Dosimetric Effect of Jaw Tracking in Volumetric-Modulated Arc Therapy

Research on the correction method for radiotherapy verification plans based on displaced electronic portal imaging device

BACKGROUND

It has been observed that under the single isocenter conditions, the potential shifts of the electronic portal imaging devices (EPID) may be introduced when executing portal dosimetry (PD) plans for bilateral breast cancer, pleural mesothelioma, and lymphoma. These shifts are relative to the calibration positions of EPID and result in significant discrepancies in the plan verification results.

PURPOSE

To explore methods including correction model and specific correction matrices to revise the data obtained from displaced EPID.

METHODS

Two methods, the correction model and the specific correction matrices, were applied to correct the data. Five experiments were designed and conducted to build correction model and to validate the effectiveness of these two methods. Gamma passing rates were calculated and data profiles along X-axis and Y-axis were captured.

RESULTS

The gamma passing rates for the EPID-displaced IMRT validation plans after applying correction model, along with the application of specific correction matrices to VMAT and IMRT validation plans, exhibit results that are comparable to the cases with non-displaced EPID. Except for the VMAT plans applied correction model which showed larger discrepancies (0.041 ± 0.028, 0.049 ± 0.030), the other three exhibit minimal differences in discrepancy values. In all profiles, the corrected data from displaced EPID exhibit a high level of agreement with data obtained from non-displaced EPID. Good consistency is observed in actual application of the correction model and the specific correction matrices between gamma passing rates of data corrected and those of non-displaced data.

CONCLUSIONS

The proposed methods involving correction model and specific correction matrices can correct the data collected from the displaced EPID, and the gamma passing rates of the corrected data show results that are comparable to some extent with those of non-displaced data. Particularly, the results corrected by specific correction matrices closely resemble the data from non-displaced EPID.

Effect of Jaw Tracking During Volumetric Modulated Arc Therapy for Facial Non-melanoma Skin Cancer

BACKGROUND/AIM

This study aimed to analyze the dosimetric effects of jaw tracking during Volumetric Modulated Arc Therapy (VMAT) planning for facial non-melanoma skin cancer (NMSC).

PATIENTS AND METHODS

This study included 50 patients with facial NMSC who underwent VMAT planning with or without jaw tracking. The target volume (TV) included the primary skin lesion with a 1-cm margin around the surface and a depth of 4 mm. A total of 55 Gy in 20 fractions was prescribed, and the plans were considered acceptable if the TV was covered by 95-105% of the isodose curve. A dosimetric comparison was performed for the volumes of the low-dose regions, which were defined as <50% of the prescription dose (V10-50%). Target coverage was evaluated using the homogeneity index (HI) and conformity index (CI).

RESULTS

The patients' mean TV was 5.137 cc (range=1.03-15.89 cc). Jaw tracking resulted in mean volume reduction rates of 3.9%, 6.6% 10.6% and 13.8% for V40%, V30%, V20%, and V10%, respectively (all p<0.001). The volume change in V50% between the two groups was 2.7% (p=0.006). No significant differences were observed in HI (p=0.449) or CI (p=0.127).

CONCLUSION

The application of jaw tracking during VMAT for facial NMSC is associated with a significant reduction in the volume of low dose delivered in the radiation field (V10-50%), while maintaining target coverage. Future analyses should assess whether this volume difference affects treatment-related cosmetic outcomes.

Estimated radiation doses to ovarian and uterine organs in breast cancer irradiation using radio-photoluminescent glass dosimeters (RPLDs)

INTRODUCTION

The well-being of breast cancer patients is essential, especially fertility in patients of reproductive age. The objective of this study was to estimate the radiation doses to the ovaries and uterus for different treatment techniques of breast cancer irradiation using radio-photoluminescent glass dosimeters (RPLDs).

METHODS

A Farmer-type ionisation chamber (IBA FC-65G) and RPLDs were used to measure in- and out-of-field radiation doses in a solid water phantom. The field sizes were set to 10 × 10 cm2 and 8 × 17 cm2 with the central axis at out-of-field measurement distances of 30 or 50 cm. The Rando phantom's left breast was planned using four different techniques: two tangential standard fields with and without electronic tissue compensator (E-comp) techniques, intensity-modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT). The radiation doses in the ipsilateral ovary, contralateral ovary and uterus were measured using RPLDs.

RESULTS

The percentage ratio of out of field to in field was affected by distance from the central axis to the point of measurement, in addition to the field sizes associated with collimator scatter. Advanced techniques such as IMRT and VMAT produced higher doses to the ovaries and uterus. The estimated results of the worst-case scenario for the ipsilateral ovary, contralateral ovary and uterus were 0.84% (42 cGy), 0.62% (31 cGy) and 0.76% (38 cGy), respectively, for a 5000 cGy prescription dose.

CONCLUSION

The lowest to highest out-of-field radiation doses to the ovarian and uterine organs from breast irradiation were the two tangential field techniques, VMAT and IMRT. These advanced techniques yielded higher radiation leakage, which potentially contributed to the out-of-field radiation dose.

Effective method to reduce the normal brain dose in single-isocenter hypofractionated stereotactic radiotherapy for multiple brain metastases

BACKGROUND AND PURPOSE

Island blocking and dose leakage problems will lead to unnecessary irradiation to normal brain tissue (NBT) in hypofractionated stereotactic radiotherapy (HSRT) for multiple brain metastases (BM) with single-isocenter volumetric modulated arc therapy (VMAT). The present study aimed at investigating whether reducing the number of metastases irradiated by each arc beam could minimize these two problems.

MATERIALS AND METHODS

A total of 32 non-small-cell lung cancer (NSCLC) patients with multiple BM received HSRT (24-36 Gy/3 fractions) with single-isocenter VMAT, where each arc beam only irradiated partial metastases (pm-VMAT), were enrolled in this retrospective study. Conventional single-isocenter VMAT plans, where each arc beam irradiated whole metastases (wm-VMAT), was regenerated and compared with pm-VMAT plans. Furthermore, the clinical efficacy and toxicities were evaluated.

RESULTS

Pm-VMAT achieved similar target coverage as that with wm-VMAT, with better dose fall-off (P < 0.001) and NBT sparing (P < 0.001). However, pm-VMAT resulted in more monitor units (MU) and longer beam-on time (P < 0.001). The intracranial objective response rate and disease control rate for all patients were 75% and 100%, respectively. The local control rates at 1 year and 2 year were 96.2% and 60.2%, respectively. The median progression-free survival and overall survival were 10.3 months (95% confidence interval [CI] 6.8-13.2) and 18.5 months (95% CI 15.9-20.1), respectively. All treatment-related adverse events were grade 1 or 2, and 3 lesions (2.31%) from 2 patients (6.25%) demonstrated radiation necrosis after HSRT.

CONCLUSION

HSRT with pm-VMAT is effective and has limited toxicities for NSCLC patients with multiple BM. Pm-VMAT could provide better NBT sparing while maintaining target dose coverage.