Dosimetric Effects of Head and Neck Immobilization Devices on Multi-field Intensity Modulated Radiation Therapy for Nasopharyngeal Carcinoma

A simple and low-cost technic for immobilization of mandible during head and heck radiotherapy: a technical note

Immobilization protocols for head and neck radiotherapy (HNR) are needed to reduce radiation dispersion in non-tumoral tissues and to reduce damage to noble structures; however, involuntary mandibular movements require additional adaptations of thermoplastic masks.

PURPOSE

Our objective was to describe an easy and low-cost disposition to help thermoplastic masks immobilize the mandible during HNR.

MATERIALS AND METHODS

We created Styrofoam models to stabilize the mandible which was used to make adaptation to the thermoplastic masks. The Styrofoam model was inserted into alginate to construct a mold and a self-curing acrylic resin was added into the mold. After the acrylic resin hardened, the dispositive mandibular immobilizer was removed from the alginate mold and adjusted with Maxicut and Minicut or other drills and sandpaper. The alginate mold can be used on the same day to create backup devices.

RESULTS AND CONCLUSIONS

Our device was made in a simple way, has a low cost, and helps the thermoplastic masks to immobilize the mandible, leading to a more precise individualization of head and neck immobilization that can reduce unanticipated radiation scatter and improve radiation distribution.

Effect of Body Size Change on Off-Center Cervical Point and Face Doses in Patients Undergoing Nasopharyngeal Carcinoma Radiotherapy

Nasopharyngeal carcinoma (NPC) is a clinically multiple malignant tumor. At present, with the increase in the infection rate of Epstein-Barr virus, the incidence of nasopharyngeal carcinoma is also increasing day by day. To explore the effect of body size change on off-center cervical point and face doses in patients with nasopharyngeal carcinoma (NPC) undergoing radiotherapy, in total, 100 patients with NPC from January 2019 to May 2020 in our hospital were selected for retrospective analysis, and they all received intensity-modulated radiation therapy. Bodyweight, horizontal longitudinal diameter of the odontoid process, longitudinal diameter of the third cervical spine, maximum radiation dose, and average radiation dose of normal organs in the first and last treatments were assessed, and the correlation between normal organ irradiation dose and body size was analyzed. Bodyweight, horizontal longitudinal diameter of the odontoid process, and longitudinal diameter of the third cervical spine in the last treatment were lower than those in the first treatment, with a statistically significant difference. There was no statistically significant difference in the maximum normal organ irradiation dose to the left eyeball, right eyeball, left crystalline lens, right crystalline lens, and maximum irradiation dose to optic nerve between the last treatment and the first treatment. In the last treatment, the maximum dose to the left parotid gland, right parotid gland, spinal cord, and brain stem was higher than that in the first treatment. The average irradiation dose to the left eye bulb, right eye bulb, left lens, right lens, optic nerve in the last treatment, and that in the first treatment showed no significant difference. The average dose to the left parotid gland, right parotid gland, spinal cord, and brain stem in the last treatment was higher than that in the first treatment. The irradiation dose to the left parotid gland, right parotid gland, spinal cord, and brain stem was significantly negatively correlated with body weight, horizontal longitudinal diameter of the odontoid process, and longitudinal diameter of the third cervical spine. After NPC radiotherapy, the body size of patients can change, which can have different effects on irradiation doses. Therefore, the target area and dose should be corrected during treatment to ensure the efficacy and safety of the treatment.

The effect of carbon fibre treatment couch with and without immobilisation devices on radiotherapy dose calculation using three different planning algorithms and photon beam energies

Introduction:

The objective of radiotherapy immobilisation devices is to improve the reproducibility of patient positioning during treatment sessions. The inclusion of these devices in the treatment protocol may increase the skin dose. In practice, these devices are not systematically taken into account in the dose calculation.

Material and methods:

In this study, the dosimetric effects of the carbon fibre couch iBEAM Evo Extension 415, with and without three different immobilisation devices (a Klarity Breastboard R610-2ECF, a Bionix Butterfly Board and CIVCO Vac-Lok vacuum bag), were calculated and evaluated on the dose calculation for conformal three-dimensional radiation therapy. The measurements were carried out by comparing the measured dose with the one calculated for three different algorithms, FFT convolution, fast superposition and superposition algorithms, which are implemented in Xio treatment planning system (TPS).

Results:

Dosimetric tolerance levels have been respected for specific dose calculations, which do not include the fibre couch with or without immobilisation devices. Errors of up to 8% in the dose calculation were obtained for the beams passing through the fibre couch and the breast board base support region.

Conclusion:

According to the significant attenuation differences of the beam by the fibre couch and immobilisation devices, it was concluded that ignoring the device in the dose calculation can change patient’s skin and target doses. The fibre couch and immobilisation device should be included within external body contour to account for the TPS calculation algorithms dose attenuation.

Dosimetric effects of supine immobilization devices on the skin in intensity-modulated radiation therapy for breast cancer: a retrospective study

Background

The dose perturbation effect of immobilization devices is often overlooked in intensity-modulated radiation therapy (IMRT) for breast cancer (BC). This retrospective study assessed the dosimetric effects of supine immobilization devices on the skin using a commercial treatment planning system.

Methods

Forty women with BC were divided into four groups according to the type of primary surgery: groups A and B included patients with left and right BC, respectively, who received 50 Gy radiotherapy in 25 fractions after radical mastectomy, while groups C and D included patients with left and right BC, respectively, who received breast-conservation surgery (BCS) and 40.05 Gy in 15 fractions as well as a tumor bed simultaneous integrated boost to 45 Gy. A 0.2-cm thick skin contour and two sets of body contours were outlined for each patient. Dose calculations were conducted for the two sets of contours using the same plan. The dose differences were assessed by comparing the dose-volume histogram parameter results and by plan subtraction.

Results

The supine immobilization devices for BC resulted in significantly increased skin doses, which may ultimately lead to skin toxicity. The mean dose increased by approximately 0.5 and 0.45 Gy in groups A and B after radical mastectomy and by 2.7 and 3.25 Gy in groups C and D after BCS; in groups A–D, the percentages of total normal skin volume receiving equal to or greater than 5 Gy (V₅) increased by 0.54, 1.15, 2.67, and 1.94%, respectively, while the V₁₀ increased by 1.27, 1.83, 1.36, and 2.88%; the V₂₀ by 0.85, 1.87, 2.76, and 4.86%; the V₃₀ by 1.3, 1.24, 10.58, and 11.91%; and the V₄₀ by 1.29, 0.65, 10, and 10.51%. The dose encompassing the planning target volume and other organs at risk, showed little distinction between IMRT plans without and with consideration of immobilization devices.

Conclusions

The supine immobilization devices significantly increased the dose to the skin, especially for patients with BCS. Thus, immobilization devices should be included in the external contour to account for dose attenuation and skin dose increment.

Trial registration

This study does not report on interventions in human health care.