Impact of geometric variations on delivered dose in highly focused single vocal cord IMRT

Early-Stage Glottic Squamous Cell Carcinoma in the Era of Image-Guided Radiotherapy

Early-stage squamous cell cancer (SCC) of the glottis has a good prognosis. Therefore, patients have long survival outcomes and may potentially suffer from late toxicities of radiotherapy. Radiotherapy with a conventional parallel-opposed-pair or anterior-oblique beam arrangements for stage 1 and 2 glottic SCC have field borders that traditionally cover the entire larynx, exposing organs-at-risk (e.g. carotid arteries, contralateral vocal cord, contralateral arytenoid and inferior pharyngeal constrictor muscles) to high radiation doses. The potential long-term risk of cerebrovascular events has attracted much attention to the dose that carotid arteries receive. Swallow and respiratory motion of laryngeal structures has been an important factor that previously limited reduction of the radiation treatment volume. Motion has been evaluated using multiple imaging modalities and this information has been used to calculate PTV margins for generation of more limited target volumes. This review discusses the current literature surrounding dose-effect relationships for various organs-at-risk and the late toxicities that are associated with them. This article also reviews the currently available data and effects of laryngeal motions on dosimetry to the primary target. We also review the current limitations and benefits of a more targeted approach of radiotherapy for early-stage glottic SCCs and the evolution of CT-based IGRT and MR-guided radiotherapy techniques that may facilitate a shift away from a conventional 3D-conformal radiotherapy approach.

Vocal-cord Only vs. Complete Laryngeal radiation (VOCAL): a randomized multicentric Bayesian phase II trial

BACKGROUND

Radiotherapy, along with laser surgery, is considered a standard treatment option for patients with early glottic squamous cell cancer (SCC). Historically, patients have received complete larynx radiotherapy (CL-RT) due to fear of swallowing and respiratory laryngeal motion and this remains the standard approach in many academic institutions. Local control (LC) rates with CL-RT have been excellent, however this treatment can carry significant toxicities include adverse voice and swallowing outcomes, along with increased long-term risk of cerebrovascular morbidity. A recent retrospective study reported improved voice quality and similar local control outcomes with focused vocal cord radiotherapy (VC-RT) compared to CL-RT. There is currently no prospective evidence on the safety of VC-RT. The primary objective of this Bayesian Phase II trial is to compare the LC of VC-RT to that of CL-RT in patients with T1N0 glottic SCC.

METHODS

One hundred and fifty-five patients with T1a-b N0 SCC of the true vocal cords that are n ot candidate or declined laser surgery, will be randomized in a 1:3 ratio the control arm (CL-RT) and the experimental arm (VC-RT). Randomisation will be stratified by tumor stage (T1a/T1b) and by site (each site will be allowed to select one preferred radiation dose regimen, to be used in both arms). CL-RT volumes will correspond to the conventional RT volumes, with the planning target volume extending from the top of thyroid cartilage lamina superiorly to the bottom of the cricoid inferiorly. VC-RT volumes will include the involved vocal cord(s) and a margin accounting for respiration and set-up uncertainty. The primary endpoint will be LC at 2-years, while secondary endpoints will include patient-reported outcomes (voice impairment, dysphagia and symptom burden), acute and late toxicity radiation-induced toxicity, overall survival, progression free survival, as well as an optional component of acoustic and objective measures of voice analysis using the Consensus Auditory-Perceptual Evaluation of Voice.

DISCUSSION

This study would constitute the first prospective evidence on the efficacy and safety of VC-RT in early glottic cancer. If positive, this study would result in the adoption of VC-RT as standard approach in early glottic cancer.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT03759431 Registration date: November 30, 2018.

Larynx motion considerations in partial larynx volumetric modulated arc therapy for early glottic cancer

INTRODUCTION

To assess laryngeal motion in early glottic cancer in order to determine safe margins for partial larynx volumetric modulated arc therapy (PL-VMAT), and to quantify dosimetric advantages of PL-VMAT.

METHODS

This prospective study included T1-2N0 glottic cancers treated with whole larynx VMAT (WL-VMAT). Pre- and mid-treatment 4D-computed tomography (4D-CT) and dynamic magnetic resonance imaging (MRI) allowed for assessment of larynx swallowing and respiratory motion. For 10 patients with lateralized lesions, PL-VMAT plans were calculated using margins derived from 4D-CT analysis.

RESULTS

Twenty patients were accrued from 2014 to 2016. Mean amplitude of larynx swallowing excursion was 23 mm and 6 mm in the superior and anterior directions, respectively. Mean respiratory motion reached 4 mm and 2 mm in superior-inferior and antero-posterior directions, respectively. Pre-treatment 4D-CT analysis identified one patient with planning CT acquired during swallowing. Mid-treatment 4D-CT revealed larynx shift relative to vertebrae in 30% of cases. PL-VMAT allowed for significant reduction of mean doses to ipsilateral carotid, contralateral carotid, thyroid gland, contralateral arytenoid and larynx. Using 8 mm internal margin for PL-VMAT, swallowing resulted in clinical target volume excursion beyond 95% isodose line during ≤1.5% of total treatment time in all patients.

CONCLUSION

Although swallowing motion is rare, rapid and easily suppressed by patients, there is a risk of systematic miss-targeting if planning CT is acquired during swallowing. Larynx position shift relative to vertebrae occurs in 1/3 of patients over the course of radiotherapy. With soft-tissue image guidance and margins accounting for respiratory motion, PL-VMAT allows safe reduction of dose to organs at risk.

Intensity-modulated radiotherapy for early glottic cancer: transition to a new standard of care?

For decades, the standard of care for radiation treatment of early larynx cancers has been conventional treatment using opposed lateral fields encompassing the larynx and overlying neck structures, including the adjacent carotid arteries. While intensity-modulated radiotherapy (IMRT) has replaced conventional radiotherapy for all other head/neck cancer situations, the use of IMRT to treat early glottic cancers remains controversial. The article reviews the published experience with IMRT for this clinical situation and provides a detailed review of the literature on radiation-induced carotid toxicity and how it might apply to the controversy. Finally, we discuss whether the radiation oncology community should transition to IMRT as a new standard of care for the treatment of early glottic cancers.

Volumetric modulated arc therapy for carotid sparing in the management of early glottic cancer

Purpose

Radiotherapy of the neck is known to cause carotid artery stenosis. We compared the carotid artery dose received between volumetric modulated arc therapy (VMAT) and conventional fixed-field intensity-modulated radiotherapy (IMRT) plans in patients with early glottic cancer.

Materials and Methods

Twenty-one early glottic cancer patients who previously underwent definitive radiotherapy were selected for this study. For each patient, double arc VMAT, 8-field IMRT, 3-dimensional conformal radiotherapy (3DCRT), and lateral parallel-opposed photon field radiotherapy (LPRT) plans were created. The 3DCRT plan was generated using lateral parallel-opposed photon fields plus an anterior photon field. VMAT and IMRT treatment plan optimization was performed under standardized conditions to obtain adequate target volume coverage and spare the carotid artery. Dose-volume specifications for the VMAT, IMRT, 3DCRT, and LPRT plans were calculated with radiotherapy planning system. Monitor units (MUs) and delivery time were measured to evaluate treatment efficiency.

Results

Target volume coverage and homogeneity results were comparable between VMAT and IMRT; however, VMAT was superior to IMRT for carotid artery dose sparing. The mean dose to the carotid arteries in double arc VMAT was reduced by 6.8% compared to fixed-field IMRT (p < 0.001). The MUs for VMAT and IMRT were not significantly different (p = 0.089). VMAT allowed an approximately two-fold reduction in treatment delivery time in comparison to IMRT (3 to 5 minutes vs. 5 to 10 minutes).

Conclusion

VMAT resulted in a lower carotid artery dose compared to conventional fixed-field IMRT, and maintained good target coverage in patients with early glottic cancer.

Volumetric modulated arc radiotherapy of the whole larynx, followed by a single affected vocal cord, for T1a glottic cancer: Dosimetric analysis of a case

Radiation therapy (RT) and endolaryngeal surgery are standard treatments for early-stage glottic cancer. They have closely matched oncological outcomes; however, it is debatable which method is superior in terms of functional outcomes. Several dosimetric studies have demonstrated that, compared with conventional RT, intensity-modulated RT (IMRT) reduces unnecessary radiation of the adjacent normal tissues, including the carotid artery and thyroid gland. However, RT targets the whole larynx, whereas endolaryngeal surgery is a highly focused treatment involving the en bloc resection of a tumor with safety margins. For T1a glottic cancer, in which the tumor is limited to one vocal cord, the technical feasibility of targeting IMRT on the single vocal cord affected has been investigated; however, the clinical feasibility and the possibility of inferior local control remain to be elucidated. In the present case study, IMRT was used to treat the whole larynx first, and then to treat a single vocal cord. The patient in the present study had T1a glottic cancer, and received volumetric modulated arc therapy with a total dose of 63 Gy/28 fractions. The first treatment phase (40.5 Gy/18 fractions) targeted the whole larynx to eliminate subclinical disease. The second treatment phase (22.5 Gy/10 fractions) targeted only the involved vocal cord. During this treatment phase, the exposure of the non-involved right vocal cord, the right carotid artery and the thyroid gland to the radiation was lower compared with the continuation of the initial treatment approach. These findings suggested that changing the target volume from the whole larynx to the affected vocal cord during the course of IMRT is feasible for T1a glottic cancer, and that it may reduce functional side effects while maintaining oncological outcomes.

Inter- and Intrafraction Target Motion in Highly Focused Single Vocal Cord Irradiation of T1a Larynx Cancer Patients

PURPOSE

The purpose of this study was to verify clinical target volume--planning target volume (CTV-PTV) margins in single vocal cord irradiation (SVCI) of T1a larynx tumors and characterize inter- and intrafraction target motion.

METHODS AND MATERIALS

For 42 patients, a single vocal cord was irradiated using intensity modulated radiation therapy at a total dose of 58.1 Gy (16 fractions × 3.63 Gy). A daily cone beam computed tomography (CBCT) scan was performed to online correct the setup of the thyroid cartilage after patient positioning with in-room lasers (interfraction motion correction). To monitor intrafraction motion, CBCT scans were also acquired just after patient repositioning and after dose delivery. A mixed online-offline setup correction protocol ("O2 protocol") was designed to compensate for both inter- and intrafraction motion.

RESULTS

Observed interfraction, systematic (Σ), and random (σ) setup errors in left-right (LR), craniocaudal (CC), and anteroposterior (AP) directions were 0.9, 2.0, and 1.1 mm and 1.0, 1.6, and 1.0 mm, respectively. After correction of these errors, the following intrafraction movements derived from the CBCT acquired after dose delivery were: Σ = 0.4, 1.3, and 0.7 mm, and σ = 0.8, 1.4, and 0.8 mm. More than half of the patients showed a systematic non-zero intrafraction shift in target position, (ie, the mean intrafraction displacement over the treatment fractions was statistically significantly different from zero; P<.05). With the applied CTV-PTV margins (for most patients 3, 5, and 3 mm in LR, CC, and AP directions, respectively), the minimum CTV dose, estimated from the target displacements observed in the last CBCT, was at least 94% of the prescribed dose for all patients and more than 98% for most patients (37 of 42). The proposed O2 protocol could effectively reduce the systematic intrafraction errors observed after dose delivery to almost zero (Σ = 0.1, 0.2, 0.2 mm).

CONCLUSIONS

With adequate image guidance and CTV-PTV margins in LR, CC, and AP directions of 3, 5, and 3 mm, respectively, excellent target coverage in SVCI could be ensured.

Carotid-Sparing TomoHelical 3-Dimensional Conformal Radiotherapy for Early Glottic Cancer

Purpose

The purpose of this study was to investigate the dosimetric benefits and treatment efficiency of carotid-sparing TomoHelical 3-dimensional conformal radiotherapy (TH-3DCRT) for early glottic cancer.

Materials and Methods

Ten early-stage (T1N0M0) glottic squamous cell carcinoma patients were simulated, based on computed tomography scans. Two-field 3DCRT (2F-3DCRT), 3-field intensity-modulated radiation therapy (3F-IMRT), TomoHelical-IMRT (TH-IMRT), and TH-3DCRT plans were generated with a 67.5-Gy total prescription dose to the planning target volume (PTV) for each patient. In order to evaluate the plan quality, dosimetric characteristics were compared in terms of conformity index (CI) and homogeneity index (HI) for PTV, dose to the carotid arteries, and maximum dose to the spinal cord. Treatment planning and delivery times were compared to evaluate treatment efficiency.

Results

The median CI was substantially better for the 3F-IMRT (0.65), TH-IMRT (0.64), and TH-3DCRT (0.63) plans, compared to the 2F-3DCRT plan (0.32). PTV HI was slightly better for TH-3DCRT and TH-IMRT (1.05) compared to 2F-3DCRT (1.06) and 3F-IMRT (1.09). TH-3DCRT, 3F-IMRT, and TH-IMRT showed an excellent carotid sparing capability compared to 2F-3DCRT (p < 0.05). For all plans, the maximum dose to the spinal cord was < 45 Gy. The median treatment planning times for 2F-3DCRT (5.85 minutes) and TH-3DCRT (7.10 minutes) were much lower than those for 3F-IMRT (45.48 minutes) and TH-IMRT (35.30 minutes). The delivery times for 2F-3DCRT (2.06 minutes) and 3F-IMRT (2.48 minutes) were slightly lower than those for TH-IMRT (2.90 minutes) and TH-3DCRT (2.86 minutes).

Conclusion

TH-3DCRT showed excellent carotid-sparing capability, while offering high efficiency and maintaining good PTV coverage.