Sparing Plans Research Articles

Dosimetric Evaluation of Hippocampus Sparing Intensity Modulated Radiation Therapy in Patients With Stage T1-T2 and Stage T3-T4 Nasopharyngeal Carcinoma

PurposeTo compare the hippocampus (HPC) dose reduced by HPC-sparing IMRT plans between nasopharyngeal carcinoma (NPC) patients of stages T1-T2 and T3-T4, and to investigate the correlation between the dose of the hippocampus and the volume of PTVnx70 (the planning target volume of the primary tumor in the nasopharynx received 70 Gy). Methods and MaterialsFifty-eight NPC patients were retrospectively evaluated. HPC non-sparing IMRT or sparing IMRT for each patient was designed according to the protocol for NPC. Dose-volume histogram (DVH) was used to evaluate the IMRT plans for each patient. The difference values of HPC parameters (e.g., Dmin[NS] - Dmin[S]) between HPC sparing and non-sparing plans in stage T1-T2 Group and stage T3-T4 Group were compared. The correlations between the dose of the hippocampus and the volume of PTVnx70 were analyzed. ResultsThere was no significance between HPC-sparing and non-sparing IMRT plans. Compared with the HPC non-sparing plans, the HPC sparing plans significantly decreased both dosimetric and volumetric parameters for the HPC (P < 0.05), except for Dmin, D98%, and V5. The medians of Dmedian[NS] - Dmedian[S], Dmean[NS] - Dmean[S], D40%[NS] - D40%[S], V30[NS] - V30[S], V40[NS] - V40[S] and V50[NS] - V50[S] in the T1-T2 Group were significantly lower than in the T3-T4 Group (P < 0.05), respectively. Both dosimetric and volumetric parameters for the HPC were positively correlated with the volume of PTVnx70 in HPC-sparing and non-sparing plans (P < 0.05). The volume of PTVnx70 was positively correlated with Dmedian[NS] - Dmedian[S], Dmean[NS] - Dmean[S], D40%[NS] - D40%[S], V40[NS] - V40[S] and V50[NS] - V50[S] (P < 0.05). ConclusionsHPC-sparing IMRT plans may play a more significant role in decreasing Dmedian, Dmean, D40%, and V30 ∼ V50 of HPC in NPC patients with stages T3-T4 than those in stages T1-T2. PTVnx70 volume of NPC patients are positively correlated with all dosimetric and volumetric parameters of HPC and the reduction of specific dosage parameters by HPC-sparing IMRT plans.

Spatiotemporal Optimization of Pelvic Radiation Therapy

Conformal and adaptive Radiation Therapy (RT) remains critical in the treatment of locally advanced pelvic cancers despite acute and late bowel complications. This work introduces spatiotemporal optimization of pelvic RT including mathematical modelling of intestinal radiobiology to evaluate a novel treatment planning approach for fractionated pelvic RT. A single-plan fractionated RT delivery has n-fractions at dose/fraction = d and has biologically effective dose BED = nd ( 1 + d / [α/β] ) - ln(2) ([T - Tk] / [αTp]), where Tk and Tp are the kickoff time and doubling time of repopulation, and T is the time to deliver all fractions (including off-days). We present a parameterization of the repopulation BED-term to estimate a range of daily recovered BED including uncertainties in α and Tp. Recovery of BED as currently formulated is independent of the delivered dose; however, the assumption of spatiotemporal optimization is that a regional reduction of dose during RT by mixing treatment plans is critical to allow for normal tissue healing. We evaluate an achievable spatiotemporal optimization treatment planning strategy for an advanced stage prostate and a cervical cancer patient using 25-fraction delivery over 5-weeks and 4-weekends (T = 33 days) and propose 3 treatment plans: (1) a conformal "clinical standard" plan, (2) a right-bowel sparing plan, and (3) a left-bowel sparing plan. The plans are optimized to ensure no increased dose in opposing bowel or normal organs at risk (OARs). For radiobiological parameters ranging from α = 0.2-0.35/Gy, α/β = 2-3 Gy, Tk = 5-10 days, and Tp = 3-7 days, parameterization shows daily BED recovery of 0.44±0.08 Gy (range = 0.28-0.69 Gy), or an average of 4Gy BED recovery for 5-days treatment surrounded by 2 weekends. Conservatively evaluating the 3.6Gy BED distribution in treatment planning may identify regions for complete recovery (after Tk days). In a prostate 3-plan set, the right- and left- bowel sparing plans deliver equivalent mean dose to targets and OARs to within 5cGy/fx. The clinical plan includes 298cc of bowel under 14Gy, but this same bowel will be spared throughout treatment. Delivery of the clinical plan in week 1 to achieve Tk days, followed by alternating the left and right plan each week has the potential to reduce BED to zero for an additional 116cc of bowel using the left-sparing plan in weeks 2 and 4, and 91cc using the right-bowel sparing plan in weeks 3 and 5. The cervical cancer plan-set was more challenging due to right-sided nodal volumes and higher clinical bowel dose; OAR mean dose variations were >8cGy/fx in a few organs, but the left- and right- sided sparing plans results in 394 cc and 139cc, respectively of unique spared bowel at the 3.6Gy/week threshold to potentially reach zero BED during treatment. A novel spatiotemporal optimization of pelvic RT has the potential to preserve bowel, allow for increased intra-treatment intestinal tissue regeneration, and reduce radiation-induced complications.

Prioritizing Toxicities in Automated Outcome-Directed Treatment Plan Optimization

We recently reported on quality of life (QOL)-guided radiotherapy including NTCP-based treatment plan optimization. This method employs NTCP objective functions, prioritized based on their impact on QOL, instead of conventional dose-based objective functions for individual organs at risk (OAR). We recently expanded this method to include simple models for tumor control probability (TCP) and general plan quality (e.g., hotspot reduction, target dose homogeneity), allowing for fully automated planning. The aim of this study was to test the feasibility of this method and to study predicted toxicity profiles of various plans by prioritizing toxicities differently during treatment plan optimization. Thirty patients, representative of our population receiving definitive (chemo)radiotherapy for head and neck cancer, were selected for this study. Fully automated VMAT treatment plans were created using a mix of NTCP and TCP objective functions. TCP objective functions were based on a PTV D98 ≥ 95% of the prescribed dose. Automated plans were created for each patient using different priorities for 20 toxicities including salivary, swallowing and speech problems, pain and general symptoms. Toxicities were prioritized in different ways: 1) minimizing the impact on QOL; 2) minimizing salivary toxicity; 3) minimizing swallowing related toxicity; 4) equal weights for each toxicity. Manually created clinical VMAT plans were used as reference. Toxicity priorities were easily selected prior to optimization and resulting plans did not need any manual post-processing. Fully automated NTCP-TCP-based optimization consistently resulted in superior plans when compared to the clinical reference plans and always in lower NTCPs for swallowing, speech, pain and general toxicities. Dose reductions in OARs depended on which toxicities were prioritized during plan optimization, e.g., the mean dose in the parotid glands was on average 13 Gy lower in salivary sparing plans compared with plans focused on swallowing sparing, and the average NTCP of salivary toxicities in those plans was 7% lower. However, these reductions were at the cost of higher NTCPs for dysphagia and moderate-to-severe general symptoms such as fatigue. Plans optimized based on the QOL model had the highest QOL scores. Automated outcome-directed treatment plan optimization is feasible and consistently outperforms manually generated VMAT plans in terms of NTCP and QOL scores. Predicted toxicity profiles depend on toxicity priority settings prior to planning. The highest QOL score was obtained when toxicities were prioritized according to relative impact on QOL.

Reducing the Risk of Post-RT Aspiration by Sparing Specific Aspiration Related Organs at Risk

Radiotherapy (RT)-induced aspiration is a serious and potentially life-threatening side effect. We identified functional swallowing units (FSUs), musculature involved in larynx elevation and tongue base retraction (R&O 2019; 130:62). FSUs were strongly related to aspiration. We developed a prediction model with baseline aspiration status and dose in FSUs as factors which performed well in predicting videofluoroscopy assessed RT-induced aspiration, at 6 months. FSUs are not yet recognized as Swallowing Organs at Risk (SWOARs) in daily practice and in treatment plan optimization. In the current study, we aimed to apply the aspiration prediction model in RT treatment plan optimization to actively spare the FSUs and establish the potential to reduce the risk of late RT-induced aspiration. The planning study included 30 HNC patients treated with definitive radiotherapy. We optimized VMAT plans directly on logistic regression models (R&O 2021; 162:85), including normal tissue complication probability (NTCP) models for xerostomia, dysphagia and aspiration, and logistic models for target volume coverage and overall plan quality. Automated plans were created with and without using the NTCP-model for aspiration, i.e., with and without specific sparing of the previously identified FSU, including musculature associated with aspiration. The FSU sparing plans were created with and without assuming baseline aspiration. All NTCP models were weighted equally during optimization. The NTCP of toxicities including salivary, swallowing and speech problems, pain and general symptoms were evaluated for each plan. All plans complied with all treatment planning quality criteria and were successful in limiting the risk of xerostomia and dysphagia. Plans optimized using the NTCP model for aspiration significantly reduced the risk of late aspiration (p < 0.001) and in all 30 patients. The optimizer of the planning system increased its efforts to spare the FSUs even more when baseline aspiration was assumed. Therefore, the aspiration risk reduction was greatest in these plans (average risk was reduced from 73.9 to 60.6%, versus from 37.8 to 29.5% in plans assuming no aspiration at baseline). Aspiration risk reduction by sparing FSUs did not reduce overall plan quality. A slight redistribution of dose was seen as a result of FSU sparing. The largest observed NTCP increase was 0.6% for moderate-to-severe xerostomia. FSU sparing unintentionally reduced the average NTCP of moderate-to-severe hoarseness and speech problems with 2.1% and 1.2%, respectively. Sparing all specific aspiration related organs at risk, including FSUs, during RT has the potential to significantly reduce the risk of RT-induced aspiration, especially in patients with pre-existing baseline aspiration. With automatic planning, FSUs sparing does not increase complexity of the treatment planning process and does not compromise treatment plan quality.

Comparison of non-coplanar optimization of static beams and arc trajectories for intensity-modulated treatments of meningioma cases

Two methods for non-coplanar beam direction optimization, one for static beams and another for arc trajectories, were proposed for intracranial tumours. The results of the beam angle optimizations were compared with the beam directions used in the clinical plans. Ten meningioma cases already treated were selected for this retrospective planning study. Algorithms for non-coplanar beam angle optimization (BAO) and arc trajectory optimization (ATO) were used to generate the corresponding plans. A plan quality score, calculated by a graphical method for plan assessment and comparison, was used to guide the beam angle optimization process. For each patient, the clinical plans (CLIN), created with the static beam orientations used for treatment, and coplanar VMAT approximated plans (VMAT) were also generated. To make fair plan comparisons, all plan optimizations were performed in an automated multicriteria calculation engine and the dosimetric plan quality was assessed. BAO and ATO plans presented, on average, moderate global plan score improvements over VMAT and CLIN plans. Nevertheless, while BAO and CLIN plans assured a more efficient OARs sparing, the ATO and VMAT plans presented a higher coverage and conformity of the PTV. Globally, all plans presented high-quality dose distributions. No statistically significant quality differences were found, on average, between BAO, ATO and CLIN plans. However, automated plan solution optimizations (BAO or ATO) may improve plan generation efficiency and standardization. In some individual patients, plan quality improvements were achieved with ATO plans, demonstrating the possible benefits of this automated optimized delivery technique.

Which Bone Marrow Sparing Strategy and Radiotherapy Technology Is Most Beneficial in Bone Marrow-Sparing Intensity Modulated Radiation Therapy for Patients With Cervical Cancer?

BackgroundTo evaluate the dosimetric parameters of different bone marrow sparing strategies and radiotherapy technologies and determine the optimal strategy to reduce hematologic toxicity associated with concurrent chemoradiation (cCRT) for cervical cancer.MethodsA total of 15 patients with Federation International of Gynecology and Obsterics (FIGO) Stage IIB cervical cancer treated with cCRT were re-planned for bone marrow (BM)-sparing plans. First, we determined the optimal BM sparing strategy for intensity modulated radiotherapy (IMRT), including a BMS-IMRT plan that used total BM sparing (IMRT-BM) as the dose-volume constraint, and another plan used os coxae (OC) and lumbosacral spine (LS) sparing (IMRT-LS+OC) to compare the plan without BM-sparing (IMRT-N). Then, we determined the optimal technology for the BMS-IMRT, including fixed-field IMRT (FF-IMRT), volumetric-modulated arc therapy (VMAT), and helical tomotherapy (HT). The conformity and homogeneity of PTV, exposure volume of OARs, and efficiency of radiation delivery were analyzed.ResultsCompared with the IMRT-N group, the average volume of BM that received ≥10, ≥20, ≥30, and ≥40 Gy decreased significantly in both two BM-sparing groups, especially in the IMRT-LS+OC group, meanwhile, two BMS-IMRT plans exhibited the similar effect on PTV coverage and other organs at risk (OARs) sparing. Among three common IMRT techniques in clinic, HT was significantly less effective than VMAT and FF-IMRT in the aspect of BM-Sparing. Additionally, VMAT exhibited more efficient radiation delivery.ConclusionWe recommend the use of VMAT with OC and LS as separate dose-volume constraints in cervical cancer patients aiming at reducing hematologic toxicity associated with cCRT, especially in developing countries.

Incorporating sensitive cardiac substructure sparing into radiation therapy planning

PurposeRising evidence suggests that cardiac substructures are highly radiosensitive. However, they are not routinely considered in treatment planning as they are not readily visualized on treatment planning CTs (TPCTs). This work integrated the soft tissue contrast provided by low‐field MRIs acquired on an MR‐linac via image registration to further enable cardiac substructure sparing on TPCTs.MethodsSixteen upper thoracic patients treated at various breathing states (7 end‐exhalation, 7 end‐inhalation, 2 free‐breathing) on a 0.35T MR‐linac were retrospectively evaluated. A hybrid MR/CT atlas and a deep learning three‐dimensional (3D) U‐Net propagated 13 substructures to TPCTs. Radiation oncologists revised contours using registered MRIs. Clinical treatment plans were re‐optimized and evaluated for beam arrangement modifications to reduce substructure doses. Dosimetric assessment included mean and maximum (0.03cc) dose, left ventricular volume receiving 5Gy (LV‐V5), and other clinical endpoints. As metrics of plan complexity, total MU and treatment time were evaluated between approaches.ResultsCardiac sparing plans reduced the mean heart dose (mean reduction 0.7 ± 0.6, range 0.1 to 2.5 Gy). Re‐optimized plans reduced left anterior descending artery (LADA) mean and LADA0.03cc (0.0–63.9% and 0.0 to 17.3 Gy, respectively). LV0.03cc was reduced by >1.5 Gy for 10 patients while 6 cases had large reductions (>7%) in LV‐V5. Left atrial mean dose was equivalent/reduced in all sparing plans (mean reduction 0.9 ± 1.2 Gy). The left main coronary artery was better spared in all cases for mean dose and D0.03cc. One patient exhibited >10 Gy reduction in D0.03cc to four substructures. There was no statistical difference in treatment time and MU, or clinical endpoints to the planning target volume, lung, esophagus, or spinal cord after re‐optimization. Four patients benefited from new beam arrangements, leading to further dose reductions.ConclusionsBy introducing 0.35T MRIs acquired on an MR‐linac to verify cardiac substructure segmentations for CT‐based treatment planning, an opportunity was presented for more effective sparing with limited increase in plan complexity. Validation in a larger cohort with appropriate margins offers potential to reduce radiation‐related cardiotoxicities.

Hippocampal sparing radiotherapy in adults with primary brain tumours: a comparative planning and dosimetric study using IMPT, IMRT and 3DCRT.

Abstract Introduction We assessed the feasibility of hippocampal sparing in adults with primary brain tumours using Intensity Modulated Proton Therapy (IMPT) and compared this with Intensity Modulated Radiotherapy (IMRT) and 3D-Conformal Radiotherapy (3DCRT). Methods and Materials 20 patients were identified, and each patient underwent a radiotherapy planning CT scan and 2 MRI scans. A pre-operative diagnostic MRI scan was fused with the planning CT and used for target delineation and a dedicated 3T MRI scan at the time of planning was fused with the CT for hippocampus delineation. 3 hippocampal sparing plans were generated for each patient with specific prescriptions (54Gy/30 fractions, 60Gy/30 fractions and 59.4Gy/33 fractions) using IMPT, IMRT and 3DCRT. Hippocampal sparing was defined as median dose to contralateral hippocampus ≤25Gy without compromising target coverage and organ at risk dose constraints. Results Hippocampal sparing was achieved in 19 patients (95%) with IMPT, 16 patients (80%) with IMRT and 13 patients (65%) with 3DCRT. The largest median hippocampal dose reduction was seen with IMPT, with a mean median hippocampal dose of 4.8Gy (range: 0.0Gy-24.9Gy), 14.6Gy (range: 1.9Gy-21.7Gy), and 16.2Gy (range: 2.3Gy-25.0Gy) for IMPT, IMRT and 3DCRT respectively. Hippocampal sparing IMPT failed in one case with the largest tumour volume (650cc) where 2/3 of the hippocampus overlapped the target volume. Conclusion IMPT as compared to IMRT and 3DCRT plans showed a trend towards significant and effective hippocampal sparing in adult patients with primary brain tumours. We are currently evaluating this in a larger patient cohort and comparing IMPT with VMAT.

Optimal dose limitation strategy for bone marrow sparing in intensity-modulated radiotherapy of cervical cancer

BackgroundTo quantify the dosimetric parameters of different bone marrow sparing strategies and to determine the optimal strategy for cervical cancer patients undergoing postoperative intensity-modulated radiotherapy (IMRT).MethodsFifteen patients with cervical cancer were selected for analysis. The planning target volume (PTV) and the organs at risks (OAR) including small bowel, bladder, rectum, femoral heads, os coxae (OC), lumbosacral spine (LS) and bone marrow (BM) were contoured. For each patient, four IMRT plans with different strategies were generated, including one plan without BM as the dose-volume constraint, namely IMRT (N) plan, and three bone marrow sparing (BMS-IMRT) plans. The three BMS-IMRT plans used the BM, OC, OC and LS respectively, as the BM OAR, namely as IMRT (BM), IMRT (OC) and IMRT (OC + LS) plans. Dose volumes for the target and the OARs were compared using analysis of variance (ANOVA).ResultsCompared with IMRT (N) plans, the dose to the small bowel, bladder, rectum and femoral heads showed no increase in the three BMS-IMRT plans. However, the irradiated dose to BM, OC and LS significantly decreased. In particular, the mean dose of BM, OC and LS decreased by about 5Gy (p < 0.05) in IMRT (BM) plans while the average volume receiving ≥20, ≥30, ≥40Gy decreased by 7.1–24.2%. The LS volume receiving 40Gy showed the highest decrease (about 31.2%, p < 0.05) in IMRT (OC + LS) plans. On the other hand, in comparison with IMRT (BM), IMRT (OC) reduced the dose volume of to the OC, but increased the dose to LS while IMRT (OC + LS) plans reduced both the OC and the LS volume at all dose levels. Specifically, the V20 of OC and LS in the IMRT (OC + LS) plan decreased by 11.5 and 11.2%, respectively.ConclusionBy introducing the os coxae and lumbosacral spine as the dose–volume constraints, the IMRT plans exhibited the best sparing of the bone marrow without compromising the dose to surrounding normal structures. Therefore, we recommend adding the os coxae and lumbosacral spine as the BM OAR in such plans.

Evaluation of organ motion-based robust optimisation for VMAT planning for breast and internal mammary chain radiotherapy

AimsIn patients undergoing locoregional radiotherapy (RT) for breast cancer including the internal mammary chain (IMC), VMAT has been shown to be superior to tangential-field radiotherapy in terms of target coverage and minimising dose to heart and lungs. In this study we describe and validate organ motion-based robust optimisation for generating breast and locoregional lymph node VMAT plans that are robust to inter-fractional changes. Materials and methodsIn this retrospective study of five patients with left-sided breast cancer requiring locoregional breast radiotherapy including the IMC, non-robust plans were generated in the nominal scenario (planning-CT) and corresponding robust plans were created by optimising over a range of simulated CTs representing worst-case scenario shape changes to the breast. Both plans were re-calculated on CBCT images (n = 67) acquired prior to RT to generate estimates of delivered fractional dose. Plan robustness to inter-fractional changes was assessed in terms of the estimated target coverage and OAR dose. ResultsOrgan motion-based robust optimisation was able to generate clinically acceptable treatment plans in the nominal scenario on the planning CT with no significant differences to OAR dose between the robust and non-robust planning techniques. All plans (robust and non-robust) achieved the mandatory target coverage requirements. Estimates of delivered dose demonstrated a significant improvement in breast target coverage for the robust plans compared to non-robust plans. For the breast CTV, 92% of the robust plans achieved the optimal D98% > 95% clinical goal as compared to 71% of the non-robust plans (p < 0.01). 94% of robust plans achieved acceptable superficial breast coverage, as compared to 55% for the non-robust technique. ConclusionsOrgan motion-based robust optimisation VMAT is able to produce clinically acceptable organ-at-risk sparing plans for locoregional breast radiotherapy (including the IMC) that are robust to inter-fractional changes, therefore reducing the likelihood of reactive adaptive re-planning.

Utilizing skin sparing technique in HN VMAT treatment planning

This study aims to demonstrate a specific treatment planning method and its effectiveness of sparing high dose to superficial skin for head and neck volumetric-modulated arc therapy (VMAT) treatments. A total of 10 patients with bilateral disease encompassing superficial neck nodes were planned as VMAT. Standard VMAT plans were created for each patient per written directive from physician, including target prescriptions, normal structure goals, and plan parameters. After these plans were created, low isodose levels were converted into dose structures, which are used to create a new skin sparing (SS) structure. With the new SS structure made, the original plan was copied to create a new SS plan. The new SS plan was re-optimized utilizing the newly created SS structure to help in decreasing dose to the superficial regions on the skin surface. The result of this treatment planning technique, comparing SS plans to original nonskin sparing (NSS) plans, provides a reduced dose of 10% to 15% to the skin surface in all cases. Although this technique reduces the dose to the skin, it can also reduce the dose to treatment volumes. Depending on physician intent, diagnosis, and disease, this method should only be used when the skin is not at risk, otherwise it could potentially cause underdosing to the disease site. All SS plans were considered clinically acceptable and provided adequate coverage to planning target volume (PTV), while reducing unwarranted high dose to superficial skin tissue.

Hippocampal sparing in stereotactic radiotherapy for brain metastases: To contour or not contour the hippocampus?

PurposeThe aim of our study was to evaluate hippocampal irradiation in patients treated with fractionated stereotactic brain radiotherapy. Patients and methodsRetrospective hippocampal dosimetric analysis performed on 22 patients with one to four brain metastases treated with fractionated stereotactic radiotherapy using volumetric intensity-modulated arc therapy. Original plans did not include hippocampus as avoidance structure in optimization criteria; hippocampus was retrospectively delineated on magnetic resonance coregistered with planning CT and using as reference the RTOG 0933 atlas. Hippocampus was defined both as a single and as pair organ. Constraints analysed were: Dmax<16Gy, D40%<7.3Gy, D100%=Dmin<9Gy. Assuming a α/β ratio of 2Gy, biologically equivalent dose in 2Gy fractions was calculated. Hippocampal-sparing plans were developed in cases where hippocampal constraints were not respected in the original plan. ResultsAmong constraints analysed Dmax and D40% have been exceeded in ten out of 22 cases. The constraints were not respected in patients with more than one metastatic lesion and in three patients with only one lesion. Considering all exceeded constraints values in non-hippocampal sparing plans, the 50% of them was respected after replanning. No significant differences were found among conformity and homogeneity index between non-hippocampal sparing and hippocampal sparing plans. ConclusionVolumetric intensity-modulated arc therapy hippocampal sparing plans significantly decreases dose to hippocampus assuring an equal target coverage and organs at risk avoiding.

Neurovascular bundle-sparing radiotherapy for prostate cancer using MRI-CT registration: A dosimetric feasibility study.

128 Background: Sexual dysfunction after radiotherapy for prostate cancer remains an important late adverse toxicity. The neurovascular bundles (NVB) that lie postero-laterally to the prostate are typically spared during prostatectomy, but in traditional radiotherapy planning they are not contoured as an organ-at-risk with dose constraints. Our goal was to determine the feasibility in sparing the NVB while still delivering adequate dose to the prostate. Methods: Twelve consecutive patients with intermediate- or high-risk prostate cancer (with no gross disease within 2mm of either NVB) who were treated with external beam radiotherapy, with the same primary PTV margins, to a dose of 79.2 Gy were evaluated. Pelvic MRI and simulation CT scans were registered using dedicated software to allow for bilateral NVB target delineation on T2-weighted MRI. A volumetric modulated arc therapy (VMAT) plan was generated using the NVB bilaterally with 2mm margin as an organ to spare (with goal of reducing V70Gy by 30%) and compared to the patient’s delivered plan. Dose-volume histogram (DVH) endpoints for NVB, rectum, bladder, and PTV79.2 were compared between the two plans using a 2-tailed paired t-test. Results: The V70 for the NVB was significantly lower on the NVB sparing plan (p &lt;0.01), while rectum and bladder endpoints were similar. Target V100% was lower (but still &gt; 95%), and V105 was higher for the NVB Sparing plans (p &lt;0.01). The table summarizes the DVH comparisons between the two plans. Conclusions: “NVB-sparing” radiotherapy is dosimetrically feasible using VMAT technology; target coverage is acceptable without increased dose to other normal structures, but with higher target dose inhomogeneity. The clinical impact of “NVH-sparing” radiotherapy is currently under study at our institution. [Table: see text]

Treatment planning strategy for whole-brain radiotherapy with hippocampal sparing and simultaneous integrated boost for multiple brain metastases using intensity-modulated arc therapy

PurposeTo retrospectively evaluate the accuracy, plan quality and efficiency of intensity-modulated arc therapy (IMAT) for hippocampal sparing whole-brain radiotherapy (HS-WBRT) with simultaneous integrated boost (SIB) in patients with multiple brain metastases (m-BM). Materials and methodsA total of 5 patients with m-BM were retrospectively replanned for HS-WBRT with SIB using IMAT treatment planning. The hippocampus was contoured on diagnostic T1-weighted magnetic resonance imaging (MRI) which had been fused with the planning CT image set. The hippocampal avoidance zone (HAZ) was generated using a 5-mm uniform margin around the paired hippocampi. The m-BM planning target volumes (PTVs) were contoured on T1/T2-weighted MRI registered with the 3D planning computed tomography (CT). The whole-brain planning target volume (WB-PTV) was defined as the whole-brain tissue volume minus HAZ and m-BM PTVs. Highly conformal IMAT plans were generated in the Eclipse treatment planning system for Novalis-TX linear accelerator consisting of high-definition multileaf collimators (HD-MLCs: 2.5-mm leaf width at isocenter) and 6-MV beam. Prescription dose was 30Gy for WB-PTV and 45Gy for each m-BM in 10 fractions. Three full coplanar arcs with orbit avoidance sectors were used. Treatment plans were evaluated using homogeneity (HI) and conformity indices (CI) for target coverage and dose to organs at risk (OAR). Dose delivery efficiency and accuracy of each IMAT plan was assessed via quality assurance (QA) with a MapCHECK device. Actual beam-on time was recorded and a gamma index was used to compare dose agreement between the planned and measured doses. ResultsAll 5 HS-WBRT with SIB plans met WB-PTV D2%, D98%, and V30Gy NRG-CC001 requirements. The plans demonstrated highly conformal and homogenous coverage of the WB-PTV with mean HI and CI values of 0.33 ± 0.04 (range: 0.27 to 0.36), and 0.96 ± 0.01 (range: 0.95 to 0.97), respectively. All 5 hippocampal sparing patients met protocol guidelines with maximum dose and dose to 100% of hippocampus (D100%) less than 16 and 9Gy, respectively. The dose to the optic apparatus was kept below protocol guidelines for all 5 patients. Highly conformal and homogenous radiosurgical dose distributions were achieved for all 5 patients with a total of 33 brain metastases. The m-BM PTVs had a mean HI = 0.09 ± 0.02 (range: 0.07 to 0.19) and a mean CI = 1.02 ± 0.06 (range: 0.93 to 1.2). The total number of monitor units (MU) was, on average, 1677 ± 166. The average beam-on time was 4.1 ± 0.4 minute . The IMAT plans demonstrated accurate dose delivery of 95.2 ± 0.6%, on average, for clinical gamma passing rate with 2%/2-mm criteria and 98.5 ± 0.9%, on average, with 3%/3-mm criteria. ConclusionsAll hippocampal sparing plans were considered clinically acceptable per NRG-CC001 dosimetric compliance criteria. IMAT planning provided highly conformal and homogenous dose distributions for the WB-PTV and m-BM PTVs with lower doses to OAR such as the hippocampus. These results suggest that HS-WBRT with SIB is a clinically feasible, fast, and effective treatment option for patients with a relatively large numbers of m-BM lesions.

Evaluation of the Dosimetric Feasibility of Hippocampal Sparing Intensity-Modulated Radiotherapy in Patients with Locally Advanced Nasopharyngeal Carcinoma

PurposeThe objective of this study was to evaluate the dosimetric feasibility of using hippocampus (HPC) sparing intensity-modulated radiotherapy (IMRT) in patients with locally advanced nasopharyngeal carcinoma (NPC).Materials/MethodsEight cases of either T3 or T4 NPC were selected for this study. Standard IMRT treatment plans were constructed using the volume and dose constraints for the targets and organs at risk (OAR) per Radiation Therapy Oncology Group (RTOG) 0615 protocol. Experimental plans were constructed using the same criteria, with the addition of the HPC as an OAR. The two dose-volume histograms for each case were compared for the targets and OARs.ResultsAll plans achieved the protocol dose criteria. The homogeneity index, conformity index, and coverage index for the planning target volumes (PTVs) were not significantly compromised by the avoidance of the HPC. The doses to all OARs, excluding the HPC, were similar. Both the dose (Dmax, D2%, D40%, Dmean, Dmedian, D98% and Dmin) and volume (V5, V10, V15, V20, V30, V40 and V50) parameters for the HPC were significantly lower in the HPC sparing plans (p<0.05), except for Dmin (P = 0.06) and V5 (P = 0.12).ConclusionsIMRT for patients with locally advanced NPC exposes the HPC to a significant radiation dose. HPC sparing IMRT planning significantly decreases this dose, with minimal impact on the therapeutic targets and other OARs.

Preventive sparing of spinal cord and brain stem in the initial irradiation of locally advanced head and neck cancers

Since reirradiation in recurrent head and neck patients is limited by previous treatment, a marked reduction of maximum doses to spinal cord and brain stem was investigated in the initial irradiation of stage III/IV head and neck cancers. Eighteen patients were planned by simultaneous integrated boost, prescribing 69.3 Gy to PTV1 and 56.1 Gy to PTV2. Nine 6 MV coplanar photon beams at equispaced gantry angles were chosen for each patient. Step‐and‐shoot IMRT was calculated by direct machine parameter optimization, with the maximum number of segments limited to 80. In the standard plan, optimization considered organs at risk (OAR), dose conformity, maximum dose <45 Gy to spinal cord and <50 Gy to brain stem. In the sparing plans, a marked reduction to spinal cord and brain stem were investigated, with/without changes in dose conformity. In the sparing plans, the maximum doses to spinal cord and brain stem were reduced from the initial values (43.5±2.2 Gy and 36.7±14.0 Gy), without significant changes on the other OARs. A marked difference (−15.9±1.9 Gy and −10.1±5.7 Gy) was obtained at the expense of a small difference (−1.3%±0.9%) from initial PTV195% coverage (96.6%±0.9%). Similar difference (−15.7±2.2 Gy and −10.2±6.1 Gy) was obtained compromising dose conformity, but unaffecting PTV195% and with negligible decrease in PTV295% (−0.3%±0.3% from the initial 98.3%±0.8%). A marked spinal cord and brain stem preventive sparing was feasible at the expense of a decrease in dose conformity or slightly compromising target coverage. A sparing should be recommended in highly recurrent tumors, to make potential reirradiation safer.PACS number: 87.55.D

SU-E-T-558: Assessing the Effect of Inter-Fractional Motion in Esophageal Sparing Plans.

To compare esophageal dose distributions in esophageal sparing IMRT plans with predicted dose distributions which include the effect of inter-fraction motion. Seven lung cancer patients were used, each with a standard and an esophageal sparing plan (74Gy, 2Gy fractions). The average max dose to esophagus was 8351cGy and 7758cGy for the standard and sparing plans, respectively. The average length of esophagus for which the total circumference was treated above 60Gy (LETT60) was 9.4cm in the standard plans and 5.8cm in the sparing plans. In order to simulate inter-fractional motion, a three-dimensional rigid shift was applied to the calculated dose field. A simulated course of treatment consisted of a single systematic shift applied throughout the treatment as well a random shift for each of the 37 fractions. Both systematic and random shifts were generated from Gaussian distributions of 3mm and 5mm standard deviation. Each treatment course was simulated 1000 times to obtain an expected distribution of the delivered dose. Simulated treatment dose received by the esophagus was less than dose seen in the treatment plan. The average reduction in maximum esophageal dose for the standard plans was 234cGy and 386cGY for the 3mm and 5mm Gaussian distributions, respectively. The average reduction in LETT60 was 0.6cm and 1.7cm, for the 3mm and 5mm distributions respectively. For the esophageal sparing plans, the average reduction in maximum esophageal dose was 94cGy and 202cGy for 3mm and 5mm Gaussian distributions, respectively. The average change in LETT60 for the esophageal sparing plans was smaller, at 0.1cm (increase) and 0.6cm (reduction), for the 3mm and 5mm distributions, respectively. Interfraction motion consistently reduced the maximum doses to the esophagus for both standard and esophageal sparing plans.

Coal seam gas production: the operational and maintenance challenges

Compared to conventional gas, coal seam gas (CSG) production introduces additional complexities in operation due to the remoteness, number and distribution of the producing wells, the quantity of associated water and the variability in characteristics of each well. The potential irregularity in gas production is further highlighted when integrating with relatively steady state downstream facilities such as LNG production and power generation. Margins are particularly sensitive to operating cost because of the remoteness and distributed nature of the producing assets. Cost effective and reliable production of gas to the flow tolerances needed downstream requires that operations management, ongoing field development, asset maintenance and regulatory compliance be systematic and highly integrated. Experience shows that for operations reliant on distributed and remote assets, including CSG, that the ratio of predictive/preventive to corrective maintenance correlates directly to surety of gas production, with little margin to compensate for less than required reliability or low maintenance productivity. For CSG production it is difficult to avoid significant lost production as a consequence of higher levels of unplanned asset downtime. The remoteness, number of assets and operating cost constraints simply makes it difficult to overcompensate for lower reliability. Corrective actions are significantly more expensive when compared with centrally located assets because of the heightened pressure to correct in the shortest possible time. Further, deterioration of the maintenance regime compounds the effect as the emerging backlog of critical issues reduces the ability of crews to keep up with the preventative maintenance programme. Achieving a reliable outcome in CSG operations is therefore more reliant on a robust operations and maintenance management arrangement that also effectively incorporates the ongoing discrete activities of drilling, well completions, start-up and abandonment. To achieve these outcomes operators must ensure: the early development of maintenance philosophies, systems and processes during the project phase that consider the frequency of field variations for life of asset; a well planned system start up, commissioning and transition into first production; and, a constant focus on efficient deployment of resources, fit for purpose reliability and maintenance routines, logistics support and sparing plans, use of appropriate remote monitoring technologies and an incorporated regulatory compliance management plan. In its presentation PSN will demonstrate, through case study examples from its operations supporting coal seam gas and gas production assets in the United States and distributed and remote assets for the Western Australian Water Corporation, the challenges and management focal points critical to effective CSG production. The case studies will demonstrate the benefits achieved by PSN’s operations and maintenance management approach in the following areas: operations and maintenance readiness; distributed asset management; managing field development and modifications; implementing high performance maintenance; environmental monitoring and compliance; resource utilisation models; and, workforce training and development

The operational status of the IMS radionuclide particulate network

A worldwide radionuclide network of 80 stations, 40 of which with noble-gas-detection capability, and part of the International Monitoring System (IMS) has been designed to monitor compliance to the Comprehensive Nuclear-Test-Ban Treaty (CTBT). Pending entry into force of the Treaty, the certified stations are provisionally operated. With about 70% of the network operational global coverage has been reached and an overall operating experience of more than 250 station year for the particulate network has been accumulated. To ensure minimum down time any operational problems are addressed within the organizations’ Operations and Maintenance Support System. Within this support system the operational performance is continuously assessed as well as particular problems identified. To address these problems, equipment failures, operational procedures, maintenance and sparing plans, and equipment obsolescence are continuously reviewed and improved. The current operational status of the network as well as the support strategy is presented.