The effect of positional realignment on dose delivery to the prostate and organs-at-risk for 3DCRT.

In this study, we evaluate the impact of daily image-guided patient repositioning on dose delivery to prostate and sensitive organs in the treatment of prostate carcinoma with 3-dimensional conformal radiation therapy (3DCRT). Five patients with substantial ultrasound-documented interfractional prostate motion during their 3DCRT treatment course were selected. Starting with the original treatment plan, 2 additional plans were retrospectively generated for each patient. In one set, organ contours were moved for each fraction, thus simulating positioning with misalignment caused by organ motion if ultrasound guidance were not used. In a second set of plans, the isocenter was shifted, as were the organ contours, simulating realignment based on the ultrasound image. In all cases, the number of planned monitor units was set to those of the original plan. For a given patient, isodose distributions, dose-volume histograms (DVHs), equivalent uniform dose (EUD) for prostate, and generalized equivalent uniform dose (gEUDs) for bladder and rectum were calculated for each fraction and then combined for each shift condition. In all reconstructed plans, the results show no substantial changes in dose coverage of the prostate <0.21% change in EUD) compared to the original plan. However, in some cases with no realignment, a larger volume of the bladder or rectum gets higher dose, with the consequent gEUD for each organ significantly greater compared to the original plan.     

Is simulation necessary for each high-dose-rate tandem and ovoid insertion in carcinoma of the cervix?

PURPOSE: To evaluate the dose variation in high-dose-rate (HDR) intracavitary brachytherapy for cancer of the cervix when treatment planning is performed prior to each applicator insertion versus when the initial plan is used for each treatment. METHODS AND MATERIALS: Fourteen patients with carcinoma of the cervix were treated with chemoradiotherapy followed by five intracavitary tandem and ovoid insertions of 600 cGy/fraction. We modified the actual plans to calculate the dose each dose point would have received using only the treatment plan created for the initial fraction. RESULTS: An increase in the percent dose to the rectum, bladder, and vaginal surface of 5%, cGy (p = 0.038), 6% (p = 0.006), and 11%, respectively, were observed when the initial treatment plan was used versus using the optimized treatment plan for each insertion. The greatest single change resulted in a percent increase of 35%, 30%, and 45% to the rectum, bladder, and vaginal surface points, respectively. CONCLUSIONS: Increased dose to at-risk structures occurred when individualized treatment planning was not performed. Since a significant increase in dose to the rectum (p = 0.038) and bladder (p = 0.006) was obtained without customized treatment planning, we continue to advocate individualized treatment planning in HDR tandem and ovoid insertions for the treatment of cervix cancer.     

Exploring the effect of marked normal structure volume on normal tissue complication probability

Radiation therapy dosimetry software now frequently incorporates biological predictions of the probability of normal tissue complications. This study investigates whether the length of normal structure outlined affects a normal tissue complication probability (NTCP) for that structure. It also researches the effect of any change in the dose parameter used to produce a 50% probability of a complication (the TD⁵⁰) on the calculated NTCP, as this is related to the clinical observations. An NTCP was calculated for rectum and bladder on a sample of prostate cases receiving external beam radiation therapy. The length of the organs at risk was varied and the NTCP recalculated for each different length using the same treatment plan. Large variations of up to 80% in NTCP for different delineated lengths of organ for a given TD⁵⁰ were observed. Changing the TD⁵⁰ dose altered the calculated NTCP and the relative size of the variation in the values. This parameter will need further investigation; a standardized delineated length of 2 cm beyond the beam edge for normal structures is recommended. Interpatient and interinstitution plan comparison using dose volume histograms and/or normal tissue complication probabilities will be compromised until such standardization occurs.     

A single plan approach for differentially dosing sequential target volumes

An in-house protocol for treatment of malignant astrocytomas requires development of a single treatment plan constructed to deliver different doses to three sequential target volumes. This single plan approach fundamentally differs from a previous protocol in which these sequential volumes were approached with three consecutive treatment plans, each tailored to a separate target, with the final target receiving a cumulative dose of 80 Gy. The intent of the revised protocol is to deliver doses to the two larger targets that are biologically equivalent (using the linear quadratic model) to the cumulative doses received by these targets in the earlier protocol, while escalating the final target dose to 90 Gy. This requires the treatment planner to manipulate the conformation of three different isodose levels simultaneously to produce a treatment plan fulfilling all protocol specifications. This paper will focus on the evolution of design for the current technique used to clinically implement this protocol.     

Photon optimizer (PO) vs progressive resolution optimizer (PRO): a conformality- and complexity-based comparison for intensity-modulated arc therapy plans

This study aimed to provide guidance on the advantages and limitations of a new optimizer, “photon optimizer” (PO), when compared with its predecessor, “progressive resolution optimizer” (PRO), for intensity-modulated arc therapy (IMAT) plans.Eleven study plans that included a cohort of prostate, head and neck, and brain treatment sites were optimized using both PRO and PO algorithms. A plan template using the same objectives for the same number of iterations was used for each optimized plan to obtain hypothetical treatment plans that would be comparable with a clinical plan. Analysis was performed using plan conformity-based parameters such as target volume coverage factor, conformation number and homogeneity indices, and plan complexity assessment parameters such as small aperture score, modulation indices, and monitor unit variation with arc angle for prostate, brain and head, and neck IMAT treatment plans.Plan conformality analysis demonstrated that conformation numbers, target volume coverage factors, and homogeneity indices produced by the 2 optimizers were comparable for most anatomic sites. IMAT treatment plans produced using the PRO optimizer were found to be less complex than plans produced using the PO optimizer, in terms of multileaf collimator (MLC) leaf position variability and modulation complexity scores. Similarly, the PRO optimizer was shown to produce treatment plans that used fewer monitor units (and generally fewer monitor unit per degree of arc rotation) than PO optimizer.This study demonstrated that the PO optimizer can produce IMAT treatment plans with a similar degree of dose conformity to the target volume and generally improved organ at risk sparing, compared with the PRO optimizer. Better coverage to organs at risk produced by plans optimized using PO was observed to have higher MLC variability and monitor units. Therefore, careful evaluation of treatment plan conformity and complexity before assessing its deliverability is recommended when implementing the routine use of PO optimizer.     

Independent dose calculations for the PEACOCK System.

An independent dose calculation method has been developed to validate intensity-modulated radiation therapy (IMRT) plans from the NOMOS PEACOCK System. After the plan is generated on the CORVUS planning system, the beam parameters are imported into an independent workstation. The beam parameters consist of intensity maps at each gantry angle and each arc position. In addition, CT scans of the patient are imported into the independent workstation to obtain the external contour of the patient. The coordinate system is defined relative to the alignment point chosen in the CORVUS plan. The independent calculation uses the pencil beam data viz tissue maximum ratio (TMR) and beam profiles for a single 1 x 0.8-cm beamlet formed by the NOMOS multileaf intensity-modulating collimator (MIMiC) leaf. The pencil beam data were measured for the 6-MV photon beam from Siemens PRIMUS linear accelerator using film dosimetry. The dose at a point is calculated using the depth and off-axis distance from a given pencil beam, corrected for its beam intensity. Isodose distributions are generated using the independent dose calculations and compared to the CORVUS plans. Isodose distributions show good agreement with the CORVUS plans for a number of clinical cases. The independent dose calculation algorithm is described in this paper.     

Mantle fields in the era of dynamic multileaf collimation: field shaping and electronic tissue compensation.

Mantle field radiotherapy for Hodgkin's disease is complicated by significant dose gradient (up to 10-20%) across the large fields required. Many different strategies of tissue compensation have been investigated, including custom physical compensators to provide better dose distributions. We present a method using dynamic multileaf collimator (dMLC) fluence modulation to simultaneously shape the treatment field and give homogeneous dose at depth throughout the classic mantle field. Five patients were treated for early-stage Hodgkin's disease with a conventional anterior-posterior-posterior-anterior (AP-PA) mantle field. The patients were planned using the Varian Eclipse treatment planning system, version 6.1.3, and treated on a Varian 2300CD. An AP-PA dynamic MLC beam-shaped and dose-compensated plan was created for each, and compared with the conventional blocked plan. Nine dose points were calculated at midplane in each plan. Chamber measurements were taken to confirm accurate dMLC delivery of the planned doses. The mean dose per fraction, relative to a central axis dose of 1.8 Gy, was increased in the conventional plans compared with the dMLC plans in the right (R) neck, left (L) neck, R supraclavicular, L supraclavicular, and L axillary points. The mediastinum tended to be underdosed relative to central axis, with the mid-mediastinal and lower mediastinal points showing improved coverage with the dMLC plans. Measurements showed excellent agreement between planned doses and delivered doses, with less than 2% in-field variation. Dynamic MLC fluence modulation was used to effectively deliver a mantle field that is both shape- and electronically-dose-compensated with sliding window MLC. Homogeneity was significantly improved throughout the treatment field, and measurements confirmed accurate dose delivery using this technique.     

非小细胞肺癌不同调强放疗方案的剂量学对比研究

目的 探讨非小细胞肺癌调强放疗计划设计的合理方案。方法 对11例非小细胞肺癌患者分别制定2种放疗计划:PTV60计划的PTV为(GTV+6～8mm)+呼吸动度+摆位误差,对PTV获得60Gy处方剂量进行归一;PTV70计划的PTV为GTV+呼吸动度+摆位误差,对PTV获得70Gy处方剂量进行归一。通过剂量体积直方图分析2种治疗计划的靶区剂量分布和危及器官受量,并进行剂量学的对比研究。结果 PTV70计划接受60Gy剂量的靶区体积明显高于PTV60计划,两组在靶区剂量均匀性方面相似。PTV70计划的肺V₂₀较PTV60计划平均下降(1.69±0.42)%,两组相比差异有统计学意义(t=0.047,P=0.002);肺V₅平均下降(1.29±1.09)%,两组相比差异无统计学意义。结论 在非小细胞肺癌调强放疗设计中,PTV70计划优于PTV60计划。     

Various cost functions evaluation of commercial biologically based treatment planning system for nasopharyngeal cancer

The aim of this study was to evaluate various combinations of Equivalent Uniform Dose (EUD) based and Dose Volume based (DV) cost functions in terms of target coverage and organ sparing for Nasopharyngeal CA. Ten patients diagnosed with Nasopharyngeal CA were selected for this retrospective study. Different hybrid VMAT plans, including EUD- and DV-based cost functions, were generated for each patient to determine the optimum combination in terms of organ sparing and target coverage. The generated VMAT plans were evaluated based on physical and biological dose parameters. No statistical difference was observed among all plans in terms of target coverage. The p values were ≥ 0.005 for V95, D_mean, and tumor control probability (TCP). The MU efficiency was maximum (67%), and the number of segments (285 segments) was minimum in Hybrid plan. Hybrid plan showed a significant difference compared to others (p = 0.001) in terms of serial organs. Moreover, the combination of serial and parallel complication models provided better reduction of radiation dose in the parotid glands in Plan-3 (p = 0.001). In this study, better protection was obtained when DVH-based cost functions were defined for targets and a combination of EUD- and DVH-based cost functions were used for OARs.     

Comparative evaluation of treatment plan quality for a prototype biology-guided radiotherapy system in the treatment of nasopharyngeal carcinoma

We aimed to compare prototype treatment plans for a new biology-guided radiotherapy (BgRT) machine in its intensity-modulated radiation therapy (IMRT) mode with those using existing IMRT delivery techniques in treatment of nasopharyngeal carcinoma (NPC). We retrospectively selected ten previous NPC patients treated in 33 fractions according to the NRG-HN001 treatment protocol. Three treatment plans were generated for each patient: a helical tomotherapy (HT) plan with a 2.5-cm jaw, a volumetric modulated arc therapy (VMAT) plan using 2 to 4 6-MV arc fields, and a prototype IMRT plan for a new BgRT system which uses a 6-MV photon beam on a ring gantry that rotates at 60 rotations per minute with a couch that moves in small incremental steps. Treatment plans were compared using dosimetric parameters to planning target volumes (PTVs) and organs at risk (OARs) as specified by the NRG-HN001 protocol. Plans for the three modalities had comparable dose coverage, mean dose, and dose heterogeneity to the primary PTV, while the prototype IMRT plans had greater dose heterogeneity to the non-primary PTVs, with the average homogeneity index ranging from 1.28 to 1.50 in the prototype plans. Six of all the 7 OAR mean dose parameters were lower with statistical significance in the prototype plans compared to the HT and VMAT plans with the other mean dose parameter being comparable, and all the 18 OAR maximum dose parameters were comparable or lower with statistical significance in the prototype plans. The average left and right parotid mean doses in the prototype plans were 10.5 Gy and 10.4 Gy lower than those in the HT plans, respectively, and were 5.1 Gy and 5.2 Gy lower than those in the VMAT plans, respectively. Compared to that with the HT and VMAT plans, the treatment time was longer with statistical significance with the prototype IMRT plans. Based on dosimetric comparison of ten NPC cases, the prototype IMRT plans achieved comparable or better critical organ sparing compared to the HT and VMAT plans for definitive NPC radiotherapy. However, there was higher dose heterogeneity to non-primary targets and longer estimated treatment time with the prototype plans.     

A comparison of conventional and conformal radiotherapy of the oesophagus: work in progress

A retrospective treatment planning study was carried out in five patients to assess the effectiveness of conformal radiotherapy of the oesophagus. A two-phase conventional treatment plan was created for each patient, with a prescribed dose of 55 Gy. This plan was compared with a single-phase conformal plan consisting of the same field arrangement as the second phase of the conventional treatment, but with conformal blocks shaped to the beam's eye view of the planning target volume. A further comparison was made between the conventional plan and a two-phase plan using the same beam angles and weights as the conventional plan, but with conformal field shapes. The effectiveness of each treatment plan was assessed using dose--volume histograms and normal tissue complication probabilities for the lungs. On average, the single-phase conformal technique increased the mean lung dose from 22.5% (+/- 6.2 SD) of the prescribed dose to 29.5% (+/- 5.2 SD) compared with the conventional technique (p = 0.0001). This indicates that this technique did not offer any benefit in terms of reducing the risk of pneumonitis. However, the two-phase conformal technique reduced the mean lung dose from 22.5% (+/- 6.2 SD) of the prescribed dose to 19.8% (+/- 4.6 SD)(p = 0.03), showing that this technique should reduce the risk of pneumonitis. Further work is underway to study more patients and to investigate tumour control probability and dose escalation.     

Hippocampal-sparing whole-brain radiotherapy under coplanar or noncoplanar VMAT

Whole-brain radiotherapy (WBRT) can alleviate symptoms in patients with brain metastases. However, WBRT may damage the hippocampus. Volumetric modulated arc therapy (VMAT) can achieve a suitable coverage of the target region and a more conforming dose distribution whereas decreasing the dose to organs-at-risk (OARs). Herein, we aimed to compare the differences between treatment plans utilizing coplanar VMAT and noncoplanar VMAT in hippocampal-sparing WBRT (HS-WBRT). Ten patients were included in this study. For each patient, the Eclipse A10 treatment planning system was used to generate 1 coplanar VMAT (C-VMAT) and 2 noncoplanar VMAT treatment plans with various beam angles (noncoplanar VMAT A [NC-A] and noncoplanar VMAT B [NC-B]) for HS-WBRT. The prescribed dose was 30 Gy in 12 fractions. Treatment plans were established based on the OAR dose constraints of the Radiation Therapy Oncology Group 0933 (RTOG 0933). Parameters such as the global maximum dose, dose conformity, dose homogeneity of plans, and OAR doses were evaluated. The maximum biologically equivalent doses in 2-Gy fractions (EQD2) of OARs in C-VMAT were 9.17 ± 0.61, 42.79 ± 2.00, and 42.84 ± 3.52 Gy in the hippocampus, brain stem, and optic chiasm, respectively, which were the lowest among the 3 treatment plans. There was no significant difference in dose conformity among the 3 treatment plans. However, NC-A had a slightly better conformity than C-VMAT and NC-B. NC-A had the best homogeneity, and NC-B had the worst homogeneity (p = 0.042). NC-A and NC-B had the lowest and highest global dose maximum, respectively. Therefore, NC-A, which had an intermediate performance in terms of OAR doses, had the best quality parameters. We used the quality score table based on the p-value to evaluate the significant difference between each treatment technique from the multiparameter results. In terms of treatment plan parameters, only NC-A received a score of 2; for OAR doses, C-VMAT, NC-A, and NC-B received a score of 6, 3, and 5, respectively. For the overall evaluation, C-VMAT, NC-A, and NC-B received a total score of 6, 5, and 5, respectively. Rather than noncoplanar VMAT, 3 full-arc C-VMATs should be utilized in HS-WBRT. C-VMAT can simultaneously maintain treatment plan quality and decrease patient alignment time and total treatment time.     

Automatic organ contour check: One essential step in autonomous treatment planning

Geometric and nomenclature errors are commonly encountered in automated treatment planning. We describe a novel algorithm to extract organ geometry relationships from patient structure DICOM data to construct a database that can be used to detect organ contour inaccuracies including relational and naming errors. Twenty-five sets of head and neck patients’ treatment plan data (CT, structures) were retrospectively retrieved from our institution. For each dataset, various organs were contoured and verified by experienced physicians. The relative position and orientation between organs were extracted from each patient and the data were used to construct an organ relationship database model. The model was tested using a dataset originating from an in-house organ renaming software that often-introduced organ contour naming mismatches. As part of the validation test, the renamed organs relative positions were compared with the database model to identify mismatches. Within the forty head and neck patients, we extracted the geometric relationship between 201 organ pairs. The average number of unique types of organ pairs (for example, left parotid with left eye is one type of organ pair) stored in the database was 12. Fifteen head and neck structure sets automatically renamed using our in-house organ renaming tool was used as validation data. All of the 30 random assigned wrong name labels present in these structure sets were identified using the established organ geometry relationship database. We successfully constructed a head and neck organ geometry relationship database and validated it in a contour naming quality assurance process. This novel scheme can be expanded to the entire body and shows a great potential in automatic plan physics QA procedure. It should be one essential QA step in an autonomous treatment planning process.     

Dosimetric comparison of IMRT rectal and anal canal plans generated using an anterior dose avoidance structure

To describe a dosimetric method using an anterior dose avoidance structure (ADAS) during the treatment planning process for intensity-modulated radiation therapy (IMRT) for patients with anal canal and rectal carcinomas. A total of 20 patients were planned on the Elekta/CMS XiO treatment planning system, version 4.5.1 (Maryland Heights MO) with a superposition algorithm. For each patient, 2 plans were created: one employing an ADAS (ADAS plan) and the other replanned without an ADAS (non-ADAS plan). The ADAS was defined to occupy the volume between the inguinal nodes and primary target providing a single organ at risk that is completely outside of the target volume. Each plan used the same beam parameters and was analyzed by comparing target coverage, overall plan dose conformity using a conformity number (CN) equation, bowel dose-volume histograms, and the number of segments, daily treatment duration, and global maximum dose. The ADAS and non-ADAS plans were equivalent in target coverage, mean global maximum dose, and sparing of small bowel in low-dose regions (5, 10, 15, and 20 Gy). The mean difference between the CN value for the non-ADAS plans and ADAS plans was 0.04 ± 0.03 (p < 0.001). The mean difference in the number of segments was 15.7 ± 12.7 (p < 0.001) in favor of ADAS plans. The ADAS plan delivery time was shorter by 2.0 ± 1.5 minutes (p < 0.001) than the non-ADAS one. The ADAS has proven to be a powerful tool when planning rectal and anal canal IMRT cases with critical structures partially contained inside the target volume.     

Evaluation of a new GPU-enabled VMAT multi-criteria optimisation plan generation algorithm

To evaluate the new Varian, graphical processing unit (GPU)-enabled, volumetric-modulated arc therapy (VMAT) multi-criteria optimisation (MCO) tool for both its dosimetric accuracy and calculation time. This is a new capability within V16.0 and greater of the Varian Eclipse treatment planning system that allows VMAT optimisation and dose calculation using the GPU (termed GPU-VMAT). In versions prior to V16.0 VMAT multi-criteria optimisation calculations were only possible using central processing unit (CPU) (termed CPU-VMAT) and Hybrid-VMAT (H-VMAT). The H-VMAT method breaks down the VMAT plan into IMRT fields which utilised GPU calculations. The study consisted of a cohort of 50 patients representing a range of anatomical treatment sites; bladder (5), brain (5), gynae (5), head & neck (5), lung (7), mediastinum (7) prostate (4), oesophagus (7) and rectum (5). Each case was planned to that of a clinical standard (Base) which was compared to a CPU-VMAT, GPU-VMAT and H-VMAT approaches. The study analysed dose to organ at risk (OAR) and target coverage, plan calculation time data and plan complexity through monitor unit (MU) for each approach. Negligible dosimetric differences were found between the CPU-VMAT, GPU-VMAT and H-VMAT approaches for the cohort of patients evaluated. The largest dosimetric change were observed in the lacrimal gland for a head and neck case, where the GPU-VMAT and H-VMAT achieved a max dose of +2.8 ± 0.0 Gy and −4.6 ± 0.0 Gy, respectively, when compared to CPU-VMAT. The majority of organ at risk’s (OAR) provided indistinguishable dosimetric outcomes, namely: heart, kidneys, femur, lens, oral cavity and oesophagus. Large time savings were found using the GPU-VMAT technique compared to CPU-VMAT, a mean decrease in calculation time across all sites of 60.2% ± 15.6%. Negligible dosimetric change between the 2 techniques and large time saving were observed with the GPU-VMAT and H-VMAT approaches when compared to the CPU-VMAT. We have shown that the GPU-VMAT technique has been safely implemented with minimal differences from CPU-VMAT, but with significant optimisation and calculation times savings.     

A novel approach of dose mapping using a humanoid breast phantom in radiotherapy

A study of dose mapping techniques to investigate the dose distribution throughout a planned target volume (PTV) in a humanoid breast phantom exposed to a 6 MV photon beam similar to that of treatment conditions is described. For tangential breast irradiation using a 6 MV accelerator beam, the dose is mapped at various locations within the PTV using thermoluminescent dosemeters (TLDs) and radiographic films. An average size perspex breast phantom with the ability to hold the dosemeters was made. TLDs were exposed after packing them in various locations in a particular slice, as planned by the treatment planning system (TPS). To map the dose relative to the isocenter, films were exposed after tightly packing them in between phantom slices, parallel to the central axis of the beam. The dose received at every location was compared with the given dose as generated by the TPS. The mapped dose in each location in the isocentric slice from superficial to deep region was found to be in close agreement with the TPS generated dose to within +/-2%. Doses at greater depths and distant medial and lateral ends, however, were found to be lower by as much as 9.4% at some points. The mapped dose towards the superior region and closest inferior region from the isocenter was found to agree with those for TPS. Conversely, results for the farthest inferior region were found to be significantly different with a variance as much as 17.4% at some points, which is believed to be owing to the variation in size and shape of the contour. Results obtained from films confirmed this, showing similar trends in dose mapping. Considering the importance of accurate doses in radiotherapy, evaluating dose distribution using this technique and tool was found to be useful. This provides the opportunity to choose a technique and plan to provide optimum dose delivery for radiotherapy to the breast.     

Predictive performance of an OVH-based treatment planning quality assurance model for prostate VMAT: Assessing dependence on training cohort size and composition

Radiotherapy treatment planning quality assurance models are used to assess overall plan quality in terms of dose-volume characteristics, by predicting an optimal dosimetry based on a dataset of prior cases (the training cohort). In this study, a treatment planning quality assurance model for prostate cancer patients treated with volumetric modulated arc therapy was developed using the concept of the overlap volume histogram for geometric comparison to the training cohort. The model was developed on the publically available Erasmus iCycle dataset in order to remove the effect of plan quality/inter-planner variability on the model's predictive capabilities. The model was used to predict anus, rectum, and bladder dose volume histograms. Two versions were developed: the n = 114 case (leave-one-out method) which made predictions using the complete Erasmus dataset, and the similarity index (SI)-based model which used a smaller training cohort allocated in order of geometric similarity determined using an overlap volume histogram-derived SI. The difference in mean dose (predicted-achieved) of the SI model at cohort sizes of 10, 20, 30, 40, 50, 75, and 100 was compared to the leave-one-out method for 5 patients, in an attempt to determine the "optimum" cohort size for the SI-based model in this dataset. Performance of the optimized SI model was compared to the leave-one-out method for all patients using the following metrics: difference in mean and median dose, difference in V65Gy and V75Gy (rectum only), similarity of predicted and achieved mean dose, and mean dose volume histograms residual. The "optimum" cohort size for the SI-based model was determined to be 45. The SI-based model implementing this cohort size yielded slightly better outcomes in all performance metrics for the rectum and anus, but worse for the bladder. SI-based training cohort allocation can lead to better predictive efficacy, but the cohort size should be optimized for each individual organ.     

Dose accumulation of daily adaptive plans to decide optimal plan adaptation strategy for head-and-neck patients treated with MR-Linac

Advances in magnetic resonance linear accelerators (MR-Linacs) allow for superior visualization of soft tissue to guide online adaptive replanning for precise radiotherapy delivery. Elekta Unity MR-Linacs (Elekta AB, Stockholm, Sweden) provides 2 plan adaptation approaches, adapt-to-position (ATP), plan reoptimization based on the reference CT with the iso-shift measured from daily MR scans, and adapt-to-shape (ATS), full plan reoptimization based on the re-contoured daily MR scans. Our study aims to close the gap in knowledge regarding the use of the ATP technique in the treatment of head and neck (HN) cancers through the analysis of accumulated dose of daily ATP plans to organs at risk (OARs). Daily accumulated doses of 8 HN patients using deformable registration were analyzed to estimate the actual delivered dose versus the planned dose to evaluate the impact from daily anatomical changes and setup uncertainties. This process was completed through the collection of doses to OARs which were chosen based on the rigidity and size of the organ and the substantial dose it received. Results showed that the actual dose delivered to some OARs was significantly higher than the originally planned dose and was more pronounced in structures that were within the high-dose gradient for some subdisease sites. These findings suggest that the ATS approach should be used for plan adaptation in some specific HN diseases where OARs receive substantial dose with anatomy changes that could not be accounted for by the ATP approach. We also investigated the possibility of predicting the actual delivered dose at an early stage of the treatment course, with the intention of exploring a possibly more optimal alternative for planning through the combination of ATP and ATS approaches throughout treatment.     

131I在细胞水平分布的实验研究

目的 用131I放射自显影技术观察其在细胞水平的分布,探讨放射性药物效果预测及评价.方法 采用放射自显影和冷冻切片技术,建立银颗粒密度与放射性药物强度的刻度曲线,确定放射性药物131Ⅰ的微观分布,基于核素的微观分布数据,建立微剂量的剂量估算模式.结果 银颗粒密度与施入放射性药物比活度的相关系数为0.9963,刻度系数为1.59×10-4Bq.结论 131I在细胞水平的分布是不均匀的,银颗粒多数是分布在细胞浆中.因此在计算细胞水平的剂量时应考虑到其分布的不均匀性.