The effect of tomotherapy imaging beam output instabilities on dose calculation

Dosimetrically triggered adaptive radiotherapy for head and neck cancer: Considerations for the implementation of clinical protocols

PURPOSE

Defining dosimetric rules to automatically detect patients requiring adaptive radiotherapy (ART) is not straightforward, and most centres perform ad-hoc ART with no specific protocol. This study aims to propose and analyse different steps to design a protocol for dosimetrically triggered ART of head and neck (H&N) cancer. As a proof-of-concept, the designed protocol was applied to patients treated in TomoTherapy units, using their available software for daily MVCT image and dose accumulation.

METHODS

An initial protocol was designed by a multidisciplinary team, with a set of flagging criteria based only on dose-volume metrics, including two action levels: (1) surveillance (orange flag), and (2) immediate verification (red flag). This protocol was adapted to the clinical needs following an iterative process. First, the protocol was applied to 38 H&N patients with daily imaging. Automatic software generated the daily contours, recomputed the daily dose and flagged the dosimetric differences with respect to the planning dose. Second, these results were compared, by a sensitivity/specificity test, to the answers of a physician. Third, the physician, supported by the multidisciplinary team, performed a self-analysis of the provided answers and translated them into mathematical rules in order to upgrade the protocol. The upgraded protocol was applied to different definitions of the target volume (i.e. deformed CTV + 0, 2 and 4 mm), in order to quantify how the number of flags decreases when reducing the CTV-to-PTV margin.

RESULTS

The sensitivity of the initial protocol was very low, specifically for the orange flags. The best values were 0.84 for red and 0.15 for orange flags. After the review and upgrade process, the sensitivity of the upgraded protocol increased to 0.96 for red and 0.84 for orange flags. The number of patients flagged per week with the final (upgraded) protocol decreased in median by 26% and 18% for red and orange flags, respectively, when reducing the CTV-to-PTV margin from 4 to 2 mm. This resulted in only one patient flagged at the last fraction for both red and orange flags.

CONCLUSION

Our results demonstrate the value of iterative protocol design with retrospective data, and shows the feasibility of automatically-triggered ART using simple dosimetric rules to mimic the physician's decisions. Using a proper target volume definition is important and influences the flagging rate, particularly when decreasing the CTV-to-PTV margin.

Practical and technical key challenges in head and neck adaptive radiotherapy: The GORTEC point of view

Anatomical variations occur during head and neck (H&N) radiotherapy (RT) treatment. These variations may result in underdosage to the target volume or overdosage to the organ at risk. Replanning during the treatment course can be triggered to overcome this issue. Due to technological, methodological and clinical evolutions, tools for adaptive RT (ART) are becoming increasingly sophisticated. The aim of this paper is to give an overview of the key steps of an H&N ART workflow and tools from the point of view of a group of French-speaking medical physicists and physicians (from GORTEC). Focuses are made on image registration, segmentation, estimation of the delivered dose of the day, workflow and quality assurance for an implementation of H&N offline and online ART. Practical recommendations are given to assist physicians and medical physicists in a clinical workflow.

Stability of the Helical TomoTherapy Hi·Art II detector for treatment beam irradiations

The Hi·Art II Helical TomoTherapy (HT) unit is equipped with a built‐in onboard MVCT detector used for patient imaging and beam monitoring. Our aim was to study the detector stability for treatment beam measurements. We studied the MVCT detector response with the 6 MV photon beam over time, throughout short‐term (during an irradiation) and long‐term (two times 50 days) periods. Our results show a coefficient of variation ≤1% for detector chambers inside the beam (excluding beam gradients) for short‐ and long‐term response of the MVCT detector. Larger variations were observed in beam gradients and an influence of the X‐ray target where degradation was found. The results assume that an ‘air scan’ procedure is performed daily to recalibrate the detector with the imaging beam. On short term, the detector response stability is comparable to other devices. Long‐term measurements during two 50‐day periods show a good reproducibility.

PACS numbers: 87.55.ne, 87.55.Qr

The potential of helical tomotherapy in the treatment of head and neck cancer

A decade after its first introduction into the clinic, little is known about the clinical impact of helical tomotherapy (HT) on head and neck cancer (HNC) treatment. Therefore, we analyzed the basics of this technique and reviewed the literature regarding HT's potential benefit in HNC. The past two decades have been characterized by a huge technological evolution in photon beam radiotherapy (RT). In HNC, static beam intensity-modulated radiotherapy (IMRT) has shown superiority over three-dimensional conformal RT in terms of xerostomia and is considered the standard of care. However, the next-generation IMRT, the rotational IMRT, has been introduced into the clinic without any evidence of superiority over static beam IMRT other than being substantially faster. Of these rotational techniques, HT is the first system especially developed for IMRT in combination with image-guided RT. HT is particularly promising for the treatment of HNC because its sharp dose gradients maximally spare the many radiosensitive organs at risk nearby. In addition, HT's integrated computed tomography scan assures a very precise dose administration and allows for some adaptive RT. Because HT is specifically developed for IMRT in combination with (integrated) image-guidance, it allows for precise dose distribution ("dose painting"), patient setup, and dose delivery. As such, it is an excellent tool for difficult HNC irradiation. The literature on the clinical results of HT in HNC all show excellent short-term (≤2 years) results with acceptable toxicity profiles. However, properly designed trials are still warranted to further substantiate these results.

Impact of very long time output variation in the treatment of total marrow irradiation with helical tomotherapy

Background

Beam-on time in Total Marrow Irradiation (TMI) delivery with helical tomotherapy is more than 30 minutes. The purpose of this study was to investigate extended time output variation in tomotherapy machine without dose servo system and its impact on the dosimetry of TMI planning.

Materials and methods

The calibration procedures with 1800 seconds delivery were conducted. The slab and cylindrical phantoms were used for static and rotational output variation measurements, respectively. All measurements were performed in 0.1 second interval with an Exradin A1SL ionization chamber (Standard Imaging Inc., Madison, WI, USA) connected to the tomoelectrometer supplied by the manufacture. Simulated TMI treatment planning with a slab phantom was delivered and verified with ion chamber and EDR-2 films.

Results

The static output variations during 30 min averaged −2.9% ± 0.2%, -3.4% ± 0.3%, and −3.4% ± 0.3% at 10 min, 20 min, and 30 min, respectively. The rotational output variations from start averaged −2.5% ± 0.7%, -3.1% ± 0.7%, and −3.5% ± 0.8% at 10 min, 20 min, and 30 min, respectively. The maximum output variation was up to 4.5%. In a TMI planning model, in which beam-on time was over 30 min, planned dose and dose measured with ion chambers in both cranial and caudal sides agreed within 3%. Film measurements in cranial and caudal sides also showed the pass rates of 97.7% and 92.2% with the criteria of 3 mm/3% in gamma analysis.

Conclusion

These results suggest that long TMI delivery by helical tomotherapy, even without dose servo system, does not pose a risk for significant deviations from the original treatment plan regardless of the output variation. However, very long time output variation should be checked before the first treatment.

Improving dose calculations on tomotherapy MVCT images

The purpose of this investigation was the creation of a new protocol allowing more precise dose calculations on megavoltage CT (MVCT) images for tomotherapy, both for adaptive and StatRT planning. Daily MVCT images offer, next to positioning purposes, the possibility for daily dose check and adaptive planning. Dose calculations use the image value to density table (IVDT) to calculate physical densities from Hounsfield Units (HUs). These measured HUs change over time, leading to a dose calculation error. We noticed dose calculation variations due to IVDT changes of: 0.2% dose during a day, up to 1.6% dose from long‐term variations, and up to 1.5% dose due to technical interventions. An analysis was performed applying the general methodology of a calibration problem. A model HU=bρc‐1020 was obtained using a weighted least squares inverse prediction method (HU as function of density) taking into account the heteroscedasticity. The b parameter is the major variable and depends also on the dose rate (DR). We demonstrate the correction for DR variations and the constance of the c parameter. Instead of scanning the whole tissue characterization phantom daily, we propose a simplified daily protocol: (a) morning airscan‐like procedure with only two inserts on the table (defining b and thus the IVDT curve), (b) DR variations throughout the day can be corrected for using the DR model. A patient‐specific protocol for which two inserts next to the patient are scanned could also be used, but results in equal uncertainties and is less practical. Therefore we recommend the morning procedure with dose rate variation correction. Applying the proposed transformations and the model, the correct IVDT of the moment can be reconstructed, with a simple measurement in the morning, and corrected with DR changes during the day. This corresponds with a linear mapping in time of the proposed IVDT function. The dosimetric variation is hereby reduced from up to 3% to 0.4 % for the tested pelvic and head‐and‐neck cases. In practice, several IVDT curves corresponding to “b” values can be entered. The correct IVDT curve of that moment can then be chosen from the list. Instead of the two high‐density inserts on table, any calibrated single density phantom could be used in order to create the IVDT curve of the day, but it should have a larger size than the current inserts.

PACS number: 87.55.Gh

Effect of jaw size in megavoltage CT on image quality and dose

PURPOSE

Recently, the jaw size for the TomoTherapy Hi-Art II(®) (TomoTherapy Inc., Madison, WI) was reduced from 4 mm (J4) to 1 mm (J1) to improve the longitudinal (IEC-Y) resolution in megavoltage computed tomography (MVCT) images. This study evaluated the effect of jaw size on the image quality and dose, as well as the dose delivered to the lens of the eye, which is a highly radiosensitive tissue.

METHODS

MVCT image quality (image noise, uniformity, contrast linearity, high-contrast resolution, and full width at half-maximum) and multiple scan average dose (MSAD) were measured at different jaw sizes. A head phantom and photoluminescence glass dosimeters (PLDs) were used to measure the exposed lens dose (cGy). Different MVCT scan modes (pitch = 1, 2, and 3) and scan lengths (108 mm, 156 mm, and 204 mm) were applied in the MSAD and PLDs measurements.

RESULTS

The change in jaw size from J4 to J1 produced no change or only a slight improvement in image noise, uniformity, contrast linearity, and high-contrast resolution. However, the full-width at half-maximum reduced from approximately 7.2 at J4 to 4.5 mm at J1, which represents an enhancement in the longitudinal resolution. The MSAD at the center point changed from approximately 0.69-2.32 cGy (peripheral: 0.83-2.49 cGy) at J4 to 0.85-2.81 cGy (peripheral: 1.05-2.86 cGy) at J1. The measured lens dose increased from 0.92-3.36 cGy at J4 to 1.06-3.91 cGy at J1.

CONCLUSIONS

The change in jaw size improved longitudinal resolution. The MVCT imaging dose of approximately 3.86 cGy, 1.92 cGy, and 1.22 cGy was delivered at a pitch of 1, 2, and 3, respectively, per fraction in the head and neck treatment plans. Therefore, allowance for an approximately 15% increase in lens dose over that with J4 should be provided with J1.

The effect of temporal HU variations on the uncertainty of dose recalculations performed on MVCT images

Over the course of radiation therapy, a patient's anatomy may change substantially. The relatively recent addition of frequent in-room imaging to assist with patient localization has provided a database of images that may be used to recalculate dose distributions for adaptive radiotherapy purposes. The TomoTherapy Hi-Art II unit (Accuray Inc., Sunnyvale, CA, USA) uses a helical scanning geometry and a megavoltage (MV) beam to acquire volumetric patient images. This study evaluated the uncertainty of dose calculations performed on megavoltage CT (MVCT) images as a function of temporal Hounsfield Unit (HU) variations observed in the imaging system over three years on two machines. A baseline error between dose calculations performed on kVCT and MVCT images was established using a series of phantoms. This baseline error ranged from -1.4% to 0.6%. Materials of differing densities were imaged and MVCT numbers were measured periodically. The MVCT number of solid water varied from 5 to 103 HU and consistently increased prior to target replacement. Finally, the dosimetric uncertainty of the temporal HU variation was assessed using MVCT images of typical head and neck, lung and prostate cancer patients. Worst-case MVCT recalculation errors could approach 5%, 7% and 10% for the head and neck, lung and prostate images, respectively. However, if a tolerance of ±30 HU were maintained for the MVCT number of solid water, dosimetric errors were limited to ±2.5%, ±3% and ±4%, respectively.

A planning study for palliative spine treatment using StatRT and megavoltage CT simulation

Megavoltage CT (MVCT) simulation on the TomoTherapy Hi·Art system is an alternative to conventional CT for treatment planning in the presence of severe metal artifact. StatRT is a new feature that was implemented on the TomoTherapy operator station for performing online MVCT scanning, treatment planning and treatment delivery in one session. The clinical feasibility of using the StatRT technique and MVCT simulation to palliative treatment for a patient with substantial spinal metallic hardware is described. A patient with metastatic non-small-cell lung cancer involving the thoracic spine underwent conventional kilovoltage CT simulation. The metal artifact due to stainless steel spine-stabilizing rods was too severe for treatment planning, despite attempts to correct using density override. The patient was then re-scanned using MVCT on a tomotherapy unit. Plans were generated using both StatRT and conventional tomotherapy planning (Tomo plan) with different settings for comparison. StatRT planning ran a total of five iterations in a short planning window (10-15 min). Two Tomo plans were generated using: (1) five iterations in the "full scatter" mode, and (2) 300 iterations in the "beamlet" mode. It was noted that the DVH of the StatRT plan was almost identical to the Tomo plan optimized by the "full scatter" mode and the same number of iterations. Dose distribution analysis reveals that these three planning methods yielded comparable doses to heart, lungs and targets. This work also demonstrated that undermodulation can result in a high degree of thread effects. The overall time for the treatment process (including 7 minutes for simulation, 15 minutes for contouring, 10 minutes for planning and 5 minutes for delivery) decreases from hours to around 40 minutes using the StatRT procedure. StatRT is a feasible treatment-planning tool for physicians to scan, contour and treat patients within one hour. This can be particularly beneficial in urgent palliative treatments.