetric uncertainty, deformable registration was performed on their daily CBCTs to generate 4DCT equivalent scans for each fraction and to map recomputed dose to a common frame. Without adaptive planning, eight patients had an undercoverage of the targets of more than 2GyE (maximum of 14.1GyE) on the recalculated treatment dose from the daily anatomy variations including respiration. In organs at risk, a maximum increase of 4.7GyE in the D1 was found in the mediastinal structures. The effect of respiratory motion alone is smaller: 1.4GyE undercoverage for targets and less than 1GyE for organs at risk. Daily anatomical variations over the course of treatment can cause considerable dose differences in the robust planned dose distribution. An advanced planning strategy including knowledge of anatomical uncertainties would be recommended to improve plan robustness against interfractional variations. For large anatomical changes, adaptive therapy is mandatory.

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

Purpose/Objectives: To optimize delivery of post-prostatectomy radiation (PPRT) with protons by examining dosimetric effects of variations in physician contouring, organ motion, and patient alignment during a course of PPRT. Material and methods: We enrolled 10 patients receiving PPRT in a prospective imaging study. All patients underwent combined computed tomography (CT)/magnetic resonance imaging (MRI) simulation with endorectal balloon (ERB) and received intensity modulated radiation therapy (IMRT) per institutional standards. Study patients underwent weekly MRI verification scans in the treatment position. Three radiation oncologists contoured clinical target volumes (CTV) on initial and verification scans using two consensus guidelines (RTOG and EORTC). We generated IMRT, double scattering (DS), and pencil beam scanning (PBS) proton plans and examined the dosimetric impact of contour variations, inter-fraction motion, and patient alignment techniques. Results: Inter-observer variations in contouring reduced median CTV coverage (D100) by 0.9% for IMRT plans, 2.8% for DS proton plans, 3.4–4.9% for PBS Proton Plans. Inter-fraction changes in target volumes due to internal organ motion resulted in a median loss of target dose coverage (D98) of 0% with IMRT, 3.5% with DS, and 8.1–8.3% with PBS. Median bladder V65Gy increased during the treatment course with all techniques (6.0–7.5%). Changes in the median rectal V60Gy remained small regardless of the treatment technique (0.5–3.1% increase). Alignment to the ERB after cranio-caudal bony alignment reduced CTV displacement compared to bony alignment alone, and as a result CTV coverage (D98) changed <2% with IMRT, DS, and PBS. Conclusion: Proton-based treatments are more sensitive to changes in inter-fraction organ motion during PPRT compared to IMRT, and therefore motion management and patient alignment methods are critical. Patient alignment using bony anatomy as well as the ERB minimizes displacement of the CTV, and reduces variation in target dose coverage particularly for PBS proton therapy.

Integrated peripheral boost in preoperative radiotherapy for the locally most advanced non-resectable rectal cancer patients

Background and Purpose. Few studies have explored the potential clinical advantages of dose escalation and integrated boosts for patients with non-resectable locally advanced rectal cancer. The possibility of escalating dose to non-resectable regions in these patients was the aim of this study. Patients and methods. Seven patients with locally very advanced rectal tumours (sacrum overgrowth or growth into pelvic side walls) were evaluated. Intensity modulated photon and pencil beam scanning proton plans with simultaneously integrated boosts (45 Gy to elective lymph nodes, 50 Gy to tumour and 62.5 Gy to boost area in 25 fractions) were compared. Results. Target coverage was achieved with both photon and proton plans. Estimated risks of acute side effects put the two patients with the largest tumours at unacceptable risk for intestinal toxicity, regardless of modality. The remaining five patients had beneficial sparing of dose to the small intestine with protons. Conclusions. Adding boost to areas where rectal tumours infiltrate adjacent non-resectable organs is an attractive option which appears possible using both photon and proton irradiation. Proton plans reduced dose to organs at risk. Integrated peripheral boosts should be considered more frequently in these very advanced tumours.