Comparison of three immobilisation systems for radiation therapy in head and neck cancer

Evaluation of decentralised model-based selection of head and neck cancer patients for a proton treatment study. DAHANCA 35

INTRODUCTION

Proton treatment can potentially spare patients with H&N cancer for substantial treatment-related toxicities. The current study investigated the reproducibility of a decentralised model-based selection of patients for a proton treatment study when the selection plans were compared to the clinical treatment plans performed at the proton centre.

METHODS

Sixty-three patients were selected for proton treatment in the six Danish Head and Neck Cancer (DAHANCA) centres. The patients were selected based on normal tissue complication probability (NTCP) estimated from local photon and proton treatment plans, which showed a ΔNTCP greater than 5%-point for either grade 2 + dysphagia or grade 2 + xerostomia at six months. The selection plans were compared to the clinical treatment plans performed at the proton centre.

RESULTS

Of the 63 patients, 49 and 25 were selected based on an estimated benefit in risk of dysphagia and xerostomia, respectively. Eleven patients had a potential gain in both toxicities. The mean ΔNTCP changed from the local selection plan comparison to the clinical comparison from 6.9 to 5.3 %-points (p = 0.01) and 7.3 to 4.9 %-points (p = 0.03) for dysphagia and xerostomia, respectively. Volume differences in both CTV and OAR could add to the loss in ΔNTCP. 61 of the 63 clinical plans had a positive ΔNTCP, and 38 had a ΔNTCP of 5%-points for at least one of the two endpoints.

CONCLUSION

A local treatment plan comparison can be used to select candidates for proton treatment. The local comparative proton plan overestimates the potential benefit of the clinical proton plan. Continuous quality assurance of the delineation procedures and planning is crucial in the subsequent randomised clinical trial setting.

Radiation therapists' perceptions of thermoplastic mask use for head and neck cancer patients undergoing radiotherapy at Ocean Road Cancer Institute in Tanzania: A qualitative study

INTRODUCTION

A thermoplastic mask is the most widely used immobilization device for head and neck cancer patients undergoing radiotherapy. The radiation therapist is the staff responsible to prepare these masks and set-up the patients for treatment, a procedure that requires time, patience, and precision. An understanding of Radiation therapists' perceptions regarding thermoplastic mask use will help design interventions to address challenges encountered in its use. This study explored Radiation therapists' perceptions of thermoplastic mask use for head and neck cancer patients undergoing radiotherapy at Ocean Road Cancer Institute in Tanzania.

MATERIAL AND METHODS

An exploratory qualitative study design was used to explore thermoplastic mask use for head and neck cancer patients undergoing radiotherapy. Semi-structured in-depth interviews were conducted, involving fifteen Radiation therapists from Ocean Road Cancer Institute in Tanzania between March and May 2021. A thematic analysis method was used to identify themes from data scripts.

RESULTS

Four themes emerged that reflected radiation therapists' perceptions of thermoplastic mask use for head and neck cancer immobilization among patients undergoing radiotherapy. Emerged themes were (1) Perceived benefits and limitations of thermoplastic mask use, (2) Refresher training and supervision requirements for effective use, (3) Proper storage for quality maintenance, and (4) Increased financial support and proper budgeting.

CONCLUSION

Participants perceived better patient immobilization with a thermoplastic mask use. However, too often recycling of thermoplastic masks and the long waiting time between thermoplastic mask preparation and treatment delivery limits their effective use. For efficient use of thermoplastic masks, there is a need for Radiation therapists' refresher training and proper supervision, improving the storage system and increasing financial support for procuring new thermoplastic masks.

Delineation uncertainties of tumour volumes on MRI of head and neck cancer patients

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Role of target delineation uncertainties in head and neck cancer patients.

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Knowing contouring variations for MRI allows better adaptation of MRLinac for H&N cancers.

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An interobserver variation for GTV among 8 observers was below 2 mm using MRI.

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Variability between observers might improve using other imaging modalities.

Background

During the last decade, radiotherapy using MR Linac has gone from research to clinical implementation for different cancer locations. For head and neck cancer (HNC), target delineation based only on MR images is not yet standard, and the utilisation of MRI instead of PET/CT in radiotherapy planning is not well established. We aimed to analyse the inter-observer variation (IOV) in delineating GTV (gross tumour volume) on MR images only for patients with HNC.

Material/methods

32 HNC patients from two independent departments were included. Four clinical oncologists from Denmark and four radiation oncologists from Australia had independently contoured primary tumour GTVs (GTV-T) and nodal GTVs (GTV-N) on T2-weighted MR images obtained at the time of treatment planning. Observers were provided with sets of images, delineation guidelines and patient synopsis. Simultaneous truth and performance level estimation (STAPLE) reference volumes were generated for each structure using all observer contours. The IOV was assessed using the DICE Similarity Coefficient (DSC) and mean absolute surface distance (MASD).

Results

32 GTV-Ts and 68 GTV-Ns were contoured per observer. The median MASD for GTV-Ts and GTV-Ns across all patients was 0.17 cm (range 0.08–0.39 cm) and 0.07 cm (range 0.04–0.33 cm), respectively. Median DSC relative to a STAPLE volume for GTV-Ts and GTV-Ns across all patients were 0.73 and 0.76, respectively. A significant correlation was seen between median DSCs and median volumes of GTV-Ts (Spearman correlation coefficient 0.76, p < 0.001) and of GTV-Ns (Spearman correlation coefficient 0.55, p < 0.001).

Conclusion

Contouring GTVs in patients with HNC on MRI showed that the median IOV for GTV-T and GTV-N was below 2 mm, based on observes from two separate radiation departments. However, there are still specific regions in tumours that are difficult to resolve as either malignant tissue or oedema that potentially could be improved by further training in MR-only delineation.

Plan quality in radiotherapy treatment planning - Review of the factors and challenges

A high-quality treatment plan aims to best achieve the clinical prescription, balancing high target dose to maximise tumour control against sufficiently low organ-at-risk dose for acceptably low toxicity. Treatment planning (TP) includes multiple steps from simulation/imaging and segmentation to technical plan production and reporting. Consistent quality across this process requires close collaboration and communication between clinical and technical experts, to clearly understand clinical requirements and priorities and also practical uncertainties, limitations and compromises. TP quality depends on many aspects, starting from commissioning and quality management of the treatment planning system (TPS), including its measured input data and detailed understanding of TPS models and limitations. It requires rigorous quality assurance of the whole planning process and it links to plan deliverability, assessable by measurement-based verification. This review highlights some factors influencing plan quality, for consideration for optimal plan construction and hence optimal outcomes for each patient. It also indicates some challenges, sources of difference and current developments. The topics considered include: the evolution of TP techniques; dose prescription issues; tools and methods to evaluate plan quality; and some aspects of practical TP. The understanding of what constitutes a high-quality treatment plan continues to evolve with new techniques, delivery methods and related evidence-based science. This review summarises the current position, noting developments in the concept and the need for further robust tools to help achieve it.

A Multi-Center Study of CT-Based Neck Nodal Radiomics for Predicting an Adaptive Radiotherapy Trigger of Ill-Fitted Thermoplastic Masks in Patients with Nasopharyngeal Carcinoma

Significant lymph node shrinkage is common in patients with nasopharyngeal carcinoma (NPC) throughout radiotherapy (RT) treatment, causing ill-fitted thermoplastic masks (IfTMs). To deal with this, an ad hoc adaptive radiotherapy (ART) may be required to ensure accurate and safe radiation delivery and to maintain treatment efficacy. Presently, the entire procedure for evaluating an eligible ART candidate is time-consuming, resource-demanding, and highly inefficient. In the artificial intelligence paradigm, the pre-treatment identification of NPC patients at risk for IfTMs has become greatly demanding for achieving efficient ART eligibility screening, while no relevant studies have been reported. Hence, we aimed to investigate the capability of computed tomography (CT)-based neck nodal radiomics for predicting IfTM-triggered ART events in NPC patients via a multi-center setting. Contrast-enhanced CT and the clinical data of 124 and 58 NPC patients from Queen Elizabeth Hospital (QEH) and Queen Mary Hospital (QMH), respectively, were retrospectively analyzed. Radiomic (R), clinical (C), and combined (RC) models were developed using the ridge algorithm in the QEH cohort and evaluated in the QMH cohort using the median area under the receiver operating characteristics curve (AUC). Delong’s test was employed for model comparison. Model performance was further assessed on 1000 replicates in both cohorts separately via bootstrapping. The R model yielded the highest “corrected” AUC of 0.784 (BCa 95%CI: 0.673–0.859) and 0.723 (BCa 95%CI: 0.534–0.859) in the QEH and QMH cohort following bootstrapping, respectively. Delong’s test indicated that the R model performed significantly better than the C model in the QMH cohort (p < 0.0001), while demonstrating no significant difference compared to the RC model (p = 0.5773). To conclude, CT-based neck nodal radiomics was capable of predicting IfTM-triggered ART events in NPC patients in this multi-center study, outperforming the traditional clinical model. The findings of this study provide valuable insights for future study into developing an effective screening strategy for ART eligibility in NPC patients in the long run, ultimately alleviating the workload of clinical practitioners, streamlining ART procedural efficiency in clinics, and achieving personalized RT for NPC patients in the future.

Dosimetric evaluation of a patient-specific 3D-printed oral positioning stent for head-and-neck radiotherapy

As head-and-neck radiotherapy treatments become more complex and sophisticated, and the need to control and stabilise the positioning of intra-oral anatomy becomes more important, leading the increasing use of oral positioning stents during head-and-neck radiotherapy simulation and delivery. As an alternative to the established practice of creating oral positioning stents using wax, this study investigated the use of a 3D printing technique. An Ender 5 3D printer (Creality 3D, Shenzhen, China) was used, with PLA+ "food-safe" polylactic acid filament (3D Fillies, Dandenong South, Australia), to produce a low-density 3D printed duplicate of a conventional wax stent. The physical and dosimetric effects of the two stents were evaluated using radiochromic film in a solid head phantom that was modified to include flexible parts. The Varian Eclipse treatment planning system (Varian Medical Systems, Palo Alto, USA) was used to calculate the dose from two different head-and-neck treatment plans for the phantom with each of the two stents. Examination of the resulting four dose distributions showed that both stents effectively pushed sensitive oral tissues away from the treatment targets, even though most of the phantom was solid. Film measurements confirmed the accuracy of the dose calculations from the treatment planning system, despite the steep density gradients in the treated volume, and demonstrated that the 3D print could be a suitable replacement for the wax stent. This study demonstrated a useful method for dosimetrically testing novel oral positioning stents. We recommend the development of flexible phantoms for future studies.

Accuracy of automatic structure propagation for daily magnetic resonance image-guided head and neck radiotherapy

BACKGROUND AND PURPOSE

Deformable image registration (DIR) and contour propagation are used in daily online adaptation for hybrid MRI linac (MRL) treatments. The accuracy of the propagated contours may vary depending on the chosen workflow (WF), affecting the amount of required manual corrections. This study investigated the impact of three different WFs of contour propagations produced by a clinical treatment planning system for a high-field MRL on head and neck cancer patients.

METHODS

Seventeen patients referred for curative radiotherapy for oropharyngeal cancer underwent standard CT-based dose planning and MR scans in the treatment position for planning (pMR), and at the 10th (MR10), 20th (MR20) and 30th (MR30) fraction (±2). The primary tumour, a metastatic lymph node and 8 organs at risk were manually delineated on each set of T2 weighted images. Delineations were repeated one month later on the pMR by the same observer to determine the intra-observer variation (IOV). Three WFs were used to deform images in the treatment planning system for the high-field MRL: In WF1, only the planning image and contours were used as a reference for DIR and propagation to MR_10,20,30. The most recently acquired image set prior to the daily images was deformed and uncorrected (WF2) versus manually corrected (WF3) structures propagated to the session image. Dice similarity coefficient (DSC), mean surface distance (MSD) and Hausdorff distance (HD) were calculated for each structure in each model.

RESULTS

Population median DSC, MSD and HD for WF1 and WF3 were similar and slightly better than for WF2. WF3 provided higher accuracy than WF1 for structures that are likely to shrink. All DIR workflows were less accurate than the IOV.

CONCLUSIONS

WF1 and WF3 provide higher accuracy in structure propagation than WF2. Manual revision and correction of propagated structures are required for all evaluated workflows.

A review of Image Guided Radiation Therapy in head and neck cancer from 2009-201 - Best Practice Recommendations for RTTs in the Clinic

Radiation therapy (RT) is beneficial in Head and Neck Cancer (HNC) in both the definitive and adjuvant setting. Highly complex and conformal planning techniques are becoming standard practice in delivering increased doses in HNC. A sharp falloff in dose outside the high dose area is characteristic of highly complex techniques and geometric uncertainties must be minimised to prevent under dosage of the target volume and possible over dosage of surrounding critical structures. CTV-PTV margins are employed to account for geometric uncertainties such as set up errors and both interfraction and intrafraction motion. Robust immobilisation and Image Guided Radiation Therapy (IGRT) is also essential in this group of patients to minimise discrepancies in patient position during the treatment course. IGRT has evolved with increased 2-Dimensional (2D) and 3-Dimensional (3D) IGRT modalities available for geometric verification. 2D and 3D IGRT modalities are both beneficial in geometric verification while 3D imaging is a valuable tool in assessing volumetric changes that may have dosimetric consequences for this group of patients. IGRT if executed effectively and efficiently provides clinicians with confidence to reduce CTV-PTV margins thus limiting treatment related toxicities in patients. Accumulated exposure dose from IGRT vary considerably and may be incorporated into the treatment plan to avoid excess dose. However, there are considerable variations in the application of IGRT in RT practice. This paper aims to summarise the advances in IGRT in HNC treatment and provide clinics with recommendations for an IGRT strategy for HNC in the clinic.

Prediction of radiation-induced mucositis of H&N cancer patients based on a large patient cohort

PURPOSE/OBJECTIVE

Radiation-induced mucositis is a severe acute side effect, which can jeopardize treatment compliance and cause weight loss during treatment. The study aimed to develop robust models to predict the risk of severe mucositis.

MATERIALS/METHODS

Mucosal toxicity scores were prospectively recorded for 802 consecutive Head and Neck (H&N) cancer patients and dichotomised into non-severe event (grade 0-2) and severe event (grade 3+) groups. Two different model approaches were utilised to evaluate the robustness of the models. These used LASSO and Best Subset selection combined with 10-fold cross-validation performed on two-thirds of the patient cohort using principal component analysis of DVHs. The remaining one-third of the patients were used for validation. Model performance was tested through calibration plot and model performance metrics.

RESULTS

The main predicted risk factors were treatment acceleration and the first two principal dose components, which reflect the mean dose and the balance between high and low doses to the oral cavity. For the LASSO model, gender and current smoker status were also included in the model. The AUC values of the two models on the validation cohort were 0.797 (95%CI: 0.741-0.857) and 0.808 (95%CI: 0.749-0.859), respectively. The two models predicted very similar risk values with an internal Pearson coefficient of 0.954, indicating their robustness.

CONCLUSIONS

Robust prediction models of the risk of severe mucositis have been developed based on information from the entire dose distribution for a large cohort of patients consisting of all patients treated H&N for within our institution over a five year period.

The influence of a six degrees of freedom couch and an individual head support in patient positioning in radiotherapy of head and neck cancer

Reproducible patient positioning is important in radiotherapy (RT) of head-and-neck cancer. We therefore compared set-up errors in head-and-neck RT resulting from three different patient positioning systems. Patients were either treated with a standard head support (SHS) and conventional treatment couch (SHS-3, n = 10), a SHS and rotational couch (SHS-6, n = 10), or an individual head support (IHS) and rotational couch (IHS-6, n = 10). Interfraction mean translation vector lenghts were significantly lower for IHS-6 compared to SHS-3 (0.8 ± 0.3 mm vs. 1.4 ± 0.7 mm, P = 0.001). Intrafraction displacement was comparable among cohorts. This study showed that the use of a six degrees of freedom couch combined with an IHS in head-and-neck RT resulted in better interfraction reproducibility.

Effectiveness of base-of-skull immobilization system in a compact proton therapy setting

PURPOSE

The purpose of this study was to investigate daily repositioning accuracy by analyzing inter- and intra-fractional uncertainties associated with patients treated for intracranial or base of skull tumors in a compact proton therapy system with 6 degrees of freedom (DOF) robotic couch and a thermoplastic head mask indexed to a base of skull (BoS) frame.

MATERIALS AND METHODS

Daily orthogonal kV alignment images at setup position before and after daily treatments were analyzed for 33 patients. The system was composed of a new type of thermoplastic mask, a bite block, and carbon-fiber BoS couch-top insert specifically designed for proton therapy treatments. The correctional shifts in robotic treatment table with 6 DOF were evaluated and recorded based on over 1500 planar kV image pairs. Correctional shifts for patients with and without bite blocks were compared.

RESULTS

Systematic and random errors were evaluated for all 6 DOF coordinates available for daily vector corrections. Uncertainties associated with geometrical errors and their sources, in addition to robustness analysis of various combinations of immobilization components were presented.

CONCLUSIONS

Analysis of 644 fractions including patients with and without a bite block shows that the BoS immobilization system is capable of maintaining intra-fraction localization with submillimeter accuracy (in nearly 83%, 86%, 95% of cases along SI, LAT, and PA, respectively) in translational coordinates and subdegree precision (in 98.85%, 98.85%, and 96.4% of cases for roll, pitch, and yaw respectively) in rotational coordinates. The system overall fares better in intra-fraction localization precision compared to previously reported particle therapy immobilization systems. The use of a mask-attached type bite block has marginal impact on inter- or intra-fraction uncertainties compared to no bite block.

Automatic treatment planning facilitates fast generation of high-quality treatment plans for esophageal cancer

BACKGROUND

The quality of radiotherapy planning has improved substantially in the last decade with the introduction of intensity modulated radiotherapy. The purpose of this study was to analyze the plan quality and efficacy of automatically (AU) generated VMAT plans for inoperable esophageal cancer patients.

MATERIAL AND METHODS

Thirty-two consecutive inoperable patients with esophageal cancer originally treated with manually (MA) generated volumetric modulated arc therapy (VMAT) plans were retrospectively replanned using an auto-planning engine. All plans were optimized with one full 6MV VMAT arc giving 60 Gy to the primary target and 50 Gy to the elective target. The planning techniques were blinded before clinical evaluation by three specialized oncologists. To supplement the clinical evaluation, the optimization time for the AU plan was recorded along with DVH parameters for all plans.

RESULTS

Upon clinical evaluation, the AU plan was preferred for 31/32 patients, and for one patient, there was no difference in the plans. In terms of DVH parameters, similar target coverage was obtained between the two planning methods. The mean dose for the spinal cord increased by 1.8 Gy using AU (p = .002), whereas the mean lung dose decreased by 1.9 Gy (p < .001). The AU plans were more modulated as seen by the increase of 12% in mean MUs (p = .001). The median optimization time for AU plans was 117 min.

CONCLUSIONS

The AU plans were in general preferred and showed a lower mean dose to the lungs. The automation of the planning process generated esophageal cancer treatment plans quickly and with high quality.

Analysis of CT-verified loco-regional recurrences after definitive IMRT for HNSCC using site of origin estimation methods

INTRODUCTION

A significant part of patients with head and neck squamous cell carcinoma (HNSCC) develop recurrent disease after curative radiotherapy. We aimed to analyze loco-regional recurrence pattern by identifying possible points of recurrence origin by three different methods in relation to treatment volumes.

MATERIAL AND METHODS

A total of 455 patients completed IMRT-based treatment for HNSCC from 2006 to 2012. A total of 159 patients had remaining cancer after IMRT, developed loco-regional recurrence or distant disease during follow-up. Among those, 69 patients with loco-regional recurrences had affirmative CT or PET/CT scan. Possible points of origin (POs) of the recurrences were identified on scans by two independent observers, estimated as center of mass and as maximum surface distance. The recurrence position was analyzed in relation to high-dose treatment volume (CTV1) and 95% of prescription dose.

RESULTS

In total, 104 loco-regional recurrences (54 in T-site and 50 in N-site) were identified in 69 patients. Median time to recurrence for the 69 patients was 10 months. No clinically relevant difference was found between the four POs, with standard deviation between POs in x, y and z axes of 3, 3 and 6 mm. For recurrences inside CTV1, 0-5 mm and 5-10 mm outside CTV1 the standard deviation of dose differences between the POs were 1, 1.4 and 1 Gy, respectively. 56% and 25% of T-site and N-site recurrences were inside CTV1, respectively. Coverage by 95% prescription dose to high-dose treatment volume was achieved in 78% of T-site and 39% of N-site recurrences.

CONCLUSIONS

For recurrences identified by possible points of recurrence origin, no significant difference between observer-based or mathematically estimated methods was found. More than half of T-site recurrences were inside high-dose treatment volume, whereas N-site recurrences were mainly outside.

A Pilot Study Evaluating the Effectiveness of Dual-Registration Image-Guided Radiotherapy in Patients with Oropharyngeal Cancer

PURPOSE

The purpose of the article was to determine the impact of Dual Registration (DR) image-guided radiotherapy (IGRT) on clinical judgement and treatment delivery for patients with oropharyngeal cancer before implementation.

METHODS

Ninety cone beam computed tomography images from 10 retrospective patients were matched using standard clipbox registration (SCR) and DR. Three IGRT specialist radiographers performed all registrations and evaluated by intraclass correlation to determine inter-rater agreement, Bland-Altman with 95% limits of agreement to determine differences between SCR and DR procedures, changes in clinical judgment, time taken to perform registrations, and radiographer satisfaction.

RESULTS

Inter-rater agreement between radiographers using both SCR and DR was high (0.867 and 0.917, P ≤ .0001). The 95% limits of agreement between SCR and DR procedures in the mediolateral, cranial-caudal, and ventrodorsal translational directions were -6.40 to +4.91, -7.49 to +6.05, and -7.00 to +5.44 mm, respectively. The mediolateral direction demonstrated significant proportional bias (P ≤ .001) suggesting non-agreement between SCR and DR. Eighty percent of DR matches resulted in a change in clinical judgement to ensure maximum target coverage. Mean registration times for SCR and DR were 94 and 115 seconds, respectively, and radiographers found DR feasible and satisfactory.

CONCLUSION

The standard method using SCR in patients with oropharyngeal cancer underestimates the deviation in the lower neck. In these patients, DR is an effective IGRT tool to ensure target coverage of the inferior neck nodes and has demonstrated acceptability to radiotherapy clinical practice.

Consequences of introducing geometric GTV to CTV margin expansion in DAHANCA contouring guidelines for head and neck radiotherapy

BACKGROUND AND PURPOSE

Defining margins around the Gross Tumour Volume (GTV) to create a Clinical Target Volume (CTV) for head and neck cancer radiotherapy has traditionally been based on presumed knowledge of anatomical routes of spread. However, using a concentric geometric expansion around the GTV may be more reproducible. The purpose of this study was to analyse the inter-observer consistency of geometric CTV delineation with adaptation for anatomical boundaries versus anatomically defined CTVs.

MATERIAL AND METHODS

Radiation oncologists at four Danish cancer centres delineated high, intermediate and elective dose CTVs (CTV1, CTV2 and CTV3, respectively) in a patient-case template (stage IV squamous cell carcinoma of the oropharynx), first using mainly anatomical margins (original standard) and then using concentric geometric expansion (new standard). Each centre made a dummy-run radiotherapy plan based on the delineated CTVs. The difference between the CTV contours and the radiotherapy plans was evaluated across the centres.

RESULTS

Anatomy-based contours were significantly more heterogenous and showed larger volume differences between centres than geometric margins. Dice similarity coefficient increased by 0.29 and mean surface distance decreased by 4mm for CTV1. Use of consistent CTV volumes resulted in more consistent irradiated volumes between centres.

CONCLUSION

Introduction of geometric margins resulted in more uniform CTV1 and CTV2 delineation. Geometric CTV expansion was easier, left less room for misinterpretation, and resulted in more uniform treatment plans with similar irradiated high and intermediate dose volumes across all centres.

Contouring and dose calculation in head and neck cancer radiotherapy after reduction of metal artifacts in CT images

BACKGROUND

Delineation accuracy of the gross tumor volume (GTV) in radiotherapy planning for head and neck (H&N) cancer is affected by computed tomography (CT) artifacts from metal implants which obscure identification of tumor as well as organs at risk (OAR). This study investigates the impact of metal artifact reduction (MAR) in H&N patients in terms of delineation consistency and dose calculation precision in radiation treatment planning.

MATERIAL AND METHODS

Tumor and OAR delineations were evaluated in planning CT scans of eleven oropharynx patients with streaking artifacts in the tumor region preceding curative radiotherapy (RT). The GTV-tumor (GTV-T), GTV-node and parotid glands were contoured by four independent observers on standard CT images and MAR images. Dose calculation was evaluated on thirty H&N patients with dental implants near the treated volume. For each patient, the dose derived from the clinical treatment plan using the standard image set was compared with the recalculated dose on the MAR image dataset.

RESULTS

Reduction of metal artifacts resulted in larger volumes of all delineated structures compared to standard reconstruction. The GTV-T and the parotids were on average 22% (p < 0.06) and 7% larger (p = 0.005), respectively, in the MAR image plan compared to the standard image plan. Dice index showed reduced inter-observer variations after reduction of metal artifacts for all structures. The average surface distance between contours of different observers improved using the MAR images for GTV and parotids (p = 0.04 and p = 0.01). The median volume receiving a dose difference larger than ±3% was 2.3 cm³ (range 0-32 cm³).

CONCLUSIONS

Delineation of structures in the head and neck were affected by metal artifacts and volumes were generally larger and more consistent after reduction of metal artifacts, however, only small changes were observed in the dose calculations.

Comparison of set up accuracy among three common immobilisation systems for intensity modulated radiotherapy of nasopharyngeal carcinoma patients

Introduction

In intensity modulated radiotherapy (IMRT) of nasopharyngeal carcinoma (NPC) patients, an effective immobilisation system is important to minimise set up deviation. This study evaluated the effectiveness of three immobilisation systems by assessing their set up deviations.

Methods

Patients were randomly assigned to one of the three immobilisation systems: (1) supine on head rest and base plate (HB); (2) supine with alpha cradle supporting the head and shoulder (AC); (3) supine with vacuum bag supporting the head and shoulder (VB). CBCT was conducted weekly for each patient on the linear accelerator. Image registration was conducted at the nasopharynx (NP) and cervical regions. The translational displacements (latero‐medial, antero‐posterior and cranio‐caudal), rotational displacements (pitch, yaw and roll) and 3D vectors obtained at the NP and cervical regions were recorded and compared among the three systems.

Results

The mean translational and rotational deviations were within 3 mm and 2°, respectively, and the range of 3D vector was 1.53–3.47 mm. At the NP region, the AC system demonstrated the smallest translational and rotational deviations and 3D vector. The differences were significant except for the latero‐medial, yaw and roll directions. Similarly, at the cervical region, the AC system showed smaller translational and rotational deviations and 3D vector, with only the cranio‐caudal and yaw deviations that did not reach statistical significance.

Conclusions

Set up deviation was greater in the neck than the NP region. The set up accuracy of the AC system was better than the other two systems, and it is recommended for IMRT of NPC patients in our institution.

Automatic planning of head and neck treatment plans

Treatment planning is time‐consuming and the outcome depends on the person performing the optimization. A system that automates treatment planning could potentially reduce the manual time required for optimization and could also provide a method to reduce the variation between persons performing radiation dose planning (dosimetrist) and potentially improve the overall plan quality. This study evaluates the performance of the Auto‐Planning module that has recently become clinically available in the Pinnacle³ radiation therapy treatment planning system. Twenty‐six clinically delivered head and neck treatment plans were reoptimized with the Auto‐Planning module. Comparison of the two types of treatment plans were performed using DVH metrics and a blinded clinical evaluation by two senior radiation oncologists using a scale from one to six. Both evaluations investigated dose coverage of target and dose to healthy tissues. Auto‐Planning was able to produce clinically acceptable treatment plans in all 26 cases. Target coverages in the two types of plans were similar, but automatically generated plans had less irradiation of healthy tissue. In 94% of the evaluations, the autoplans scored at least as high as the previously delivered clinical plans. For all patients, the Auto‐Planning tool produced clinically acceptable head and neck treatment plans without any manual intervention, except for the initial target and OAR delineations. The main benefit of the method is the likely improvement in the overall treatment quality since consistent, high‐quality plans are generated which even can be further optimized, if necessary. This makes it possible for the dosimetrist to focus more time on difficult dose planning goals and to spend less time on the more tedious parts of the planning process.

PACS number: 87.55.de