Split-field vs extended-field intensity-modulated radiation therapy plans for oropharyngeal cancer: Which spares the larynx? Which spares the thyroid?

Dosimetric accuracy of dual isocenter irradiation in low magnetic field resonance guided radiotherapy system for extended abdominal tumours

PURPOSE

Due to limited field size of Magnetic Resonance Linear Accelerators (MR-Linac), some treatments could require a dual-isocenter planning approach to achieve a complete target coverage and thus exploit the benefits of the online adaptation. This study evaluates the dosimetric accuracy of the dual-isocenter intensity modulated radiation therapy (IMRT) delivery technique for MR-Linac.

MATERIAL AND METHODS

Dual-isocenter multi leaf collimator (MLC) and couch accuracy tests have been performed to evaluate the delivery accuracy of the system. A mono-isocenter plan delivered in clinical practice has then been retrospectively re-planned with dual-isocenter technique. The dual-isocenter plan has been re-calculated and delivered on a 3-dimensional (3D) ArcCHECK phantom and 2-dimensional (2D) films to assess its dosimetric accuracy in terms of gamma analysis. Clinical and planning target volume (CTV and PTV respectively) coverage robustness was then investigated after the introduction of ± 2 mm and ± 5 mm positioning errors by shifting the couch.

RESULTS

MLC and couch accuracy tests confirmed the system accuracy in delivering a dual-isocenter irradiation. 2D/3D gamma analysis results occurred always to be above 95% if considered a gamma criteria 1%/2 mm and 1%/1 mm respectively for the 2D and 3D analysis. The mean variations for CTV D98% and PTV V95% were 0.2% and 1.1% respectively when positioning error was introduced separately in each direction, while the maximum observed variations were 0.9% (CTV) and 3.7% (PTV).

CONCLUSION

The dosimetric accuracy of dual-isocenter irradiation has been verified for MR-Linac, achieving accurate and robust treatment strategy and improving dose conformality also in presence of targets whose extension exceeds the nominal maximum field size.

Split-field versus extended-field step-and-shoot IMRT techniques in nasopharyngeal cancer: a report of acute and late toxicities

Aim:

This study aimed to evaluate acute and late toxicities in nasopharyngeal cancer (NPC) patients who were treated between split-field (SF) and extended-field (EF) step-and-shoot intensity-modulated radiotherapy (IMRT) techniques.

Materials and methods:

Between January 2011 and October 2011, 21 NPC patients with stage I-IVB (7th edition American Joint Committee on Cancer Staging) were randomly assigned to undergo radiotherapy with SF or EF step-and-shoot IMRT technique.

Results:

At a median follow-up time of 60 months (range 3–77), we reported the comparable acute and late toxicities between the two techniques. One patient (9%) in SF-IMRT arm developed grade 3 acute skin toxicity.

Findings:

Both SF and EF step-and-shoot IMRT techniques for NPC patients did not produce any statistically significant differences in both acute and late toxicities. Although no difference in toxicity was observed, technical problems due to field matching management were the obstacles in utilisation of SF-IMRT in our routine practice.

Validation of previously reported predictors for radiation-induced hypothyroidism in nasopharyngeal cancer patients treated with intensity-modulated radiation therapy, a post hoc analysis from a Phase III randomized trial

This study aimed to validate previously reported dosimetric parameters, including thyroid volume, mean dose, and percentage thyroid volume, receiving at least 40, 45 and 50 Gy (V40, V45 and V50), absolute thyroid volume spared (VS) from 45, 50 and 60 Gy (VS45, VS50 and VS60), and clinical factors affecting the development of radiation-induced hypothyroidism (RHT). A post hoc analysis was performed in 178 euthyroid nasopharyngeal cancer (NPC) patients from a Phase III study comparing sequential versus simultaneous-integrated boost intensity-modulated radiation therapy. RHT was determined by increased thyroid-stimulating hormone (TSH) with or without reduced free thyroxin, regardless of symptoms. The median follow-up time was 42.5 months. The 1-, 2- and 3-year freedom from RHT rates were 78.4%, 56.4% and 43.4%, respectively. The median latency period was 21 months. The thyroid gland received a median mean dose of 53.5 Gy. Female gender, smaller thyroid volume, higher pretreatment TSH level (≥1.55 μU/ml) and VS60 < 10 cm3 were significantly associated with RHT in univariate analyses. Only pretreatment TSH ≥ 1.55 μU/ml and VS60 < 10 cm3 were significant predictors in multivariate analysis. Our results suggested that patients with pretreatment TSH ≥ 1.55 μU/ml should be cautious about the risk of RHT. The VS60 ≥ 10 cm3 is recommended for treatment planning.

Sparing dysphagia/aspiration related structures using novel hybrid volumetric modulated arc therapy

PURPOSE

Studies using split field IMRT to spare dysphagia/aspiration related structures (DARS) have raised concern regarding dose uncertainty at matchline. This study explores the utility of hybrid VMAT in sparing the DARS and assesses matchline dose uncertainty in postoperative oral cavity cancer patients and compares it with VMAT.

METHODS & MATERIALS

Ten postoperative oral cavity cancer patients were planned with h-VMAT and VMAT using the same planning CT dataset. PTV and DARS were contoured using standard delineation guidelines. In h-VMAT 80% of the neck dose was planned using AP/PA technique and then VMAT optimization was done for the total PTV by keeping the corresponding AP/PA plan as the base dose. Planning goal for PTV was V_95%≥95% and for DARS, adequate sparing. Plans and dose volume histograms were analyzed using dosimetric indices. Absolute point and portal dose measurements were done for h-VMAT plans to verify dose at the matchline.

RESULTS

Coverage in both the techniques was comparable. Significant differences were observed in mean doses to DARS (Larynx: 24.36±2.51 versus 16.88±2.41Gy; p<0.0006, Pharyngeal constrictors: 25.16±2.41 versus 21.2±2.1Gy; p<0.005, Esophageal inlet: 18.71±2 versus 12.06±0.79Gy; p<0.0002) favoring h-VMAT. Total MU in both the techniques was comparable. Average percentage variations in point dose measurements in h-VMAT done at +3.5 and -3.5 positions were (1.47±1.48 and 2.28±1.35%) respectively. Average gamma agreement for portal dose measured was 97.07%.

CONCLUSION

h-VMAT achieves better sparing of DARS with no matchline dose uncertainty. Since these patients have swallowing dysfunction post-operatively, attempts should be made to spare these critical structures as much as possible.

Reducing radiation-related morbidity in the treatment of nasopharyngeal carcinoma

While radiation therapy is the mainstay of treatment for nasopharyngeal carcinoma, the anatomic location of the nasopharynx in close proximity to radiation-sensitive organs such as the salivary glands, optic nerves and chiasm, cochlea, brainstem and temporal lobes presents a special challenge. Technological approaches to reducing the morbidity of nasopharyngeal cancer irradiation have been historically successful with the evolution from 2D techniques to increasingly conformal forms of radiation therapy. This report reviews normal tissue dose constraints and major considerations in target delineation for patients with nasopharyngeal cancer in the intensity-modulated radiation therapy era. Furthermore, this report discusses more contemporary approaches to toxicity reduction such as the judicious reduction or omission of radiation to low-risk regions and the potential role of particle beam therapy.