Clinical Target Volume Coverage Research Articles

Original Knee Fixation Device as a Useful Fixation Method during Prostate Intensity-Modulated Radiation Therapy.

Purpose:The purpose of he study was to reduce setup errors during intensity-modulated radiation therapy (IMRT) with an original knee fixation device (KFD) and evaluate the clinical target volume (CTV) coverage.Methods:Participants were classified into two groups: knee flexion (KF) group (n = 16), wherein participants' knees were fixed in a flexed position using the proposed KFD during planning computed tomography, and knee extension group (KE; n = 15), wherein no KFD was used. We investigated the residual rotational errors and inter-fractional setup errors with or without KFD. Furthermore, inter-fractional margins were calculated using logistic regression analysis, and CTV coverage was evaluated.Results:The residual rotational errors in the yaw and roll directions (P < 0.02) and the inter-fractional error in the anterior-posterior (A-P) direction (P < 0.02) improved significantly in the KF group compared with the KE group. Repeatability was improved for the pitch direction. The inter-fractional margins were 6.68 mm and 4.87 mm in the A-P and superior-inferior (S-I) directions, respectively, in the KF group, representing reductions (mm) of 20.8% and 12.6% compared with the KE group, respectively. The odds ratios for CTV coverage in the KF group compared to the KE group were 2.76 (P < 0.001) and 1.74 (P < 0.05) in the A-P and S-I directions, respectively.Conclusions:The IMRT fixation method using an original KFD improved the residual rotational error in the three directions and the inter-fractional error in the A-P direction, reduced the interfractional margins in the A-P, and S-I directions and improved CTV coverage. Our original KFD may be a useful fixation method during prostate IMRT.

A Statistical Index for Evaluating the CTV Coverage Loss of Brain Stereotactic Radiosurgery When the PTV Margin Is Zero

Purpose: To develop a new statistical index “percent CTV (clinical target vo-lume) coverage probability” (%CCP), defined as the probability that a specific percent (e.g., 95%) of the CTV can be reliably covered by the prescription dose, for evaluating the coverage loss of brain (fractionated) stereotactic radiosurgery (SRS/fSRS) when the PTV (planning target volume) margin is zero. Methods: The random variable Q for CTV percent coverage was derived using a previously developed model for CTV random motion that follows a three-dimensional (3D) independent normal distribution with a zero mean and a standard deviation of ????(for translation) or ????(for rotation). Assuming both CTV and PTV are spherical with the same diameter dCTV, the cumulative distribution function of Q could be obtained analytically using the relation of sphere-sphere intersection. The %CCP was then derived as the reliability function of Q and was used to quantify the coverage loss for selected dCTV. Results: The 95%-95% clinical goal (95% of the times, at least 95% of the CTV is covered) is not achievable with dCTV mm. For common CTVs (dCTV mm) encountered in SRS/fSRS, only 60%-90% of the CTV could be reliably covered by the prescription dose 95% of the time. For ????=0.5mm and ???? =0.4?, the 95% CCP was the highest when the distance between the CTV and the isocenter ???????=0 and gradually decreased with the increasing ???????. Conclusions: The %CCP was successfully derived for evaluating the CTV coverage loss for brain SRS/fSRS. When the PTV margin is zero, the 95%-95% clinical goal cannot be achieved for most targets (dCTV mm).

The dosimetric advantages of perirectal hydrogel spacer in men with localized prostate cancer undergoing stereotactic ablative radiotherapy (SABR).

To evaluate the dosimetric differences for patients receiving a perirectal hydrogel spacer (PR-HS) using SpaceOAR undergoing stereotactic ablative radiotherapy (SABR) for localized prostate cancer with the CyberKnife VSI system. Gold fiducial markers and a PR-HS was inserted in 22 consecutive patients with histologically confirmed localized prostate cancer. For planning comparison, dosimetry from the clinical plans was compared against replans based on a simulated rectum volume designed to recreate a clinically appropriate spacer-less anatomy for each patient. Both sets were planned to 36.25 Gy in 5 fractions using the treatment planning system associated with the CyberKnife VSI system. The aim was to ensure equivalent target coverage for both plans and to evaluate doses to the organs-at-risk (OARs): rectum, bladder and penile bulb. The median PR-HS implant volume was 11.2 cc (range 8.8 to 14.9 cc). The maximal median perirectal separation was 15.5 mm (10.5 to 20.7 mm). Statistically significant reductions were noted for the 3 OARs, with no statistically significant difference in planning target volumes or clinical target volume coverage. All rectal dose constraints were significantly improved in the PR-HS plans with a percentage dose difference of at least 24% (rectum V18.1Gy (%)) to 60.5% (rectum V36Gy (cc)). The bladder and penile bulb dose constraints parameters were also significantly improved: the bladder V37Gy was reduced by 17.1%, V18.1Gy was reduced by 4.2%; the penile bulb D50% was reduced by 7.7%. The use of PR-HS was able to significantly reduce planned dose to the rectum, bladder and penile bulb with SABR techniques associated with the CyberKnife VSI system.

Dosimetric comparison of intensity modulated radiotherapy and intensity modulated proton therapy in the treatment of recurrent nasopharyngeal carcinoma.

To compare the dosimetric performance of Intensity Modulated Proton Therapy (IMPT) and Intensity Modulated Radiotherapy (IMRT) in terms of target volume coverage and sparing of neurological organs-at-risk (OARs) in salvaging recurrent nasopharyngeal carcinoma (rNPC). The maximum dose to the internal carotid artery (ICA) and nasopharyngeal (NP) mucosa, which are associated with potential carotid blowout and massive epistaxis, were also evaluated. IMRT and IMPT treatment plans were created for twenty patients with locally advanced rNPC. Planning Target Volume (PTV) was used to account for the setup and spatial error/uncertainty in the IMRT planning. Robust optimization on Clinical Target Volume (CTV) coverage with consideration of range and setup uncertainty was employed to produce two IMPT plans with 3-field and 4-field arrangements. The planning objective was to deliver 60 Gy to the PTV (IMRT) and CTV (IMPT) without exceeding the maximum lifetime cumulative Biologically Effective Dose (BED) of the neurological OARs (applied to the Planning organs-at-risk volume). The target dose coverage as well as the maximum dose to the neurological OARs, ICA, and NP mucosa were compared. Compared with IMRT, 3-field IMPT achieved better coverage to GTV V100% (83.3% vs. 73.2%, P <0.01) and CTV V100% (80.5% vs. 72.4%, P <0.01), and lower maximum dose to the critical OARs including the spinal cord (19.2 Gy vs. 22.3 Gy, P <0.01), brainstem (30.0 Gy vs. 32.3 Gy, P <0.01) and optic chiasm (6.6 Gy vs. 9.8 Gy, P <0.01). The additional beam with the 4-fields IMPT plans further improved the target coverage from the 3-field IMPT (CTV V98%: 85.3% vs. 82.4%, P <0.01) with similar OAR sparing. However, the target dose was highly non-uniform with both IMPT plans, leading to a significantly higher maximum dose to the ICA (∼68 Gy vs. 62.6 Gy, P <0.01) and NP mucosa (∼72 Gy vs. 62.8 Gy, P <0.01) than IMRT. IMPT demonstrated some dosimetric advantage over IMRT in treating rNPC. However, IMPT could also result in very high dose hot spots in the target volume. Careful consideration of the ICA and NP mucosal complications is recommended when applying IMPT on rNPC patients.

Efficacy and safety of a 3D-printed applicator for vaginal brachytherapy in patients with central pelvic-recurrent cervical cancer after primary hysterectomy

Intracavitary and/or interstitial brachytherapy is an integral component of the management of patients with central pelvic-recurrent cervical cancer after primary hysterectomy, and is typically delivered using conventional applicators. We investigated the efficacy and safety of three-dimensional (3D)-printed, customizable applicators for those patients. Twenty-six patients were treated with combination external beam radiotherapy and brachytherapy. Patients with lesions ≤1 and >1 cm before brachytherapy were treated with intracavitary and interstitial brachytherapy, respectively. Dosimetric plans were compared between the vaginal cylinder and 3D-printed applicator for the first 9 patients. Outcomes and treatment-related complications were also investigated. The median tumor size before brachytherapy was 0.81 cm. Intracavitary, interstitial, and combined interstitial-intracavitary brachytherapy were performed in 22 (85%), 3 (11%), and 1 (4%) of the patients, respectively. The clinical target volume (CTV) coverage goal was achieved with all 3D-printed plans but failed with three single-channel cylinder plans (33.3%). Owing to 3D-printed transvaginal applicator guidance, there was no need to adjust the needle position after implantation. The mean CTV dose for all patients was 71 ± 8.2 Gy; all met the dose constraints to the organs at risk, but 1 (4%) had a rectal D2cc overdose. The 2-year local control, progression-free survival, and overall survival rates were 87.8%, 71.0%, and 91.6%, respectively. Four patients (21%) developed early grade 3-4 hematological toxicities and 1 (4%) developed a late grade 3 adverse event. High-quality intracavitary and/or interstitial brachytherapy can be achieved using a 3D-printed applicator and yields favorable outcomes with acceptable toxicity.

In Silico Feasibility Study of Carbon Ion Radiotherapy With Simultaneous Integrated Boost for Head and Neck Adenoid Cystic Carcinoma.

PurposeIn carbon ion radiotherapy (CIRT), a simultaneous integrated boost (SIB) approach has not been fully exploited so far. The feasibility of a CIRT-SIB strategy for head and neck adenoid cystic carcinoma (ACC) patients was investigated in order to improve treatment planning dose distributions.Methods and MaterialsCIRT plans of 10 ACC patients treated at the National Center for Oncological Hadrontherapy (CNAO, Pavia, Italy) with sequential boost (SEQ) irradiation and prescription doses of 41.0 Gy [relative biological effectiveness (RBE)]/10 fractions to low-risk (LR) clinical target volume (CTV) plus 24.6 Gy(RBE)/6 fractions to the high-risk (HR) CTV were re-planned with two SIB dose levels to the LR-CTV, namely, 48.0 Gy(RBE) and 54.4 Gy(RBE). While planning with SIB, the HR-CTV coverage had higher priority, with fixed organ-at-risk dose constraints among the SIB and SEQ plans. The homogeneity and conformity indexes were selected for CTV coverage comparison. The biologically effective dose (BED) was calculated to compare the different fractionation schemes.ResultsComparable HR-CTV coverage was achieved with the treatment approaches, while superior conformality and homogeneity were obtained with the SIB technique in both CTVs. With the SEQ, SIB48.0, and SIB54.4, the LR-CTV median doses were respectively 50.3%, 11.9%, and 6.0% higher than the prescriptions. Significant reductions of the median and near-maximum BEDs were achieved with both SIB dose levels in the LR-CTV.ConclusionsThe SIB approach resulted in highly conformal dose distributions with the reduction of the unintended dose to the LR-CTV. A prescription dose range for the LR-CTV will be clinically defined to offer tailored personalized treatments, according to the clinical and imaging characteristics of the patients.

Proton Beam Therapy for Locally Advanced Head and Neck Tumors: An Analysis of Dosimetric and Clinical Outcomes.

Locally advanced tumors of the head and neck region often lie in close proximity to critical organs at risk (OARs). Providing effective treatment coverage to these malignancies while minimizing radiation dose to surrounding OARs is advantageous. Our aim is to compare dosimetric data of OARs from proton beam therapy (PBT) plans to volumetric modulated arc therapy (VMAT) treatment plans, and to evaluate clinical outcomes in patients treated with PBT. We identified patients with locally advanced head and neck tumors treated with PBT at our institution from 2016 to 2019. Study endpoints included mean and maximum doses for the OAR structures for each treatment plan, overall survival, time to local-regional or distant progression, and presence of acute and late toxicities. Mean and maximum doses to OAR structures were compared between treatment modalities using a paired Wilcoxon signed-rank test. P-values <0.05 were considered significant. A total of 42 patients were identified. Clinical target volume coverage was >95% for both PBT and VMAT plans. PBT plans showed a significant reduction to the mean doses to all OARs, and max doses to most OARs (P<0.05). The largest reduction mean dose was seen in the contralateral cochlea and parotid glands at 71% and 75%, respectively. Median follow-up was 27 months. Overall survival at 4 years was 44.75%. Freedom from local-regional progression was 73.28% at 2 years. The majority of patients developed Common Terminology Criteria for Adverse Events (CTCAE) grade I dermatitis, mucositis, or both. PBT resulted in meaningful dose reductions to OARs while maintaining comparable target coverage when compared with VMAT plans. Further refinements to proton therapy may have the potential to further minimize dose to critical structures.

Single-institution clinical experience using robust intensity modulated proton therapy in chordoma and chondrosarcoma of the mobile spine and sacrum: Feasibility and need for plan adaptation

BackgroundDue to its specific physical characteristics, proton irradiation is especially suited for irradiation of chordomas and chondrosarcoma in the axial skeleton. Robust plan optimization renders the proton beam therapy more predictable upon individual setup errors. Reported experience with the planning and delivery of robustly optimized plans in chordoma and chondrosarcoma of the mobile spine and sacrum, is limited. In this study, we report on the clinical use of robustly optimized, intensity modulated proton beam therapy in these patients. MethodsWe retrospectively reviewed patient, treatment and acute toxicity data of all patients with chordoma and chondrosarcoma of the mobile spine and sacrum, treated between 1 April 2019 and 1 April 2020 at our institute. Anatomy changes during treatment were evaluated by weekly cone-beam CTs (CBCT), supplemented by scheduled control-CTs or ad-hoc control-CTs. Acute toxicity was scored weekly during treatment and at 3 months after therapy according to CTCAE 4.0. Results17 chordoma and 3 chondrosarcoma patients were included. Coverage of the high dose clinical target volume was 99.8% (range 56.1–100%) in the nominal and 80.9% (range 14.3–99.6%) in the voxel-wise minimum dose distribution. Treatment plan adaptation was needed in 5 out of 22 (22.7%) plans. Reasons for plan adaptation were either reduced tumor coverage or increased dose to the OAR. ConclusionsRobustly optimized intensity modulated proton beam therapy for chordoma and chondrosarcoma of the mobile spine is feasible. Plan adaptations due to anatomical changes were required in approximately 23 percent of treatment courses.

Feasibility of Same-Day Prostate Fiducial Markers, Perirectal Hydrogel Spacer Placement, and Computed Tomography and Magnetic Resonance Imaging Simulation for External Beam Radiation Therapy for Low-Risk and Intermediate-Risk Prostate Cancer

PurposeThe use of prostate fiducial markers and perirectal hydrogel spacers can reduce the acute and late toxic effects associated with prostate radiation therapy. These procedures are usually performed days to weeks before simulation during a separate clinic visit to ensure resolution of procedure-related inflammation. The purpose of this study was to assess whether same-day intraprostatic fiducial marker placement, perirectal hydrogel injection, and computed tomography (CT) and magnetic resonance imaging (MRI) simulation were feasible without adversely affecting hydrogel volume, perirectal spacing, or rectal dose. If feasible, performing these procedures on the same day as simulation would expedite the start of radiation therapy, improve patient convenience, and reduce costs. Methods and MaterialsTwenty-one patients with clinically localized prostate cancer who were enrolled on a prospective clinical trial (NCT01617161) underwent same-day marker placement, hydrogel injection, and CT and MRI simulation, then underwent T2 MRI verification scans 3 to 4 weeks later. The MRI scans were fused to the CT planning scans by clinical target volumes (CTVs) to generate comparison treatment plans (70 Gy in 28 fractions). Hydrogel volume and symmetry, perirectal spacing, CTV dose, and organ-at-risk dose were evaluated. ResultsVerification scans occurred a mean of 24.9 ± 4.6 days after simulation and 9.3 ± 4.9 days after treatment start. Prostate volume did not change between scans (median, 67.3 ± 22.1 cm3 vs 64.1 ± 21.8 cm3; P = .64). The median hydrogel change between simulation and verification was −1.8% ± 4.5% (P = .27). No significant differences in perirectal spacing (midgland: 1.33 ± 0.45 cm vs 1.3 ± 0.7 cm; 1 cm superior: 1.25 ± 0.95 cm vs 1.43 ± 0.91 cm; 1 cm inferior: 1.16 ± 0.28 cm vs 1.41 ± 0.49 cm) were identified. No significant differences in rectal V66 (median 2.3 ± 2.18% vs 2.3 ± 2.28%; P = .99), V35 (median 14.79 ± 7.61 vs 14.67 ± 8.4; P = .73), or D1cc (65.7 ± 9.2 Gy vs 68.2 ± 9.0 Gy; P = .80) were found. All plans met CTV and organ-at-risk constraints. ConclusionSame-day placement of intraprostatic fiducial markers, perirectal hydrogel, and simulation scans was feasible and did not significantly affect hydrogel volume, position, CTV coverage, or rectal dose.

Dosimetric Study of Proton Therapy Using Scanning Method for Localized Prostate Cancer Patients in Comparison With Wobbler Method and Volumetric-Modulated Arc Therapy

<h3>Purpose/Objective(s)</h3> Proton therapy (PT) for localized prostate cancer using the wobbler method (passive scattering) has been reported to have favorable biochemical control and late toxicities rates. In recent years, PT has been gradually shifting to scanning methods from wobbler methods. We performed a planning study to compare the dose distribution of the scanning method with that of the wobbler and the volumetric modulated arc therapy (VMAT) for patients with localized prostate cancer. <h3>Materials/Methods</h3> Thirty prostate cancer patients treated in 2016-2018 were randomly selected. Three radiotherapy plans were created for each patient with the same prescribed dose (76.0 Gy or Gy [RBE] in 38 fractions) and compared using dose-volume histograms. Dose constraints were clinical target volume (CTV) D98 ≥ 73.0 Gy (RBE), rectal wall V65 < 17%, V40 < 35%, and bladder wall V65 < 25% and V40 < 50%, respectively. CTV doses, bladder and rectum wall dose volumes (V10 - V75), Dmean, and Dmax were calculated and tested by Wilcoxon's rank-sum test. <i>P</i> < 0.05 was determined to be statistically significant. <h3>Results</h3> The dose coverage of CTV was favorable with the scanning and the wobbler than with the VMAT. Wide ranges of the rectal and bladder wall volumes of V10 - V70 were significantly lower with the scanning method compared with the wobbler method and the VMAT (<i>P</i> < 0.05). The wobbler method yielded better dose distribution of the rectum and bladder wall in the low dose range (V10-20) and near the maximum dose compared with the VMAT. <h3>Conclusion</h3> Compared with the wobbler method and the VMAT, the scanning method enables further reduction of the rectal and bladder doses while maintaining the CTV dose.

Small Non-Rigid Variations Can Assist Robust IMPT Plan Selection for Head and Neck Patients

Interfraction anatomical variability increases dosimetric discrepancy during delivery of intensity-modulated proton therapy (IMPT). Small non-rigid variations (sNRVs) such as neck extension, spinal flexion, jaw and shoulder position occur during IMPT for head & neck cancer. We assess dosimetric discrepancies caused by sNRVs and suggest a population-based model to incorporate sNRVs into robust evaluation of IMPT plans.A total of 20 nasopharynx and 15 oropharynx radiotherapy patients were retrospectively analyzed, each with weekly CT scans. Anatomical variations during the first week of treatment are predominately due to sNRV, when progressive changes (weight-loss, tumor shrinkage) are less significant. Using deformable image registration, we obtained deformations between week 0/week 1 of treatment for all 35 patients, creating a distribution of possible sNRVs. The deformations from this distribution are then applied to the planning CT of two patients. Patient 1 has a laterally located gross tumor volume (GTV), patient 2 has a central GTV, both with bilateral low-risk clinical target volume (CTV). For both patients, IMPT plans using 3-field, 4-field, and 5-field beam arrangements were generated (nominal dose) and dose was forward calculated on the 35 deformed CT images representing the possible sNRV dose (n = 210) to assess dose degradation. Dose-volume histogram (DVH) bandwidths for sNRV doses were calculated per patient and beam arrangement to evaluate the robustness of the nominal plans with respect to the sNRVs.Among both patients (210 scenarios), we observe a maximum difference to the nominal dose of: 7% degradation of CTV D95, increase in parotid Dmean 10.6 Gy (RBE), increase in brainstem Dmax 14.8 Gy (RBE), increase in chiasm Dmax 18 Gy (RBE), increase in larynx Dmean15 Gy (RBE). For both patients, the 4-field plan shows the narrowest bandwidth and thus best robustness against sNRV in terms of CTV coverage (tab 1). For patient 1, the 4-field plan appears most robust in terms of organ at risk dose. For patient 2, the 3-field plan shows the best parotid sparing while dose metrics for brainstem and chiasm are similar for all beam arrangements.We demonstrate that sNRVs can substantially degrade the dose to the target volume and increase OAR doses. Including the influence of sNRVs during robust evaluation may assist in plan selection, providing information on clinical uncertainties additional to the conventional rigid isocenter shift and CT number uncertainty.

Using CBCT to Monitor Treatment Dose Delivery Accuracy in Head and Neck Proton Radiotherapy

<h3>Purpose/Objective(s)</h3> In comparison with photon radiotherapy, intensity modulated proton therapy (IMPT) has higher sensitivities to anatomical and geometrical changes. In IMPT, variations in tumor size, patient geometry and setup can compromise target coverage or augment dose to nearby organs at risk (OAR). As a result, weekly quality assurance CT scans (QACTs) are commonly used to monitor treatment accuracy and provide indicators for adaptive radiotherapy (ART). This study presents the validation of a tool that applies deformable image registration and image processing techniques to generate accurate noise-free CBCT-based virtual CTs used for proton dose calculation to monitor treatment accuracy in head and neck (HN) patients. <h3>Materials/Methods</h3> A single-institution retrospective IRB-approved analysis was conducted on 25 randomly selected HN patients (15 [58%] males, 10 [42%] females) treated with IMPT at our institution between January 2019 and August 2020. Patients had same-day QACT and CBCTs (total of 194 scans) which were imported into a research version of the treatment planning system. A vendor-developed prototype (RaySearch Laboratories, Sweden) tool was used to correct CBCT artifacts and subsequently create a virtual-CT (vCTs) by deforming the planning CT (pCT). The treatment plan was subsequently recomputed on both the QACT and vCT scans. Differences for dose parameters of clinical target volumes (CTVs) and primary OARs such as oral cavity, larynx, spinal cord, mandible, and parotids were compared between vCT and the reference QACT image sets. <h3>Results</h3> Patient averaged differences between CTV coverage parameters estimated from vCT and QACTs were within a 3% standard deviation; CTV mean doses were 0.4% ± 0.7%, and D_2% and D_95% were 0.8% ± 1.6%, and 0.9% ± 1.7%, respectively. Similarly, good correspondence was observed for CTV V_95% (difference = 0.3% ± 1.0%). High agreement was also observed for OARs mean dose (parotid 0.6% ± 1.0%) and max doses (D_0.01cc): oral cavity (0.9% ± 1.5%), larynx (0.8% ± 1.2%), spinal cord (0.7% ± 1.4%), and mandible (1.0% ± 1.2%). Except for 5 cases where the vCT suffered from uncorrected CBCT artifacts, vCT and QACT estimated changes in target coverage metrics (in percent) with respect to the planning CT were within 3% agreement. Virtual-CTs also accurately indicated scenarios where adaptive radiotherapy (e.g., replan) was initiated based on the QACTs. <h3>Conclusion</h3> For all patients, the dose distributions computed on the vCTs closely matched the reference QACT scans. Our results suggest that CBCT-based dosimetry can be used to evaluate treatment plans, and guide the decision for ART over the treatment course.

Knowledge-Based Planning for Robustly Optimized Intensity-Modulated Proton Therapy of Head and Neck Cancer Patients.

PurposeTo assess the performance of a proton-specific knowledge-based planning (KBP) model in the creation of robustly optimized intensity-modulated proton therapy (IMPT) plans for treatment of advanced head and neck (HN) cancer patients.MethodsSeventy-three patients diagnosed with advanced HN cancer previously treated with volumetric modulated arc therapy (VMAT) were selected and replanned with robustly optimized IMPT. A proton-specific KBP model, RapidPlanPT (RPP), was generated using 53 patients (20 unilateral cases and 33 bilateral cases). The remaining 20 patients (10 unilateral and 10 bilateral cases) were used for model validation. The model was validated by comparing the target coverage and organ at risk (OAR) sparing in the RPP-generated IMPT plans with those in the expert plans. To account for the robustness of the plan, all uncertainty scenarios were included in the analysis.ResultsAll the RPP plans generated were clinically acceptable. For unilateral cases, RPP plans had higher CTV_primary V100 (1.59% ± 1.24%) but higher homogeneity index (HI) (0.7 ± 0.73) than had the expert plans. In addition, the RPP plans had better ipsilateral cochlea Dmean (−5.76 ± 6.11 Gy), with marginal to no significant difference between RPP plans and expert plans for all other OAR dosimetric indices. For the bilateral cases, the V100 for all clinical target volumes (CTVs) was higher for the RPP plans than for the expert plans, especially the CTV_primary V100 (5.08% ± 3.02%), with no significant difference in the HI. With respect to OAR sparing, RPP plans had a lower spinal cord Dmax (−5.74 ± 5.72 Gy), lower cochlea Dmean (left, −6.05 ± 4.33 Gy; right, −4.84 ± 4.66 Gy), lower left and right parotid V20Gy (left, −6.45% ± 5.32%; right, −6.92% ± 3.45%), and a lower integral dose (−0.19 ± 0.19 Gy). However, RPP plans increased the Dmax in the body outside of CTV (body-CTV) (1.2 ± 1.43 Gy), indicating a slightly higher hotspot produced by the RPP plans.ConclusionIMPT plans generated by a broad-scope RPP model have a quality that is, at minimum, comparable with, and at times superior to, that of the expert plans. The RPP plans demonstrated a greater robustness for CTV coverage and better sparing for several OARs.

Variation of optimization techniques for high dose rate brachytherapy in cervical cancer treatment

High dose rate (HDR) brachytherapy treatment planning usually involves optimization methods to deliver uniform dose to the target volume and minimize dose to the healthy tissues. Four optimizations were used to evaluate the high-risk clinical target volume (HRCTV) coverage and organ at risk (OAR). Dose-volume histogram (DVH) and dosimetric parameters were analyzed and evaluated. Better coverage was achieved with PGO (mean CI = 0.95), but there were no significant mean CI differences than GrO (p = 0.03322). Mean EQD2 doses to HRCTV (D90) were also superior for PGO with no significant mean EQD2 doses than GrO (p = 0.9410). The mean EQD2 doses to bladder, rectum, and sigmoid were significantly higher for NO plan than PO, GrO, and PGO. PO significantly reduced the mean EQD2 doses to bladder, rectum, and sigmoid but compromising the conformity index to HRCTV. PGO was superior in conformity index (CI) and mean EQD2 doses to HRCTV compared with the GrO plan but not statistically significant. The mean EQD2 doses to the rectum by PGO plan slightly exceeded the limit from ABS recommendation (mean EQD2 dose = 78.08 Gy EQD2). However, PGO can shorten the treatment planning process without compromising the CI and keeping the OARs dose below the tolerance limit.

Strategies for Motion Robust Proton Therapy With Pencil Beam Scanning for Esophageal Cancer.

Proton therapy of esophageal cancer is superior to photon radiation therapy in terms of normal tissue sparing. However, respiratory motion and anatomical changes may compromise target dose coverage owing to density changes, geometric misses, and interplay effects. Here we investigate the combined effect on clinical target volume (CTV) coverage and compare proton therapy with intensity modulated radiation therapy (IMRT). This study includes 26 patients with esophageal cancer previously treated with IMRT planned on 4-dimensional computed tomography (4D-CT). For each patient, 7 proton pencil beam scanning (PBS) plans were created with different field configurations and optimization strategies. The effect of respiration was investigated by calculating the phase doses, 4D dose, and 4D dynamic dose (including interplay effects). The effect of anatomical changes was investigated by recalculating all plans on all phases of a 4D-CT surveillance scan. The most robust PBS plans were achieved using 2 posterior beams requiring coverage of planning target volume (PTV) and simultaneously using robust optimization (RO) of CTV (2PAPTVRO), resulting in only 1 patient showing V95%CTV <97% in 1 or more phases of the planning CT. For the least robust PBS plans obtained using lateral+posterior beams and CTV-RO, but not requiring PTV coverage (2LPRO), 10 patients showed underdosage. For IMRT, 2 patients showed underdosage. Interplay effects reduced V95%CTV significantly when delivering only 1 fraction, but the effects generally averaged out after 10 fractions. The effect of interplay was significantly larger for RO-only plans compared with plans optimized with RO combined with PTV coverage. Combining the effect of anatomical changes and respiration on the 4D-CT surveillance scan resulted in V95%CTV <97% for 3 2PAPTVRO, 16 2LPRO, and 8 IMRT patients. PBS using posterior beam angles was more robust to anatomical changes and respiration than IMRT. The effect of respiration was enhanced when anatomical changes were present. Single fraction interplay effects deteriorated the dose distribution but were averaged out after 10 fractions.

Treatment plan comparison of proton vs photon radiotherapy for lower-grade gliomas.

Background and purposePatients with lower-grade gliomas are long-term survivors after radiotherapy and may benefit from the reduced dose to normal tissue achievable with proton therapy. Here, we aimed to quantify differences in dose to the uninvolved brain and contralateral hippocampus and compare the risk of radiation-induced secondary cancer for photon and proton plans for lower-grade glioma patients.Materials and methodsTwenty-three patients were included in this in-silico planning comparative study and had photon and proton plans calculated (50.4 Gy(RBE = 1.1), 28 Fx) applying similar dose constraints to the target and organs at risk. Automatically calculated photon plans were generated with a 3 mm margin from clinical target volume (CTV) to planning target volume. Manual proton plans were generated using robust optimisation on the CTV. Dose metrics of organs at risk were compared using population mean dose-volume histograms and Wilcoxon signed-rank test. Secondary cancer risk per 10,000 persons per year (PPY) was estimated using dose-volume data and a risk model for secondary cancer induction.ResultsCTV coverage (V95%>98%) was similar for the two treatment modalities. Mean dose (Dmean) to the uninvolved brain was significantly reduced from 21.5 Gy (median, IQR 17.1–24.4 Gy) with photons compared to 10.3 Gy(RBE) (8.1–13.9 Gy(RBE)) with protons. Dmean to the contralateral hippocampus was significantly reduced from 6.5 Gy (5.4–11.7 Gy) with photons to 1.5 Gy(RBE) (0.4–6.8 Gy(RBE)) with protons. The estimated secondary cancer risk was reduced from 6.7 PPY (median, range 3.3–10.4 PPY) with photons to 3.0 PPY (1.3–7.5 PPY) with protons.ConclusionA significant reduction in mean dose to uninvolved brain and contralateral hippocampus was found with proton planning. The estimated secondary cancer risk was reduced with proton therapy.

PO-1620 Quantification of Incidental Clinical Target Volume Coverage in Linear Accelerator Based Lung SABR

PO-1620 Quantification of Incidental Clinical Target Volume Coverage in Linear Accelerator Based Lung SABR

Feasibility of Conebeam CT-based online adaptive radiotherapy for neoadjuvant treatment of rectal cancer

BackgroundOnline adaptive radiotherapy has the potential to reduce toxicity for patients treated for rectal cancer because smaller planning target volumes (PTV) margins around the entire clinical target volume (CTV) are required. The aim of this study is to describe the first clinical experience of a Conebeam CT (CBCT)-based online adaptive workflow for rectal cancer, evaluating timing of different steps in the workflow, plan quality, target coverage and patient compliance.MethodsTwelve consecutive patients eligible for 5 × 5 Gy pre-operative radiotherapy were treated on a ring-based linear accelerator with a multidisciplinary team present at the treatment machine for each fraction. The accelerator is operated using an integrated software platform for both treatment planning and delivery. In all directions for all CTVs a PTV margin of 5 mm was used, except for the cranial/caudal borders of the total CTV where a margin of 8 mm was applied. A reference plan was generated based on a single planning CT. After aligning the patient the online adaptive procedure started with acquisition of a CBCT. The planning CT scan was registered to the CBCT using deformable registration and a synthetic CT scan was generated. With the support of artificial intelligence, structure guided deformation and the synthetic CT scan contours were adapted by the system to match the anatomy on the CBCT. If necessary, these contours were adjusted before a new plan was generated. A second and third CBCT were acquired to validate the new plan with respect to CTV coverage just before and after treatment delivery, respectively. Treatment was delivered using volumetric modulated arc treatment (VMAT). All steps in this process were defined and timed.ResultsOn average the timeslot needed at the treatment machine was 34 min. The process of acquiring a CBCT, evaluating and adjusting the contours, creating the new plan and verifying the CTV on the CBCT scan took on average 20 min. Including delivery and post treatment verification this was 26 min. Manual adjustments of the target volumes were necessary in 50% of fractions. Plan quality, target coverage and patient compliance were excellent.ConclusionsFirst clinical experience with CBCT-based online adaptive radiotherapy shows it is feasible for rectal cancer.Trial registration Medical Research Involving Human Subjects Act (WMO) does not apply to this study and was retrospectively approved by the Medical Ethics review Committee of the Academic Medical Center (W21_087 # 21.097; Amsterdam University Medical Centers, Location Academic Medical Center, Amsterdam, The Netherlands).

Dosimetric impact of rotational setup errors in volumetric modulated arc therapy for postoperative cervical cancer.

We aimed to evaluate the impact of rotational setup errors on the doses received during postoperative volumetric-modulated arc therapy (VMAT) for cervical cancer. Overall, 121 cone-beam computed tomography (CBCT) sets from 20 patients were rigidly registered to reference computed tomography (CT) sets based on bony landmarks. The rotational setup errors (pitch, yaw and roll) were calculated. Then, 121 CT sets involving rotational setup errors were created, and the dose distribution in these CT sets were recalculated. The recalculated dosimetric parameters for the clinical target volume (CTV) and organs at risk (OAR) were compared to the reference values, and the correlation coefficients between the dosimetric parameter differences and rotational setup errors were calculated. Only the pitch setup error was moderately correlated with CTV coverage (r ≥ 0.40) and strongly correlated with V45 for the bladder (r ≥ 0.91) and V40 for the rectum, small bowel and bone marrow (r ≥ 0.91). The maximum dosimetric difference in a single fraction and overall fractions was −1.59% and −0.69% in D98 for the CTV, 11.72% and 5.17% in V45 for the bladder and −8.03% and −4.68% in V40 for the rectum, respectively. In conclusion, rotational setup errors only slightly impact dose coverage during postoperative cervical cancer VMAT. However, the pitch setup error occasionally affected the doses received by the bladder or the rectum in the overall fraction when the error was systematic. Thus, rotational setup errors should be corrected by adjusting six-degree-of-freedom (DOF) couches to reduce dosimetric differences in the OARs.

CTV-to-PTV margin assessment for esophageal cancer radiotherapy based on an accumulated dose analysis

PurposeThis study aimed to assess the smallest clinical target volume (CTV) to planned target volume (PTV) margins for esophageal cancer radiotherapy using daily online registration to the bony anatomy that yield full dosimetric coverage over the course of treatment. Methods29 esophageal cancer patients underwent six T2-weighted MRI scans at weekly intervals. An online bone-match image-guided radiotherapy treatment of five fractions was simulated for each patient. Multiple conformal treatment plans with increasing margins around the CTV were created for each patient. Then, the dose was warped to obtain an accumulated dose per simulated fraction. Full target coverage by 95% of the prescribed dose was assessed as a function of margin expansion in six directions. If target coverage in a single direction was accomplished, then the respective margin remained fixed for the subsequent dose plans. Margins in uncovered directions were increased in a new dose plan until full target coverage was achieved. ResultsThe smallest set of CTV-to-PTV margins that yielded full dosimetric CTV coverage was 8 mm in posterior and right direction, 9 mm in anterior and cranial direction and 10 mm in left and caudal direction for 27 out of 29 patients. In two patients the curvature of the esophagus considerably changed between fractions, which required a 17 and 23 mm margin in right direction. ConclusionAccumulated dose analysis revealed that CTV-to-PTV treatment margins of 8, 9 and 10 mm in posterior & right, anterior & cranial and left & caudal direction, respectively, are sufficient to account for interfraction tumor variations over the course of treatment when applying a daily online bone match. However, two patients with extreme esophageal interfraction motion were insufficiently covered with these margins and were identified as patients requiring replanning to achieve full target coverage.