Interfraction Motion Research Articles

Dealing with rectum motion during radiotherapy: How can we anticipate it?

IntroductionIntra- and inter-fraction rectum motion is important for pelvic radiotherapy (RT). This study assesses how RT session duration, the presence or the absence of an intra-rectal tumour, and the distance from the anorectal junction (ARJd) impact rectal motion. Materials and methodsAnalyses used cone-beam computed tomographies (CBCTs) from RT patients treated for rectal and prostate cancer. Three structures were evaluated: (1) the entire rectum in patients without a rectal tumour (RectumProstate); (2) the non-invaded portion (RectumRectum) and (3) the tumour-invaded portion (RectumTumour) in rectal cancer patients.Intrafraction motion was assessed using the Hausdorff distance 95% and the Mean distance-to-agreement between structures delineated on the first CBCT and the 2 subsequent CBCTs within a same RT session. Interfraction motion was quantified by comparing structures delineated on the planning-CT and the first CBCT of each session.Linear mixed model evaluated rectum motion in relation to time, tumour presence, and ARJd, respectively. ResultsWe included 10 patients with and 10 without rectal cancer, collecting 385 CBCTs. A significant correlation (p < 0.05) between rectum motion and RT session duration was found. Intrafraction motion was significantly higher in prostate cancer patients (RectumProstate motion > RectumRectum and RectumTumour, p < 0.01). For interfraction motion, only the mean distance to agreement was significantly higher for RectumProstate (p < 0.05). Motion increased significantly with ARJd for all three structures (p < 0.001). ConclusionsSession duration, absence of a tumour, and ARJd are associated with larger intra- and interfraction rectal motion. This highlights the need for tailored RT treatment, including online-adaptive RT, to manage intra- and interfraction variations. Rectal motion should be handled differently for patients with prostate cancer and those with rectal cancer.

2762: Is there an association between inter-fraction motion during prostate SABR and toxicity?

2762: Is there an association between inter-fraction motion during prostate SABR and toxicity?

CT-guided online adaptive stereotactic body radiotherapy for pancreas ductal adenocarcinoma: Dosimetric and initial clinical experience

Purpose/ObjectivesRetrospective analysis suggests that dose escalation to a biologically effective dose of more than 70 Gy may improve overall survival in patients with pancreatic ductal adenocarcinoma (PDAC), but such treatments in practice are limited by proximity of organs at risk (OARs). We hypothesized that CT-guided online adaptive radiotherapy (OART) can account for interfraction movement of OARs and allow for safe delivery of ablative doses. Materials/MethodsThis is a single institution retrospective analysis of patients with PDAC treated with OART on the Ethos platform (Varian Medical Systems, a Siemens Healthineers Company, Palo Alto). All patients were treated to 40 Gy in 5 fractions. PTV overlapping with a 5 mm planning risk volume expansion on the stomach, duodenum and bowel received 25 Gy. Initial treatment plans were created conventionally. For each fraction, PTV and OAR volumes were recontoured with AI assistance after initial cone beam CT (CBCT). The adapted plan was calculated, underwent QA, and then compared to the scheduled plan. A second CBCT was obtained prior to delivery of the selected plan. Total treatment time (first CBCT to end of radiation delivery) and active physician time (first to second CBCT) were recorded. PTV_4000 V95 %, PTV_2500 V9 5%, and D0.03 cc to stomach, duodenum and bowel were reported for scheduled (S) and adapted (A) plans. CTCAEv5.0 toxicities were recorded. Statistical analysis was performed using a two-sided T test and α of 0.05. Results21 patients with unresectable or locally-recurrent PDAC were analyzed, with a total of 105 fractions. Average total time was 29 min and 16 s (16:36–49:40) and average active physician time was 19:41 min (9:25–39:34). All fractions were treated with adapted plans. 97 % of adapted plans met PTV_4000 V95.0 % >95.0 % coverage goal and 100 % of adapted plans met OAR dose constraints. Median follow up was 6.6 months. Only 1 patient experienced acute grade 3+ toxicity directly attributable to radiation. Only 1 patient experienced late grade 3+ toxicity directly attributable to radiation. ConclusionsDaily CT-based OART was associated with significantly reduced dose OARs while achieving superior PTV coverage. Given the relatively quick total treatment time, radiation delivery was generally well tolerated and easily incorporated into the clinic workflow. Our initial clinical experience demonstrates OART allows for safe dose escalation in the treatment of PDAC.

Inter-fraction motion robustness in a prospective phase II trial on dose-escalated proton reirradiation for locally recurrent rectal cancer

Background and purposeIntensity modulated proton therapy (IMPT) enables generation of conformal dose plans with organ at risk (OAR) sparing potential. However, pelvic IMPT robustness is challenged by inter-fraction motion caused by constant anatomical variations. In this study, the dosimetric impact of inter-fraction motion on target coverage and dose to OAR was quantified in the prospective phase II study ReRad-II on dose-escalated proton reirradiation for locally recurrent rectal cancer (LRRC). Materials and methodsThe inter-fraction motion robustness was assessed for the initial twelve patients enrolled in the ReRad-II study. Patients with resectable LRRC were assessed for neoadjuvant IMPT (55 Gy(RBE)/44Fx) and unresectable recurrences for definitive IMPT (57.5–65 Gy(RBE)/ 46-52Fx). Target coverage and dose to OAR were assessed for robustly optimised three-field IMPT, on 12 plan computerized tomography (CT) scans (pCT) − and 47 repetitive control CT scans (cCTs) during the treatment. The target coverage and doses to OAR were re-calculated on each cCT and the mean dose ratio (pCT/cCT-ratio) and target coverage (V95%) was evaluated. ResultsThe target coverage was robust with a mean dose pCT/cCT-ratio of 1.00 (+/-1%). The V95% target coverage for every cCT were above the accepted worst-case scenario in the robust evaluation. Considerable variation in bladder-, bowel bag-, and bowel loop volume was observed. The OAR with the largest variation in ratio was the bladder (pCT/cCT-ratio: 1.3 (range: 0.5–4.7). ConclusionsIMPT for dose-escalated reirradiation of LRRC provided anatomically robust target coverage despite OAR changes. Inter-fraction motion resulted in OAR doses varying within clinically acceptable range.

Implementing Plan of the Day for Cervical Cancer: A Comparison of Target Volume Generation Methods

Background and purposeDue to substantial interfraction motion in cervix cancer, Plan-of-the-Day (PotD) adaptive radiotherapy may be of benefit to patients. Implementation is limited by uncertainty over how to generate the planning target volumes (PTVs). We compared published methods on our own patients. Materials and methodsForty patients each had three planning scans with variable bladder filling and daily cone beam CTs (CBCTs) during radiotherapy; 5-11 CBCTs were selected to represent interfraction motion. Clinical target volumes (CTVs) and organs at risk (OARs) were contoured following EMBRACE-II guidelines. A literature search identified 30 adaptive and non-adaptive solutions to PTV generation, which we applied to our patients. PTV sizes and mean coverage of the daily CTV were determined. For 11 patients, the clinically-implemented subjectively-edited plan library was also investigated. ResultsEleven studies assessed 15 PotD strategies against non-adaptive comparators on median 14 patients (range 9-23). Some PotD approaches applied margin recipes to the CTV on each planning scan, some modelled the CTV against bladder volume, and others applied incremental isotropic margins to the CTV with a single planning scan. Generally, coverage improved as PTV size increased. The fixed isotropic margin required to provide 100% coverage of all patients was 44mm, with mean PTV size 3316cc. The PotD strategy with the best coverage was a two-plan library formed by modelling the CTV against bladder volume with extrapolation; it provided 98% mean coverage with 1419cc mean PTV size. A three-plan library consisting of the CTV on each planning scan with 10mm margin provided 96% mean coverage with 1346cc mean PTV size. The clinically-implemented solution which employed subjective extrapolation, had mean 100% coverage and 1282cc PTV size on the 11-patient subset. Coverage provided by the best non-adaptive strategies was not statistically superior to the best PotD strategy (p = 0.13) but PTVs were larger (p = 0.02). ConclusionWe identified a modelled two-plan method, and a simple three-plan method, which both provided excellent coverage with small PTVs compared to non-adaptive strategies.

Evaluation of PTV margin in CBCT-based online adaptive radiation therapy for gastric mucosa-associated lymphoid tissue lymphoma.

The aim of this study was to investigate planning target volume (PTV) margin in online adaptive radiation therapy (oART) for gastric mucosa-associated lymphoid tissue (MALT) lymphomas. Four consecutive patients with gastric MALT lymphoma who received oART (30Gy in 15 fractions) on the oART system were included in this study. One hundred and twenty cone-beam computed tomography (CBCT) scans acquired pre- and post-treatment of 60 fractions for all patients were used to evaluate intra- and interfractional motions. Patients were instructed on breath-holding at exhalation during image acquisition. To assess the intrafraction gastric motion, different PTVs were created by isotropically extending the CTV contoured on a pre-CBCT image (CTVpre) at1 mm intervals. Intrafraction motion was defined as the amount of expansion covering the contoured CTV on post-CBCT images (CTVpost). Interfractional motion was defined as the amount of reference CTV expansion that could cover each CTVpre, as well as the evaluation of the intrafractional motion. PTV margins were estimated from the cumulative proportion of fraction covering the intra- and interfractional motions. The extent of expansion covering the CTVs in 90% of fractions was adopted as the PTV margin. The PTV margin for intrafractional gastric motion using the oART system with breath-holding was 14mm. In contrast, the PTV margin for interfractional gastric organ motion without the oART system was 25mm. These results indicated that the oART system can reduce the PTV margin by >10mm. Our results could be valuable data for oART cases.

Liver respiratory-induced motion estimation using abdominal surface displacement as a surrogate: robotic phantom and clinical validation with varied correspondence models

PurposeThis work presents the implementation of an RGB-D camera as a surrogate signal for liver respiratory-induced motion estimation. This study aims to validate the feasibility of RGB-D cameras as a surrogate in a human subject experiment and to compare the performance of different correspondence models.MethodsThe proposed approach uses an RGB-D camera to compute an abdominal surface reconstruction and estimate the liver respiratory-induced motion. Two sets of validation experiments were conducted, first, using a robotic liver phantom and, secondly, performing a clinical study with human subjects. In the clinical study, three correspondence models were created changing the conditions of the learning-based model.ResultsThe motion model for the robotic liver phantom displayed an error below 3 mm with a coefficient of determination above 90% for the different directions of motion. The clinical study presented errors of 4.5, 2.5, and 2.9 mm for the three different motion models with a coefficient of determination above 80% for all three cases.ConclusionRGB-D cameras are a promising method to accurately estimate the liver respiratory-induced motion. The internal motion can be estimated in a non-contact, noninvasive and flexible approach. Additionally, three training conditions for the correspondence model are studied to potentially mitigate intra- and inter-fraction motion.

Pelvic lymph node motion during cone-beam computed tomography guided stereotactic radiotherapy

Background and purposeStereotactic body radiotherapy (SBRT) is increasingly applied for pelvic lymph node recurrence. Thus far, knowledge on pelvic lymph node motion during CBCT-guided SBRT is lacking and the applied margins vary between institutions. This study evaluated pelvic lymph node motion during CBCT-guided SBRT and assessed the currently applied PTV margins of 3 and 5 mm. Material and methodsIn total, 45 pelvic lymph node metastases were included. One observer delineated 45 GTVs on planning CT, 224 GTVs on pre-fraction and 216 on post-fraction CBCT. The GTV centroid coordinates were derived from all images for inter- and intrafraction motion analysis. Additionally, we assessed the influence of treatment time and lesion location on lesion motion. The expected coverage of a 3-mm and 5-mm PTV margin was assessed using the inclusiveness index for GTVs on pre- and post-fraction CBCT. ResultsLymph node interfraction motion was limited to 5 mm in 96–97 % of fractions for all translational directions and intrafraction lesion motion was limited to 3 mm in 97–100 % of fractions. Para-rectal lesions (11 %) were associated with significantly larger inter- and intrafraction motion compared to other pelvic locations and treatment duration showed no correlation with lesion motion. The mean (sd) lesion inclusiveness index was 99 % (5 %) for the 5-mm PTV margin and 96 % (9 %) for the 3-mm margin. ConclusionPelvic lymph node motion during CBCT-guided stereotactic radiotherapy was within the widely applied PTV margin of 5 mm, providing an opportunity to reduce this margin for pelvic lymph node SBRT.

2830: Is there an association between inter-fraction motion during prostate SABR and quality of life?

2830: Is there an association between inter-fraction motion during prostate SABR and quality of life?

1802: Effect of interfractional volume and motion displacement of trigone and bladder on prostate cancer

1802: Effect of interfractional volume and motion displacement of trigone and bladder on prostate cancer

Pelvic target volume inter-fractional motion during radiotherapy for cervical cancer with daily iterative cone beam computed tomography.

Tumor regression and organ movements indicate that a large margin is used to ensure target volume coverage during radiotherapy. This study aimed to quantify inter-fractional movements of the uterus and cervix in patients with cervical cancer undergoing radiotherapy and to evaluate the clinical target volume (CTV) coverage. This study analyzed 303 iterative cone beam computed tomography (iCBCT) scans from 15 cervical cancer patients undergoing external beam radiotherapy. CTVs of the uterus (CTV-U) and cervix (CTV-C) contours were delineated based on each iCBCT image. CTV-U encompassed the uterus, while CTV-C included the cervix, vagina, and adjacent parametrial regions. Compared with the planning CTV, the movement of CTV-U and CTV-C in the anterior-posterior, superior-inferior, and lateral directions between iCBCT scans was measured. Uniform expansions were applied to the planning CTV to assess target coverage. The motion (mean ± standard deviation) in the CTV-U position was 8.3 ± 4.1mm in the left, 9.8 ± 4.4mm in the right, 12.6 ± 4.0mm in the anterior, 8.8 ± 5.1mm in the posterior, 5.7 ± 5.4mm in the superior, and 3.0 ± 3.2mm in the inferior direction. The mean CTV-C displacement was 7.3 ± 3.2mm in the left, 8.6 ± 3.8mm in the right, 9.0 ± 6.1mm in the anterior, 8.4 ± 3.6mm in the posterior, 5.0 ± 5.0mm in the superior, and 3.0 ± 2.5mm in the inferior direction. Compared with the other tumor (T) stages, CTV-U and CTV-C motion in stage T1 was larger. A uniform CTV planning treatment volume margin of 15mm failed to encompass the CTV-U and CTV-C in 11.1% and 2.2% of all fractions, respectively. The mean volume change of CTV-U and CTV-C were 150% and 51%, respectively, compared with the planning CTV. Movements of the uterine corpus are larger than those of the cervix. The likelihood of missing the CTV is significantly increased due to inter-fractional motion when utilizing traditional planning margins. Early T stage may require larger margins. Personal radiotherapy margining is needed to improve treatment accuracy.

The use of dose surface maps as a tool to investigate spatial dose delivery accuracy for the rectum during prostate radiotherapy.

This study aims to address the lack of spatial dose comparisons of planned and delivered rectal doses during prostate radiotherapy by using dose-surface maps (DSMs) to analyze dose delivery accuracy and comparing these results to those derived using DVHs. Two independent cohorts were used in this study: twenty patients treated with 36.25Gy in five fractions (SBRT) and 20 treated with 60Gy in 20 fractions (IMRT). Daily delivered rectum doses for each patient were retrospectively calculated using daily CBCT images. For each cohort, planned and average-delivered DVHs were generated and compared, as were planned and accumulated DSMs. Permutation testing was used to identify DVH metrics and DSM regions where significant dose differences occurred. Changes in rectal volume and position between planning and delivery were also evaluated to determine possible correlation to dosimetric changes. For both cohorts, DVHs and DSMs reported conflicting findings on how planned and delivered rectum doses differed from each other. DVH analysis determined average-delivered DVHs were on average 7.1%±7.6% (p ≤ 0.002) and 5.0±7.4% (p ≤ 0.021) higher than planned for the IMRT and SBRT cohorts, respectively. Meanwhile, DSM analysis found average delivered posterior rectal wall dose was 3.8±0.6Gy (p=0.014) lower than planned in the IMRT cohort and no significant dose differences in the SBRT cohort. Observed dose differences were moderately correlated with anterior-posterior rectal wall motion, as well as PTV superior-inferior motion in the IMRT cohort. Evidence of both these relationships were discernable in DSMs. DSMs enabled spatial investigations of planned and delivered doses can uncover associations with interfraction motion that are otherwise masked in DVHs. Investigations of dose delivery accuracy in radiotherapy may benefit from using DSMs over DVHs for certain organs such as the rectum.

ИСПОЛЬЗОВАНИЕ ВОДНОГО БОЛЮСА ПРИ ЛУЧЕВОЙ ТЕРАПИИ ПАЦИЕНТОВ С ИЗОЛИРОВАННЫМИ ПОВЕРХНОСТНЫМИ ПРОЯВЛЕНИЯМИ Т-КЛЕТОЧНОЙ ЛИМФОМЫ КОЖИ

Purpose: To demonstrate the effectiveness of water bolus, combined with individual immobilization for the treatment of lesions in the pedal region of the foot on a patient with isolated cutaneous T-cell lymphoma lesions. Material and methods: Construction of individual immobilization devices, simultaneously functioning as a bolus, in two clinical cases of T-cell cutaneous lymphoma radiation therapy in the pedal region of the foot, along with 3D-CRT and VMAT treatment planning, accompanied by retrospective evaluation of interfractional motion. Results: The proposed methodology demonstrates the effectiveness of the individual immobilization devices in minimizing interfractional motion, as well as the benefit of the bolus effect of the device in achieving optimal dose distribution in the treatment of superficial T-cell lymphoma lesions.

MRI-based inter- and intrafraction motion analysis of the pancreatic tail and spleen as preparation for adaptive MRI-guided radiotherapy in neuroblastoma

BackgroundIn pediatric radiotherapy treatment planning of abdominal tumors, dose constraints to the pancreatic tail/spleen are applied to reduce late toxicity. In this study, an analysis of inter- and intrafraction motion of the pancreatic tail/spleen is performed to estimate the potential benefits of online MRI-guided radiotherapy (MRgRT).Materials and methodsTen randomly selected neuroblastoma patients (median age: 3.4 years), irradiated with intensity-modulated arc therapy at our department (prescription dose: 21.6/1.8 Gy), were retrospectively evaluated for inter- and intrafraction motion of the pancreatic tail/spleen. Three follow-up MRIs (T2- and T1-weighted ± gadolinium) were rigidly registered to a planning CT (pCT), on the vertebrae around the target volume. The pancreatic tail/spleen were delineated on all MRIs and pCT. Interfraction motion was defined as a center of gravity change between pCT and T2-weighted images in left-right (LR), anterior-posterior (AP) and cranial-caudal (CC) direction. For intrafraction motion analysis, organ position on T1-weighted ± gadolinium was compared to T2-weighted. The clinical radiation plan was used to estimate the dose received by the pancreatic tail/spleen for each position.ResultsThe median (IQR) interfraction motion was minimal in LR/AP, and largest in CC direction; pancreatic tail 2.5 mm (8.9), and spleen 0.9 mm (3.9). Intrafraction motion was smaller, but showed a similar motion pattern (pancreatic tail, CC: 0.4 mm (1.6); spleen, CC: 0.9 mm (2.8)). The differences of Dmean associated with inter- and intrafraction motions ranged from − 3.5 to 5.8 Gy for the pancreatic tail and − 1.2 to 3.0 Gy for the spleen. In 6 out of 10 patients, movements of the pancreatic tail and spleen were highlighted as potentially clinically significant because of ≥ 1 Gy dose constraint violation.ConclusionInter- and intrafraction organ motion results into unexpected constrain violations in 60% of a randomly selected neuroblastoma cohort, supporting further prospective exploration of MRgRT.

Dosimetric Advantages of Online Adaptative Radiotherapy for Cervical Cancer on 1.5T MR-Linac

Online adaptative radiotherapy (ART) strategy can be applied to account for inter-fraction organ motion while limiting organ at risk (OAR) dose. This study aims to evaluate tumor target and OARs inter- and intrafraction motion using online MRI during the full course of MR-Linac radiotherapy fractions. Furthermore, quantify the dosimetric consequences of online adaptative compared with non-adaptative radiotherapy (non-ART) in cervical cancer. Six cervix cancer patients were treated with 150 fractions on the 1.5 T Unity MR-Linac. Each fraction, pre-treatment MRI scans were obtained at the start of every treatment session, and post-treatment MRI scans were obtained at the end of every treatment session. A total 300 MR images were included in this study and the CTV, bladder and rectum were delineated on each MRI by the same radiation oncologist. The inter-fraction and intrafraction changes of contours were evaluated by dice similarity coefficient (DSC), Hausdorff distance (HD), volume difference (ΔV). The reference treatment plans were generated using step-and-shoot IMRT and utilized 9 beam groups on original CT. Then, the online adaptative treatment plans were obtained by re-optimizing based on the contours on daily pre-treatment MRI by "adapt to shape" workflow using the same beam parameters and optimization objectives from the reference plans. Non-ART plans for each patient were generated by recalculating the dose from the reference plans on daily online MRI by "adapt to position" workflow. CTV coverage and OARs constraints were used to compare ART and non-ART plans. The results showed that large inter-fraction anatomical changes limited the efficacy of radiation therapy (CTV: DSC: 0.89±0.03, HD: 20.83±5.05mm, ΔV: 1.21%±5.44%; Bladder: DSC: 0.68±0.18, HD: 30.58±15.75mm, ΔV: -10.17%±61.19%; Rectum: DSC: 0.69±0.09, HD: 22.98±12.03mm, ΔV: 21.01%±20.59%).The intrafraction anatomical changes were smaller (CTV: DSC: 0.96±0.01, HD: 8.85±3.54mm, ΔV: -0.64%±1.90%; Bladder: DSC: 0.90±0.07, HD: 14.62±9.56mm, ΔV: 19.83%±21.71%; Rectum: DSC: 0.98±0.04, HD: 2.90±4.27mm, ΔV: 0.13%±5.04%) . Non-ART showed inadequate primary CTV coverage in 30% of the daily fractions. Online adaptative plans improved CTV coverage significantly (p<0.001) to 99%. Compared with non-ART, online ART decreased the fraction dose to bladder and rectum indicated by significant (p<0.001) improvements for daily D50%, D2% and Dmean. In cervical cancer radiotherapy, a non-adaptive strategy led to inadequate target coverage for individual patients. Online adaptative radiotherapy corrected for day-to-day anatomical variations and resulted in adequate target coverage in all fractions. The dose to rectum and bladder was decreased significantly when applying online adaptative radiotherapy.

CBCT Guided Interfraction Absolute Displacement and Setup Error Assessment during Breast Prone Radiotherapy

To calculate interfraction absolute displacement/shift and setup error using cone beam computed tomography (CBCT) during breast prone radiotherapy. Fifty-nine patients undergoing prone whole breast-irradiation after breast-conserving surgery (BCS) were studied as part of an institutional review board- approved prospective trial. Setup precision was monitored using a daily online CBCT. Translational shifts in 3 axes (AP: anterior posterior; LR: left right; and SI: superior inferior) and 3 rotations (pitch, roll, and rtn) after CBCT were analyzed for 1062 treatment fractions. The random and systematic setup errors (SE) were calculated and were analyzed for time trends during the course of radiotherapy. For absolute inter-fractional shifts, the numbers of fractions exceeding 10 mm in the AP, LR, and SI directions were 6.5%, 17.42% and 8.92%, respectively; 0%, 0% and 1.31% fractions exceeded 3°for pitch, roll, and Rtn, respectively. The population systematic errors were 1.89/2.91/1.98 mm in AP/SI/LR directions, while the random error were 2.72/3.99/3.31 mm. In pitch, roll, rtn rotations, the population systematic error were 0.64°/0.49°/0.46°, and 0.89°/0.90°/0.93° for the random error. Without correction these would correspond to a clinical to planning target volume margin of 6.64/10.08/7.26 mm in AP/SI/LR directions and 2.22°/1.79°/1.8° in pitch, roll, rtn rotations. The magnitude of inter-fraction motion was not correlated with patient treatment time accept in AP direction (P = 0.000). SE was larger in prone position in breast cancer patients, attributable mostly to random errors, which emphasize the need for on-line imaging guidance in breast prone radiotherapy. 10mm margins would adequately cover the target volume and account for setup errors in the absence of IGRT.

The Impact of Intra-Fraction Bowel Motion on Luminal Gastrointestinal Organ at Risk Dosimetry When Using Stereotactic Adaptive Radiotherapy for Abdominal Malignancies

Daily online adaptive radiotherapy (ART) provides inter-fraction motion management of the luminal gastrointestinal (GI) structures when delivering abdominal SBRT. One potential drawback of ART is the time-consuming process, and intra-fraction GI changes from completion of the ART process to the end of treatment delivery have not been thoroughly evaluated. We explored intra-fraction bowel motion for patients receiving abdominal stereotactic adaptive radiotherapy (sART) MATERIALS/METHODS: Six patients with abdominal malignancies treated with CT-guided sART on a prospective feasibility trial had additional CBCT's acquired post-treatment (pTx-CBCT). All patients were prescribed to 50 Gy/5 fractions (fx), and the constraint for all GI OARs was V33≤0.5 cc. Time from initial CBCT (I-CBCT) used for adaptive planning to pTx-CBCT was collected. The luminal GI OAR (stomach (S), duodenum (D), small bowel (SB), and large bowel (LB)) were retrospectively contoured on pTx-CBCT. The OAR doses were compared between the I-CBCT and pTx-CBCT. The adaptive plan (PA) and initial plan (PI) doses were overlayed on the pTx-CBCT contours. The PA pTx-CBCT OAR doses were then compared to the PI pTx-CBCT OAR doses. A Boolean OAR structure of all GI OARs was evaluated to remove potential differences in structure definitions between providers. The T-test was used to compare differences in instances of D0.5cc ≥ 33 and 50 Gy. Patient charts were reviewed for grades (G) ≥ 3 toxicity. Thirty fractions (fx) of sART were delivered and pTx-CBCT were acquired in 26 fx. Mean time from I-CBCT to pTx-CBCT acquisition was 66 min (38-98 min). On average at 0.5 cc the PA overdosed the S by 1.74 Gy based on pTx-CBCT anatomy compared to 2.35 Gy by the PI, the D by 0.47 Gy (PA) vs .84 Gy (PI), the SB by 1.14 Gy (PA) vs 1.43 Gy (PI), and the LB by 0.13 Gy (PA) vs 0.60 Gy (PI). The dose to the Boolean OAR structure was on average 2.51 Gy/fx higher than expected when overlaying the PA on the pTx-CBCT compared to 3.38 Gy/fx higher when overlaying the PI on the pTx-CBCT. There was no significant difference in the instance of the PA exceeding D0.5 cc ≥33 Gy vs the PI (p = 0.083), but the PA significantly reduced the instances of D0.5cc≥50 Gy (p = 0.001) compared to the PI. No patient experienced G≥3 toxicity at a median follow-up of 8 months (3-12). These data demonstrate sART led to a significant decrease in dose to GI OARs, particularly for prescription dose or greater, even after accounting for intra-fractional bowel motion. While both the PI and the PA violated the V33 luminal GI OAR constraint in approximately ½ of pTx-CBCTs, the fraction of OARs receiving at least 50 Gy was significantly higher when overlaying the PI compared to the PA. While no G3 toxicities were reported in this small cohort, further studies are needed to characterize if the increased dose to GI OARs over the expected dose is clinically significant.

Enough and Some to Spare: Improved OAR Sparing with Daily Online Cone Beam Adaptive Radiation Therapy (OnC-ART) of the Cervix

The standard of care treatment for locally advanced cervical cancer involves pelvic chemoradiation. During treatment planning, the cervix and uterus contours are expanded by 1.5 to 2 cm margins to account for inter- and intrafractional motion per the consensus guidelines. Daily online cone-beam adaptive radiation therapy (OnC-ART) accounts for interfractional motion, thus significantly reducing CTV to PTV margins. In this study, we compared the dosimetry of standard larger margin plans (IGRT) to 0.5 cm margins for patients treated with daily OnC-ART and hypothesized that daily OnC-ART with reduced margins will have improved dosimetry. A retrospective cohort of 10 patients with cervical cancer (stage IIB - IIIC2) treated with 260 definitive daily fractions of OnC-ART chemoradiation between April 2021 and December 2022 was included. Initial plans were generated with OnC-ART and IGRT CTV to PTV margins as follows: combined cervix, uterus, and GTV (0.5 cm vs. 1.5 cm), combined parametria and vagina (0.5 cm vs. 1 cm), nodal chains (0.5 cm vs. 0.5 cm), and gross nodes (0.5 cm vs. 0.5 cm). The initial IGRT plan was copied to synthetic CTs generated for each fraction in the adaptive workflow containing the daily contours of all targets and OARs. The dosimetry of each IGRT fraction was compared to the dose delivered from each daily OnC-ART fraction. Statistical significance was defined as p < 0.05. When compared with dosimetry from daily IGRT, the daily OnC-ART plans had significantly improved CTV coverage and less hotspots with improvements in D95% (+1.6%), D99% (+2.9%), and Dmax (-0.9%). Bowel bag and bowel loops showed significant improvements in Dmax (-1.4% and - 2.6%), V40 (-6.2% and -5.3%), and V45 (-6.1% and 5.5%). The daily OnC-ART plans had a mean bowel bag V40 of 177.4 cm3. Contrastingly, the mean bowel bag V40 for the large margin IGRT plans was 268.7 cm3. The bladder and rectum also showed significant improvements in Dmax (-1.7% and -1.6%), V40 (-25.2% and -36.0%), and V30 (-9.7% and -17.1%). The daily OnC-ART plans had a mean bladder and rectum V40 of 41.6 cm3 and 18.4 cm3, and the large margin IGRT plans had a mean bladder and rectum V40 of 65.1 cm3 and 40.8 cm3. Bone marrow had a significantly reduced dose with improvements in Dmean (-3.2%), V10 (-2.7%), and V20 (-3.3%). Reduced CTV to PTV margins achievable with daily OnC-ART is shown to not only improve sparing of critical OARs, especially the bladder and rectum, but also improves target coverage compared to larger margins from daily IGRT. The clinical impact of these dosimetric improvements is currently undergoing investigation.

CT-Guided Online Adaptive Stereotactic Body Radiotherapy for Pancreas Ductal Adenocarcinoma: Dosimetric and Initial Clinical Experience

Retrospective analysis suggests that dose escalation to a biologically effective dose of more than 70 Gy may improve overall survival in patients with pancreatic ductal adenocarcinoma (PDAC), but such treatments in practice are limited by proximity of organs at risk (OARs). We hypothesized that CT-guided online adaptive radiotherapy (OART) can account for interfraction movement of OARs, reduce dose to OARs, and improve coverage of targets. This is a single institution retrospective analysis of patients with PDAC treated with OART on a CBCT-based OART platform. All patients were treated to 40 Gy in 5 fractions. PTV overlapping with a 5 mm planning risk volume expansion on the stomach, duodenum and bowel received 25 Gy. Initial treatment plans were created conventionally. For each fraction, PTV and OAR volumes were recontoured with AI assistance after initial cone beam CT (CBCT). The adapted plan was calculated, underwent QA, and then compared to the scheduled plan. A second CBCT was obtained prior to delivery of the selected plan. Total treatment time (first CBCT to end of radiation delivery) and active physician time (first to second CBCT) were recorded. PTV_4000 V95%, PTV_2500 V95%, and D0.03 cc to stomach, duodenum and bowel were reported for scheduled (S) and adapted (A) plans. CTCAEv5.0 toxicities were recorded. Statistical analysis was performed using a two-sided T test and α of 0.05. Seven patients with unresectable or locally-recurrent PDAC were analyzed, with a total of 35 fractions. Average total time was 33:00 minutes (22:25-49:40) and average active time was 22:48 minutes (14:15-39:34). All fractions were treated with adapted plans. All adapted plans met PTV_4000 V95.0% > 95.0% coverage goal and OAR dose constraints. Dosimetric data for scheduled and adapted plans per fraction are in Table 1. Median follow up was 1.7 months. 5 (71%) patients experienced either Grade 1 or 2 toxicities. No patients experienced Grade 3+ toxicities. Daily OART significantly reduced dose OARs while achieving superior PTV coverage. Treatment was generally well tolerated with no grade 3+ acute toxicity, and required only 22:48 minutes on average of active physician time. Our initial clinical experience demonstrates OART allows for safe dose escalation in the treatment of PDAC.

Prostate cancer image guided radiotherapy: Why the commotion over rectal volume and motion?

IntroductionDistended rectums on pre-radiotherapy scans are historically associated with poorer outcomes in patients treated with two-dimensional IGRT. Subsequently, strict rectal tolerances and preparation regimes were implemented. Contemporary IGRT, daily online registration to the prostate, corrects interfraction motion but intrafraction motion remains. We re-examine the need for rectal management strategies when using contemporary IGRT by quantifying rectal volume and its effect on intrafraction motion. Materials and methodsPre and during radiotherapy rectal volumes and intrafraction motion were retrospectively calculated for 20 patients treated in 5-fractions and 20 treated in 20-fractions. Small (rectal volume at planning-CT ≤ median), and large (volume > median) subgroups were formed, and rectal volume between timepoints and subgroups compared. Rectal volume and intrafraction motion correlation was examined using Spearman’s rho. Intrafraction motion difference between small and large subgroups and between fractions with rectal volume < or ≥ 90 cm3 were assessed. ResultsMedian rectal volume was 74 cm3, 64 cm3 and 65 cm3 on diagnostic-MRI, planning-CT and treatment imaging respectively (ns). No significant correlation was found between patient’s rectal volume at planning-CT and median intrafraction motion, nor treatment rectal volume and intrafraction motion for individual fractions. No significant difference in intrafraction motion between small and large subgroups presented and for fractions where rectal volume breached 90 cm3, motion during that fraction was not significantly greater. ConclusionLarger rectal volumes before radiotherapy and during treatment did not cause greater intrafraction motion. Findings support the relaxation of strict rectal diameter tolerances and do not support the need for rectal preparation when delivering contemporary IGRT to the prostate.