A dosimetric analysis of volumetric-modulated arc radiotherapy with jaw width restriction vs 7 field intensity-modulated radiotherapy for definitive treatment of cervical cancer

Objective:

Radiation therapy treatment planning was performed to compare the dosimetric difference between volumetric-modulated arc radiotherapy (RapidArc™ v. 10; Varian® Medical Systems, Palo Alto, CA) and 7-field intensity-modulated radiotherapy (7f-IMRT) in the definitive treatment of cervical cancer.

Methods:

13 patients with cervical cancer were enrolled in this study. Planning target volume (PTV) 50 and PTV60 were prescribed at a dose of 50 and 60 Gy in 28 fractions, respectively. The dose to the PTV60 was delivered as a simultaneous integrated boost to the pelvic lymph nodes. Owing to the mechanical limitation of the multileaf collimator in which the maximum displacement was limited to 15 cm, two types of RapidArc with different jaw width restrictions (15 and 20–23 cm) were investigated to evaluate their dosimetric differences. The RapidArc plan type with dosimetric superiority was then compared against the 7f-IMRT on the target coverage, sparing of the organs at risk (OARs), monitor units, treatment time and delivery accuracy to determine whether RapidArc is beneficial for the treatment of cervical cancer.

Results:

The 15-cm jaw width restriction had better performance compared with the restrictions that were longer than 15 cm in the sparing of the OARs. The 15-cm RapidArc spared the OARs, that is, the bladder, rectum, small intestine, femoral heads and bones, and improved treatment efficiency compared with 7f-IMRT. Both techniques delivered a high quality-assurance passing rate (>90%) according to the Γ_3mm,3% criterion.

Conclusion:

RapidArc with a 15-cm jaw width restriction spares the OARs and improves treatment efficiency in cervical cancer compared with 7f-IMRT.

Advances in knowledge:

This study describes the dosimetric superiority of RapidArc with a 15-cm jaw width restriction and explores the feasibility of using RapidArc for the definitive treatment of cervical cancer.Volumetric-modulated arc therapy (RapidArc™ v. 10; Varian® Medical Systems, Palo Alto, CA) is a novel type of intensity-modulated radiation therapy (IMRT) in which the gantry speed, multileaf collimator (MLC) leaf position, and dose rate (DR) can vary simultaneously during delivery.¹ Compared with traditional IMRT, RapidArc is able to reduce treatment time and number of monitor units (MUs) while improving the dose to organs at risk (OARs) and healthy tissue sparing. It does this while producing similar or even better dose distributions.² Therefore, RapidArc has been widely applied in the treatment of various cancers, such as larynx carcinoma,³ peripheral lung tumour,⁴ nasopharyngeal carcinoma,^5,6 hepatocellular carcinoma,⁷ spinal metastases,⁸ total marrow irradiation,⁹ prostate cancer,¹⁰ cervical cancer^11–13 and so on. The feasibility of RapidArc has been confirmed by many studies. However, no one has reported the influence of jaw width on the dose distribution, especially for the OARs. Only Vieillot et al¹⁴ had described that the RapidArc field size should be minimized to 15 cm in the x direction, although they did not perform additional experiments to confirm whether the 15-cm field size restriction was actually more beneficial than larger restriction sizes. Owing to the mechanical limitation, the maximum leaf span of the MLC is 15 cm. When the field size is set to <15 cm, anywhere inside the field can be moderated by both sides of the MLC and this permits a higher degree of freedom for the MLC modulation to achieve better optimization results. Otherwise, when the field size is >15 cm, some areas in the field can only be reached by one side of the MLC, which may prohibit achieving better modulation for the treatment plans. Therefore, the dosimetric effect of different jaw widths needs to be further investigated.Two previous studies reported that RapidArc produced significant improvements in the coverage of the target, sparing the OARs and reducing the treatment time for cervical cancer compared with 5-field (5f)-IMRT.^11,12 However, it is still difficult to conclude that RapidArc is superior to IMRT in the treatment of cervical cancer. Because 5f-IMRT is not able to offer good conformity for the target, and taking 5f-IMRT as the benchmark is not sufficient to demonstrate the dosimetric superiority of RapidArc. The two studies were not consistent regarding the sparing of the bladder and rectum. Whether RapidArc achieves better sparing of the two organs than IMRT is still unknown. Moreover, only post-operative patients were enrolled in the two studies, and no simultaneous integrated boost (SIB) technique was used.Because cervical tumours are usually surrounded by many organs, such as the bladder, rectum, bones, femoral heads and the small intestine, it is an ideal model to study the effect of jaw width on the dose distribution and the sparing of the OARs. Therefore, we aimed to investigate the dosimetric benefit of 15-cm RapidArc in the definitive treatment of cervical cancer with SIB to the pelvic lymph nodes that required an additional 10–15 Gy of radiation.¹⁵ We also aimed to compare the 15-cm RapidArc dose distribution and the sparing of the OARs with that of 7-field (7f)-IMRT. Although 9-field (9f)-IMRT was reported to produce improved dose distribution for cervical cancer compared with 7f-IMRT,¹⁶ it required longer treatment time and was not treatment efficient. Thus 7f-IMRT was used as a benchmark in this research. We hope that in addition to clarifying the effect of jaw width, our study will help to determine whether RapidArc is truly beneficial in the treatment of cervical cancer.