Interfacility variation in treatment planning parameters in tomotherapy: field width, pitch, and modulation factor

Evaluating the Hounsfield unit assignment and dose differences between CT-based standard and deep learning-based synthetic CT images for MRI-only radiation therapy of the head and neck

BACKGROUND

Magnetic resonance image only (MRI-only) simulation for head and neck (H&N) radiotherapy (RT) could allow for single-image modality planning with excellent soft tissue contrast. In the MRI-only simulation workflow, synthetic computed tomography (sCT) is generated from MRI to provide electron density information for dose calculation. Bone/air regions produce little MRI signal which could lead to electron density misclassification in sCT. Establishing the dosimetric impact of this error could inform quality assurance (QA) procedures using MRI-only RT planning or compensatory methods for accurate dosimetric calculation.

PURPOSE

The aim of this study was to investigate if Hounsfield unit (HU) voxel misassignments from sCT images result in dosimetric errors in clinical treatment plans.

METHODS

Fourteen H&N cancer patients undergoing same-day CT and 3T MRI simulation were retrospectively identified. MRI was deformed to the CT using multimodal deformable image registration. sCTs were generated from T1w DIXON MRIs using a commercially available deep learning-based generator (MRIplanner, Spectronic Medical AB, Helsingborg, Sweden). Tissue voxel assignment was quantified by creating a CT-derived HU threshold contour. CT/sCT HU differences for anatomical/target contours and tissue classification regions including air (<250 HU), adipose tissue (-250 HU to -51 HU), soft tissue (-50 HU to 199 HU), spongy (200 HU to 499 HU) and cortical bone (>500 HU) were quantified. t-test was used to determine if sCT/CT HU differences were significant. The frequency of structures that had a HU difference > 80 HU (the CT window-width setting for intra-cranial structures) was computed to establish structure classification accuracy. Clinical intensity modulated radiation therapy (IMRT) treatment plans created on CT were retrospectively recalculated on sCT images and compared using the gamma metric.

RESULTS

The mean ratio of sCT HUs relative to CT for air, adipose tissue, soft tissue, spongy and cortical bone were 1.7 ± 0.3, 1.1 ± 0.1, 1.0 ± 0.1, 0.9 ± 0.1 and 0.8 ± 0.1 (value of 1 indicates perfect agreement). T-tests (significance set at t = 0.05) identified differences in HU values for air, spongy and cortical bone in sCT images compared to CT. The structures with sCT/CT HU differences > 80 HU of note were the left and right (L/R) cochlea and mandible (>79% of the tested cohort), the oral cavity (for 57% of the tested cohort), the epiglottis (for 43% of the tested cohort) and the L/R TM joints (occurring > 29% of the cohort). In the case of the cochlea and TM joints, these structures contain dense bone/air interfaces. In the case of the oral cavity and mandible, these structures suffer the additional challenge of being positionally altered in CT versus MRI simulation (due to a non-MR safe immobilizing bite block requiring absence of bite block in MR). Finally, the epiglottis HU assignment suffers from its small size and unstable positionality. Plans recalculated on sCT yielded global/local gamma pass rates of 95.5% ± 2% (3 mm, 3%) and 92.7% ± 2.1% (2 mm, 2%). The largest mean differences in D95, Dmean , D50 dose volume histogram (DVH) metrics for organ-at-risk (OAR) and planning tumor volumes (PTVs) were 2.3% ± 3.0% and 0.7% ± 1.9% respectively.

CONCLUSIONS

In this cohort, HU differences of CT and sCT were observed but did not translate into a reduction in gamma pass rates or differences in average PTV/OAR dose metrics greater than 3%. For sites such as the H&N where there are many tissue interfaces we did not observe large scale dose deviations but further studies using larger retrospective cohorts are merited to establish the variation in sCT dosimetric accuracy which could help to inform QA limits on clinical sCT usage.

Comparison of Different Head Tilt Angles in Tomotherapy and Volumetric Modulated Arc Therapy for Hippocampal-Avoidance Whole-Brain Radiotherapy

PURPOSE

Hippocampal-avoidance whole-brain radiotherapy (HA-WBRT) planning can present challenges. This study examines the influence of head tilt angles on the dosimetric characteristics of target and organs at risk (OARs), aiming to identify the optimal tilt angle that yields optimal dosimetric outcomes using tomotherapy (TOMO).

METHODS

Eight patients diagnosed with brain metastases underwent CT scans at five tilt angles: [0°, 10°), [10°, 20°), [20°, 30°), [30°, 40°), and [40°, 45°]. Treatment plans were generated using TOMO and volumetric modulated arc therapy (VMAT). Dosimetric parameters including conformity index (CI), homogeneity index (HI), D2cc, D98%, and Dmean of PTV, as well as Dmax, and Dmean of OARs were analyzed. Furthermore, a comparison was made between the dosimetric parameters of TOMO and VMAT plans. Finally, delivery efficiency of TOMO plans were assessed.

RESULTS

For the PTV, [40°, 45°] tilt angle demonstrated significantly better conformity, homogeneity, lower D2cc, and lower Dmean for the PTV. Regarding the OARs, the [40°, 45°] head tilt angle demonstrated significantly lower Dmax and Dmean in hippocampus, eyes, optic chiasm, and optic nerves. The [40°, 45°] tilt angle also showed significantly lower Dmax for brainstem and cochleas, as well as a lower Dmean for lens. In the [40°,45°] tilt angle for HA-WBRT, TOMO showed superior performance over VMAT for the PTV. TOMO achieved lower Dmax for brainstem, cochleas, optic nerves, and optic chiasm, as well as a lower Dmean for hippocampus. Furthermore, a significant correlation was found between delivery time and the PTV projection length in the sagittal plane.

CONCLUSION

The TOMO plan utilizing a tilt angle range of [40°, 45°] demonstrated superior PTV conformity and uniformity, along with enhanced OARs sparing. Furthermore, it exhibited a dosimetric advantage over VMAT for PTV and most OARs at the same angle range.

Impact of delivery time factor on treatment time and plan quality in tomotherapy

Delivery time factor (DTF) is a new parameter introduced by the RayStation treatment planning system for tomotherapy treatment planning. This study investigated the effects of this factor on various tomotherapy plans. Twenty-five patients with cancer (head and neck, 6; lung, 9; prostate, 10) were enrolled in this study. Helical tomotherapy plans with a field width of 2.5 cm, pitch of 0.287, and DTF of 2.0 were created. All the initial plans were recalculated by changing the DTF parameter from 1.0 to 3.0 in increments of 0.1. Then, DTF's impact on delivery efficiency and plan quality was evaluated. Treatment time and modulation factor increased monotonically with increasing DTF. Increasing the DTF by 0.1 increased the treatment time and modulation factor by almost 10%. This relationship was similar for all treatment sites. Conformity index (CI), homogeneity index, and organ at risk doses were improved compared to plans with a DTF of 1.0, except for the CI in the lung cancer case. However, the improvement in most indices ceased at a certain DTF; nevertheless, treatment time continued to increase following an increase in DTF. DTF is a critical parameter for improving the quality of tomotherapy plans.

Fractionated Total Body Irradiation on an Infant Using Tomotherapy

Total body irradiation (TBI) is used with chemotherapy to induce immunosuppression for hematopoietic cell transplantation and is often administered using lead blocks to minimize lung dose in adults and children. This technique is challenging in infants and young children. A 13-month-old female with acute lymphoblastic leukemia (ALL) was treated with fractionated TBI to a dose of 12 Gy in eight fractions delivered twice daily. Multiple TBI techniques for delivering treatment were considered. Ultimately, treatment using helical tomotherapy was selected in order to spare and accurately quantify the dose to the lung, meet lung dose constraints, and ensure adequate TBI dose coverage. With anesthesia, this technique provided a comfortable and reproducible set-up for the young child. The treatment plan was delivered with intensity-modulated radiotherapy, where 96.4% of the target volume received a prescription dose with a total beam-on time of 16.8 minutes. The mean lung dose was 7.7 Gy for a total lung volume of 245cc. This report describes the challenges faced during the treatment planning and delivery, and how they were resolved.

Impact of different fixed dental prostheses on radiation dose in helical tomotherapy as measured with metal oxide semiconductor field-effect transistor dosimetry

OBJECTIVE

This ex vivo study evaluated the effects of different fixed dental prostheses and protective materials on scattered radiation during radiation therapy (RT).

STUDY DESIGN

Natural teeth (group NT) and 4 types of prostheses (group BL: bilayer lithium disilicate glass-ceramic; group MZ: monolithic zirconia ceramic; group BZ: bilayer zirconia-based all ceramic; and group BM: bilayer metal-ceramic restorations) were examined in maxillary and mandibular arch phantoms. All groups were divided into 3 subgroups: (1) without protective material over the prostheses; (2) protected with a soft acrylic night guard; and (3) protected with polyvinylsiloxane putty. All groups were irradiated with helical RT at 2 Gy and 6 MV photon energy. Doses were measured internally and externally 3 times on each phantom. Results were significant at P < .05.

RESULTS

No statistically significant differences in doses were found between groups NT and BL in either phantom, but these groups were different from the others. The differences between groups BZ and MZ were insignificant. Doses in group BM were significantly larger than all other groups. No significant differences existed between doses with and without protective material.

CONCLUSIONS

Doses increased significantly due to scattered radiation from the prostheses with increasing material density. Protective materials had no significant effect on dose.

Influence of Modulation Factor on Treatment Plan Quality and Irradiation Time in Hippocampus-Sparing Whole-Brain Radiotherapy Using Tomotherapy

Objectives: Hippocampus-sparing whole-brain radiotherapy (HS-WBRT) using tomotherapy is known to provide a better dose distribution than volumetric-modulated arc therapy but requires an extended irradiation time. The present study aimed to investigate whether irradiation time can be shortened by reducing the modulation factor (MF) without losing the target dose distribution. Methods: Using six tilted computed tomography images in the head area, the planning target volume (PTV) and hippocampal doses, and the irradiation time was investigated with a jaw width of 1 cm, a pitch of 0.200, and the MF changed from 3.0 to 2.6, 2.2, 1.8, and 1.4. Results: No significant changes in the PTV or hippocampus were found with MF in the range from 3.0 to 1.8, but marked deterioration was found with that of 1.4. The irradiation time showed a linear relationship with the MF within the range from 3.0 to 1.8, with 1334, 1158, 986, and 817 s at modulation factors of 3.0, 2.6, 2.2, and 1.8, respectively. However, when the MF was 1.4, the irradiation time was 808 s. Conclusions: When HS-WBRT is performed with a tilted body position and a jaw width of 1 cm, with a MF of 1.8, a favorable balance between dose parameters and irradiation time is achieved, whereas with a MF of 1.4, the quality of the radiotherapy plan deteriorates, and the irradiation time is approximately the same as that with a MF of 1.8.

Stereotactic body radiotherapy using the forward-planned static-port tomotherapy for lung cancer: a novel planning technique with the newly-developed mode

With the newly-developed static-port forward-planning (FP) mode of tomotherapy, the ratio of the dose of the planning target volume (PTV) periphery to the maximum dose can be easily adjusted by modifying leaf margins when planning stereotactic body radiotherapy (SBRT). The purpose of this study was to evaluate the characteristics of FP plans compared to helical intensity-modulated radiotherapy (IMRT) and helical 3D conformal radiotherapy (3DCRT) plans of SBRT for lung tumors. The three plans were created for 14 tumors in 11 patients. For 13 tumors, 60 Gy in 7.5-Gy fractions was prescribed for a minimum coverage dose of 95% of the PTV (D95). The prescribed isodose line (PIL) was intended to be 60-80% of the maximum dose. Nine angles were used for the FP plans. The median D98 and D50 of the internal target volume for FP, helical-IMRT and helical-3DCRT plans were 70.4, 71.4 and 60.5 Gy, respectively (P < 0.001), and 77.7, 75.7 and 62.3 Gy, respectively (P < 0.0001). The median PIL and the lung volume receiving ≥20 Gy (V20) were 73.4, 73.4 and 94.3%, respectively (P < 0.0001), and 4.7, 4.0 and 5.7%, respectively (P < 0.0001). These parameters were not significantly different between the FP and helical-IMRT plans. The median beam-on times were 238.6, 418.9 and 197.1 s, respectively (P < 0.0001). The FP plans reduced the beam-on time by 43% compared to the helical-IMRT plans. The dose distribution of the FP plans was comparable to that of the helical-IMRT plans. The helical-3DCRT plans could not adjust PIL to be 60-80%.