Setup uncertainties and PTV margins at different anatomical levels in intensity modulated radiotherapy for nasopharyngeal cancer

Target movement according to cervical lymph node level in head and neck cancer and its clinical significance

PURPOSE

To evaluate set-up error for head and neck cancer (HNC) patients according to each neck lymph node (LN) level. And clinical factors affecting set-up error were analyzed.

MATERIALS AND METHODS

Reference points (RP1, RP2, RP3, and RP4) representing neck LN levels I to IV were designated. These RP were contoured on simulation computed tomography (CT) and cone-beam CT of 89 HNC patients with the same standard. After image registration was performed, movement of each RP was measured. Univariable logistic regression analyses were performed to analyze clinical factors related to measured movements.

RESULTS

The mean value of deviation of all axes was 1.6 mm, 1.3 mm, 1.8 mm, and 1.5 mm for RP1, RP2, RP3, and RP4, respectively. Deviation was over 3 mm in 24 patients. Movement of more than 3 mm was observed only in RP1 and RP3. In RP1, it was related to bite block use. Movement exceeding 3 mm was most frequently observed in RP3. Primary tumor and metastatic LN volume change were clinical factors related to the RP3 movement.

CONCLUSION

Planning target volume margin of 4 mm for neck LN level I, 3 mm for neck LN level II, 5 mm for neck LN level III, and 3 mm for neck LN level IV was required to include all movements of each LN level. In patients using bite block, changes in primary tumor volume, and metastatic LN volume were related to significant movement.

Assessing setup errors and shifting margins for planning target volume in head, neck, and breast cancer

Accurately calculating setup errors is crucial in ensuring quality assurance for patients undergoing radiation therapy treatment. This cross-sectional study aimed to determine the systematic, random, and planning target volume (PTV) margin errors for patients with head and neck cancer (n=48) and breast cancer (n=50). The treatment setup was performed using electronic portal imaging (EPIDs) and irradiated using Elekta linac. The errors were calculated using the van Herk formula. The systematic error for the head and neck was 0.89, 0.43, and 1.49 mm on the x, y, and z-axis, respectively, and 0.39, 0.74, 0.38 for the breast cases. The random error was 0.82, 0.68, 0.94 mm for the head and neck and 0.66, 0.72, 0.79 mm for the breast. The PTV margin shifting error for the head and neck were 2.79, 1.55, and 4.38 mm, while it was 1.43, 2.35, and 1.50 mm for the breast. The setup errors varied according to the tumor location. The study highlights the potential benefits of using EPIDs for reducing uncertainties in setup verification procedures.

Rotational Set Up Uncertainly in Non-6D Couch and its Effects in Clinical Target Volume- Planning Target Volume Margin Calculation for Different Sites

Purpose

The purpose of this study was to estimate and incorporate rotational error to translational error for clinical target volume (CTV) to planning target volume (PTV) margin calculations for non-6D couch.

Materials and Methods

The study involved cone-beam computed tomography (CBCT) images of the patients who already had treatment in Varian Trilogy Clinac. The different sites studied were brain (70 patients, 406 CBCT images), head and neck (72 patients, 356 CBCT images), pelvis (83 patients, 606 CBCT images), and breast (45 patients, 163 CBCT images). Rotational and translational patient shifts were measured with the help of Varian eclipse offline review. The rotational shift introduces translational shift as it resolved along craniocaudal and mediolateral directions. Both rotational and translational error follow normal distribution and their respective errors were used to calculate CTV-PTV margin using van Herk model.

Results

Rotational effect on CTV-PTV margin contribution increases with increase in size of CTV. It also increases with increase in distance between center of mass of CTV and isocenter. These margins were more pronounce in single isocenter supraclavicular fossa-Tangential Breast plans.

Conclusions

There is always rotational error in all sites and it causes shift and rotation of the target. Rotational contribution to the CTV-PTV margin depends upon geometric center of CTV and isocenter distance and also on size of CTV. CTV-PTV margins should incorporate rotational error along with transitional error.

PTV margin calculation for head and neck patients treated with VMAT: a systematic literature review

Aim:

The intent of the review was to identify different methodological approaches used to calculate the planning target volume (PTV) margin for head and neck patients treated with volumetric arc therapy (VMAT), and whether the necessary factors to calculate the margin size with the selected formula were used.

Materials and Methods:

A comprehensive, systematic search of related studies was done using the Hydi search engine and different databases: MEDLINE, PubMed, CINAHL, ProQuest (Nursing and Allied Health), Scopus, ScienceDirect and tipsRO. The literature search included studies published between January 2007 and December 2020. Eligibility screening was performed by two reviewers.

Results:

A total of seven studies were found. All the reviewed studies used the Van Herk formula to measure the PTV margin. None of the studies incorporated the systematic errors of target volume delineation in the PTV equation. Inter-fraction translational errors were assessed in all the studies, whilst intra-fraction errors were only included in the margin equation for two studies. The studies showed great heterogeneity in the key characteristics, aims and methods.

Findings:

Since systemic errors from target volume delineation were not considered and not all studies assess intra-fraction errors, PTV margins may be underestimated. The recommendations are that studies need to determine the effect of target volume variance on PTV margins. It is also recommended to compare PTV margin results using various formulas.

Effectiveness of the No action level protocol for head & neck patients - Time considerations

Background

The No Action Protocol (NAL) was used to diminish the systematic set-up error. Recently, owing to the development of image registration technologies, the on-line positioning control is more often used. This method significantly reduces the CTV-PTV margin at the expense of the lengthening of a treatment session. The efficiency of NAL in decreasing the total treatment time for Head&Neck patients was investigated.

Methods

Results of set-up control of 30 patients were analyzed. The set-up control was carried out on-line. For each patient and each fraction, the set-error and the time needed for making the set-up control procedure were measured. Next, retrospectively, the NAL was applied to this data. The number of initial errors (without interventions) and after NAL protocol were compared in terms of errors larger than 3 and 4 mm. The average and total time used for portal control was calculated and compared.

Results

The number of setup errors in the posterior-anterior, inferior-superior, and right-left directions ≥3 mm and ≥4 mm were 98, 79, and 91 sessions and 44, 38 and 30 sessions out of 884 sessions. After NAL protocol the number of errors ≥3 mm and ≥4 mm decreased to 84, 57, and 39 sessions and 31, 15 and 10 sessions, respectively. The average time needed for one set-up control was 5.1 min. NAL protocol allows saving 4049 min for the whole group.

Conclusions

For locations where the random set-up errors are small, the NAL enables a very precise treatment of patients. Implementation of this protocol significantly decreases the total treatment time.

[Uncertainties in the current concept of radiotherapy planning target volume]

The planning target volume is an essential notion in radiotherapy, that requires a new conceptualization. Indeed, the variability and diversity of the uncertainties involved or improved with the development of the new modern technologies and devices in radiotherapy suggest that random and systematic errors cannot be currently generalized. This article attempts to discuss these various uncertainties and tries to demonstrate that a redefinition of the concept of planning target volume toward its personalization for each patient and the robustness notion are likely an improvement basis to take into account the radiotherapy uncertainties.

Estimating PTV Margins in Head and Neck Stereotactic Ablative Radiation Therapy (SABR) Through Target Site Analysis of Positioning and Intrafractional Accuracy

PURPOSE

Recurrent or previously irradiated head and neck cancers (HNC) are therapeutically challenging and may benefit from high-dose, highly accurate radiation techniques, such as stereotactic ablative radiation therapy (SABR). Here, we compare set-up and positioning accuracy across HNC subsites to further optimize the treatment process and planning target volume (PTV) margin recommendations for head and neck SABR.

METHODS AND MATERIALS

We prospectively collected data on 405 treatment fractions across 79 patients treated with SABR for recurrent/previously irradiated HNC. First, interfractional error was determined by comparing ExacTrac x-ray to the treatment plan. Patients were then shifted and residual error was measured with repeat x-ray. Next, cone beam computed tomography (CBCT) was compared with ExacTrac for positioning agreement, and final shifts were applied. Lastly, intrafractional error was measured with x-ray before each arc. Results were stratified by treatment site into skull base, neck/parotid, and mucosal.

RESULTS

Most patients (66.7%) were treated to 45 Gy in 5 fractions (range, 21-47.5 Gy in 3-5 fractions). The initial mean ± standard deviation interfractional errors were -0.2 ± 1.4 mm (anteroposterior), 0.2 ± 1.8 mm (craniocaudal), and -0.1 ± 1.7 mm (left-right). Interfractional 3-dimensional vector error was 2.48 ± 1.44, with skull base significantly lower than other sites (2.22 vs 2.77; P = .0016). All interfractional errors were corrected to within 1.3 mm and 1.8°. CBCT agreed with ExacTrac to within 3.6 mm and 3.4°. CBCT disagreements and intrafractional errors of >1 mm or >1° occurred at significantly lower rates in skull base sites (CBCT: 16.4% vs 50.0% neck, 52.0% mucosal, P < .0001; intrafractional: 22.0% vs 48.7% all others, P < .0001). Final PTVs were 1.5 mm (skull base), 2.0 mm (neck/parotid), and 1.8 mm (mucosal).

CONCLUSIONS

Head and neck SABR PTV margins should be optimized by target site. PTV margins of 1.5 to 2 mm may be sufficient in the skull base, whereas 2 to 2.5 mm may be necessary for neck and mucosal targets. When using ExacTrac, skull base sites show significantly fewer uncertainties throughout the treatment process, but neck/mucosal targets may require the addition of CBCT to account for positioning errors and internal organ motion.

A study of nonuniform CTV to PTV margin expansion incorporating both rotational and translational uncertainties

PURPOSE

In this work, we implemented a method to obtain a nonuniform clinical target volume (CTV) to planning target volume (PTV) margin caused by both rotational and translational uncertainties and evaluated it in the treatment planning system (TPS).

MATERIALS AND METHOD

Based on a previously published statistical model, the relationship between a target margin and the distance d (from isocenter to target point), setup uncertainties, and significance level was established. For a single CTV, it can be thought as a combination of many small volume elements or target points. The margin of each point could be obtained using the suggested statistical model. The whole nonuniform CTV-PTV margin was determined by the union of all possible margins of the CTV boundary points. This method was implemented in the Pinnacle³ treatment planning system and compared with uniform margin algorithm. Ten vertebral metastases targets and multiple brain metastases targets were chosen for evaluation.

RESULTS

The combined CTV-PTV margin as a function of d for various initial translational margin and rotational uncertainties was calculated. The combined margin increases as d, rotational uncertainties and translational margin increase. For the same rotational uncertainty, a smaller initial translational margin requires a larger rotational margin to compensate for the rotational error. Compared with the uniform margin algorithm, the advantage of this method is that it could minimize the PTVs volume for given CTVs to obtain same significance level. Using vertebral metastases targets and multiple brain metastases targets, a series of volume difference was obtained for various translational margins and rotational uncertainties. The volume difference of PTV could be more than 17% when translational margin is 2 mm and rotational uncertainty is 1.4°.

CONCLUSION

Nonuniform margin algorithm could avoid excessive compensation for the CTV boundary points near isocenter. This method could be used for clinical margin determination and might be useful for the protection of risk organs.

Evaluation of Target Volume Location and Its Impact on Delivered Dose Using Cone-Beam Computed Tomography Scans for Patients with Head and Neck Cancer

INTRODUCTION

Within radiation oncology, treatment of head and neck cancer is known for its unique challenges with patient weight loss and body contour changes. This study sought to quantify these changes through measuring the volume and position of specific target structures over the course of radiation treatment and determining if changes in these factors affected what dose was ultimately delivered.

METHODS

This study utilized weekly cone-beam computed tomography (CBCT) images taken immediately before radiation treatments to measure the difference between the expected location and the actual location of clinical target volumes. Minimum and mean doses to planned target volumes (PTVs) were then calculated on the CBCT scans and compared with the expected planned dose.

RESULTS

In the twenty patients included in this single-institutional study, a significant average difference of 2.47% (P < .0001) and 5.06% (P < .0001) was found in the locations of the high-risk and low-risk clinical target volumes, respectively. Software limitations reduced the sample size that could be used to compare delivered and planned dose to nine patients, but of that number, a significant decrease of 10% was found in the minimum dose delivered to both the high-risk (P = .0401) and low-risk (P = .0123) PTVs. Mean doses to the PTVs did not differ significantly and no correlation was found between any volumetric and dosimetric deviations.

CONCLUSION

The results of this study support the presence of volume matching inaccuracies for patients with head and neck cancer with simultaneous altered minimum doses to PTVs. Based on these findings, it is suggested that institutions may benefit from a standardized treatment imaging protocol that would include a minimum of weekly full-trajectory CBCT scans to assess target volume location, particularly those of the inferior nodal volumes.

Assessment of the therapeutic accuracy of cone beam computed tomography-guided nasopharyngeal carcinoma radiotherapy

The aim of the present study was to determine the ability of cone beam computed tomography (CBCT) to improve the accuracy of nasopharyngeal carcinoma (NPC) radiotherapy by analyzing the setup and inter-fraction errors at different levels and directions of the target volumes. A total of 113 patients with NPC who were undergoing intensity-modulated radiotherapy were recruited for the present study. Each patient had at least three CBCT exams prior to the start of radiation therapy. Three anatomic bony landmarks, including the upper neck, lower neck and head, were used to represent the different levels of assessment. The positioning errors were registered in three planes throughout the course of radiotherapy: The right-left (RL), superior-inferior (SI) and anterior-posterior (AP) directions. The planning CT images were matched with the CBCT images to determine the naso-pharynx shifts. A receiver operating characteristic curve was plotted to establish the specificity and sensitivity of CBCT. The planning target volume margin (MPTV) for the head was 0.9 mm, 1.4 mm for the upper neck and 2.0 mm for the lower neck. MPTVs of 1.5, 0.6 and 2.2 mm in the RL, SI and AP directions, respectively, were detected. In addition, there was evidence of setup errors in the three planes (RL, SI and AP) with the greatest error observed in the AP direction. Furthermore, the setup uncertainties in the neck region were greater than those of the head. In conclusion, CBCT could greatly improve the accuracy of radiotherapy by minimizing the setup errors and MPTV.