RGPT successfully tracked fiducial marker motion for DIBH SBRT liver treatment. Despite target drift during DIBH, the uncertainty of our DIBH SBRT procedure was sufficient to cover target motion throughout treatment. Based on the target drift value, a maximum of 25 seconds for breath hold time should be employed. Utilizing a tighter gating tolerance of 3 mm SI and 2 mm radially has the potential to maintain target coverage while significantly reducing OAR dose. Aggregated RGPT-derived data may provide optimal treatment planning parameters such as variable uncertainty based on target location.

We report the first dosimetric analysis of proton SBRT for spine metastasis using variable RBE dose models. IMPT may provide improved target coverage and better sparing of adjacent OARs compared to CK though fixed RBE computation may underestimate maximum dose to adjacent OARs.

Purpose/Objective(s) With the results of the randomized STRASS trial, the benefit of preoperative radiation has been called into question in the treatment of retroperitoneal sarcoma. Given the large amount of normal tissue which often resides in close proximity to retroperitoneal sarcomas, the risk of radiation-associated toxicity is substantial, with 77% of irradiated patients experiencing grade ≥3 lymphopenia in STRASS. The exit dose and integral exposure associated with photon irradiation limits its therapeutic ratio. Our purpose was to evaluate a dosimetric comparison between ultra-hypofractionated modern scanning beam intensity-modulated proton therapy (IMPT) and intensity-modulated photon radiotherapy (IMRT) for the preoperative treatment of retroperitoneal sarcoma. We hypothesized that IMPT would significantly reduce dose to the to the liver, bowel and kidneys without sacrificing clinical target volume (CTV) coverage. Materials/Methods Ultra-hypofractionated IMRT and IMPT plans were generated using TPS on 10 patients previously treated with conventionally fractionated preoperative photon irradiation for retroperitoneal sarcoma. The prescription was 25 Gy radiobiological equivalent (GyE) to the entire CTV and 30 GyE to the margin-at-risk (CTV_Boost), all in five fractions. IMPT doses are reported in GyE = 1.1 Gy. Target coverage goals, OAR constraints, and integral dose were compared by student T-test with p < .050 considered significant. Results Target coverage goals, OAR constraints, and integral dose are reported in Table 1. Mean dose to liver, bowel, and ipsilateral and contralateral kidneys were all significantly lower with IMPT than IMRT, as was integral dose. Conclusion IMPT maintained target coverage while significantly reducing integral dose and mean dose to nearby OARs compared to IMRT. Further investigation is warranted to validate these dosimetric findings and potential clinical benefit in the management of retroperitoneal sarcoma.

Purpose/Objective(s)RGPT has the potential to provide instantaneous feedback for intrafraction target motion to maximize patient safety and inform optimal treatment planning. Our purpose was to report our early experience with RGPT and to summarize intrafraction target motion and implications on treatment delivery, including added time, for a pilot population of prostate cancer patients before expansion to other sites with greater motion.Materials/MethodsRGPT with a proton beam therapy system was commissioned and implemented at our proton center, representing the first experience in the United States. Patients being treated definitively with proton therapy to the prostate ± seminal vesicles-only and who had successful placement of platinum fiducial markers and hydrogel spacer were assessed on a prospective IRB-approved registry. Orthogonal fluoroscopic imaging was taken at a pulse rate of one image per second during fractionated proton therapy to assess positioning of a pre-selected fiducial marker. Thresholds to guide beam-on and beam-off gating were determined for a matching score component (based on quality of the image tracking and the guiding template) and for a shift tolerance component (based on our typical set-up uncertainty robustness of 3mm). The shift tolerances were defined as 2mm for left-right and anterior-posterior, and 3mm for superior-inferior. Each patient was treated with single-field optimization planning and a two lateral beam setup.ResultsTo date, 8 prostate cancer patients have been treated with a total of 152 RGPT-guided fields. The table details the mean, maximum, standard deviation, and estimated upper 90th percentile of directional shifts for the entire dataset for 1) the total RGPT-tracked treatment time and 2) the beam-on time as gated by RGPT tracking. On average, the initial imaging to set up the RGPT system required 2.5 seconds per field. The average total treatment time during RGPT guidance, including periods of both "on" and "off" gating, was 41.3 seconds per field. RGPT tracking triggered "beam-off" for an average of 7.8 seconds per field, increasing the total treatment delivery time by 20.2% across the 152 fields.ConclusionRGPT successfully tracked intrafraction fiducial marker motion during proton therapy for prostate cancer, while having minimal impact on total treatment time. Further aggregated RGPT-derived data may help to inform optimal treatment planning parameters such as robust optimization.

The 4-field IMPT beam arrangement showed the greatest reductions in dose to the bowel cavity and rectum compared to VMAT and the 2- and 3-field IMPT arrangements. These data can inform the future clinical management and delivery of proton therapy for prostate cancer in the post-prostatectomy setting.

To estimate for proton stereotactic body radiotherapy (SBRT) plan delivery uncertainty with daily cone-beam CT (CBCT) imaging for liver patients under breath-hold with active breath coordinator (ABC). Images from 11 liver patients previously treated with photon SBRT with ABC at our institution are used for this study. The prescribed dose for the proton SBRT plan is 75 Gy (RBE) in 5 fractions. There is a total of 11 patients with 113 setup CBCT images collected during the patient's previous photon SBRT treatment. The planning CT and all CBCT images are imported into Raystation 9A. A CT relative linear stopping power (RLSP) curve is calibrated for CBCT and used for proton dose calculation. A four beams proton SBRT plan is optimized with the single field optimization (SFO) technique. All 113 CBCT images are registered to planning CT with bony anatomy (rib cage and vertebrae) or liver to evaluate the delivery uncertainty. The delivery dose is estimated from the proton dose calculation on each CBCT image with the same proton plan. The dose coverages for targets are evaluated for: gross tumor volume (GTV), gtv_multABC_2mmRad_5mmSI, and ptv_5mmRad_7mmSI. Liver positions under DIBH with ABC are not consistent under bony alignment as measured. The inter-fractional and intra-fractional variation of DIBH for liver are analyzed. Proton plan delivery uncertainty can dramatically reduce under well liver alignment (up to 5% prescription dose). Daily CBCT images could be potentially used to estimate proton plan delivery uncertainty for liver patients.

Wound healing complications remains a common morbidity with pre-operative radiation for soft tissue sarcoma. Proton therapy can spare excess dose to normal structures and may have the potential to reduce this morbidity. Our aim was to evaluate dosimetric differences in modern scanning beam proton therapy (PT) versus conventional photon 3D conformal (3DCRT) and intensity modulated radiation therapy (IMRT) for the pre-operative treatment of soft tissue sarcoma (STS). The hypothesis is that proton therapy will spare dose to normal tissues when compared to photon therapy (RT). The existing 3DCRT (N = 7) and IMRT (N = 12) data of 19 adult STS patients treated pre-operatively at our institution were used to create proton plans using a treatment planning system. Volumes were delineated and doses reported consistent with ICRU reports 50, 62, and 78. Target volumes were robustly optimized for 100% CTV coverage with 3.5% range uncertainty and 3mm setup uncertainty. The prescribed dose was 50.4 Gy for 3DCRT, IMRT, and PT delivered in 28 fractions. For PT doses are reported in Gy (RBE) = 1.1 Gy. Constraints for organs at risk (OARs) were joint and bone V50<50%. Mean dose to each OAR and integral dose were compared by student T-test with P<0.05 significance. Median patient age at start of RT was 58 (range 27-90). Of the 19 patients, 13 patients had primary tumors of the lower extremity and 6 patients had upper extremity tumors. A minimum 99% CTV coverage and OAR constraints were achieved for all proton plans. PT reduced the integral dose to the body by an average of 136.47% (5.56-323.65%, p<0.01) (Table 1). The average dose to Body-CTV was 345±320 Gy (RBE) for PT and 656±349 cGy (RBE) for RT (p<0.00001). The average dose to Body-PTV was 258±278 Gy (RBE) for PT and 562±300 cGy (RBE) for RT (p<0.00001). For the proximal joint, mean dose was 1024± 1190 cGy (RBE) for PT and 1973±1402 cGy (RBE) for RT (n = 10, p<0.05). For the proximal bone, mean dose was 1715±1177 cGy (RBE) for PT and 2748±1104 cGy (RBE) for RT (n = 19, p<0.00001). Proton therapy maintained target coverage while significantly reducing the dose to the proximal organs at risk (bone and joint), normal tissues, and the integral dose compared to photon therapy. Further investigation is warranted to validate these dosimetric findings and potential clinical benefit in the management of adult soft-tissue sarcomas. Table 1: Comparative integral dose case series for photon versus scanning beam proton therapy.Abstract 2025; TableIntegral Dose to Normal Tissue (Body – CTV) (Gy*cc) x 105Patient IndexProtonPhoton10.371.2120.431.2730.220.3640.310.7350.972.8960.250.3970.781.8680.731.9890.180.26100.100.42110.150.20120.641.23131.251.67140.080.18150.291.19161.496.30170.751.35180.180.19191.051.67 Open table in a new tab

Deep inspiration breath hold (DIBH) is often used to immobilize pancreatic tumor for SBRT. Geometric reproducibility of tumor positioning between breath hold have been reported, but the corresponding dosimetric impact is not well understood. We evaluated dosimetric reproducibility of tumor coverage and OAR doses for inter-breath hold variation at treatment. Ten pancreatic cancer patients who underwent DIBH SBRT with 660cGy x 5 fractions (Rx) were selected for the study. The implanted fiducial markers were used as tumor motion surrogate at treatment. At CT sim, the contrast planning CT was acquired with 2-mm slice thickness, immediately followed by three additional CT sets. The patient specific breath-hold variation measured from the four sim CT sets was added as a margin to GTV in addition to a 2-mm set-up margin. At treatment, 4.6 ± 1.5 breath-hold CBCTs were acquired for each patient and each fraction. The first CBCT of each fraction was used as the reference to quantify the geometric and dosimetric inter-breath hold variations on the subsequent CBCT sets in the treatment planning system. The geometric variation of the GTV was quantified as the variation of fiducial centroids. The metrics used for the dosimetric variation were GTV D95 ≥ Rx, PTV D95 ≥ Rx, PTV Dmax < 4290 cGy (130% Rx), Paddick conformity index (pCI), and the ratio of the volume of the 50% isodose volume and the PTV volume (R50%) for tumor, V20Gy < 20% for duodenum, stomach and bowel, V12% < 50% for liver, and V12% < 25% for combined kidneys, in addition to Dmax for each OAR. The tumor position showed an average variation of 0.0 ± 1.6 mm, -0.3 ± 1.5 mm, and -0.4 ± 2.6 mm in the LR, AP, and SI directions, respectively. The GTV D95, PTV D95, and PTV Dmax were changed by 0.8 ± 5.3%, -3.1 ± 14.0%, and -0.1 ± 3.5%. The pCI and R50% varied by 1.7 ± 6.5% and 1.3 ± 10.8%. The V20Gy of duodenum, stomach, and bowel varied by -0.2 ± 3.1%, -0.3 ± 2.5%, and -0.2 ± 4.8%. There was no sizable difference with V12% for liver and combined kidneys. The Dmax for duodenum, stomach, bowel, liver, and combined kidneys were changed by -0.7 ± 4.8%, -0.4 ± 8.2%, 0.8 ± 5.6%, 0.4 ± 4.3%, and -0.6 ± 3.5%, respectively. Inter-breath hold variation in pancreatic tumor position affects tumor coverage, dose conformity, dose fall-off and OAR doses, more significantly for Dmax of duodenum, stomach and bowel which are closely located to the tumor. Future work is to evaluate whether a robust optimization can account for the inter-breath hold variation to minimize the corresponding dosimetric deviations.