Elsevier

Radiotherapy and Oncology

Volume 89, Issue 2, November 2008, Pages 180-191
Radiotherapy and Oncology

Cervix cancer radiotherapy
A treatment planning study comparing volumetric arc modulation with RapidArc and fixed field IMRT for cervix uteri radiotherapy

https://doi.org/10.1016/j.radonc.2008.06.013Get rights and content

Abstract

Purpose

A treatment planning study was performed to evaluate the performance of the novel volumetric modulated single arc radiotherapy on cervix uteri cancer patients. Conventional fixed field IMRT was used as benchmark.

Methods and materials

CT datasets of eight patients were included in the study. Plans were optimised with the aim to assess organs at risk and healthy tissue sparing while enforcing highly conformal target coverage. Planning objectives for PTV were: maximum significant dose lower than 52.5 Gy and minimum significant dose higher than 47.5 Gy. For organs at risk, the median and maximum doses were constrained to be lower than 30 (rectum), 35 (bladder) and 25 Gy (small bowel) and 47.5 Gy; additional objectives were set on various volume thresholds. Plans were evaluated on parameters derived from dose volume histograms and on NTCP estimates. Peripheral doses at 5, 10 and 15 cm from the PTV surface were recorded to assess the low-level dose bath. The MU and delivery time were scored to measure expected treatment efficiency.

Results

Both RapidArc and IMRT resulted in equivalent target coverage but RapidArc had an improved homogeneity (D5%D95% = 3.5 ± 0.6 Gy for RapidArc and 4.3 ± 0.8 Gy for IMRT) and conformity index (CI90% = 1.30 ± 0.06 for RapidArc and 1.41 ± 0.15 for IMRT). On rectum the mean dose was reduced by about 6 Gy (10 Gy for the rectum fraction not included in the PTV). Similar trends were observed for the various dose levels with reductions ranging from ∼3 to 14.4 Gy. For the bladder, RapidArc allowed a reduction of mean dose ranging from ∼4 to 6 Gy and a reduction from ∼3 to 9 Gy w.r.t. IMRT. Similar trends but with smaller absolute differences were observed for the small bowel and left and right femur. NTCP calculations on bladder and rectum confirmed the DVH data with a potential relative reduction ranging from 30 to 70% from IMRT to RapidArc. The healthy tissue was significantly less irradiated in the medium to high dose regions (from 20 to 30 Gy) and the integral dose reduction with RapidArc was about 12% compared to IMRT. Concerning peripheral dose, the relative difference between IMRT and RapidArc was of 9 ± 2%, 43 ± 11% and 36 ± 5% at 5, 10 and 15 cm from the PTV surface, respectively. The MU/Gy from RapidArc was 245 ± 17 corresponding to an expected average beam on time of 73 ± 10 s per fractions of 2 Gy. IMRT plans presented higher values with an average of MU/Gy = 479 ± 63.

Conclusion

RapidArc was investigated for cervix uteri cancer showing significant improvements in organs at risk and healthy tissue sparing with uncompromised target coverage leading to better conformal avoidance of treatments w.r.t. conventional IMRT. This, in combination with the confirmed short delivery time, can lead to clinically significant advances in the management of this highly aggressive cancer type. Clinical protocols are now advised to evaluate prospectively the potential benefit observed at the planning level.

Section snippets

Patient selection and planning objectives

CT data for a cohort of eight patients from the TMH centre were selected for this comparative analysis. All patients were affected by carcinoma of the cervix uteri to be treated with curative intent. For all patients, plans were designed on CT scans acquired with 5 mm slice thickness extending the scan from T11 to include the proximal third of the femur’s diaphysis. The OAR considered were: rectum, bladder, femoral heads and the small bowel. For rectum and bladder, two sets of OARs were defined

Results

Dose distributions are shown for one example in Fig. 1 for axial and coronal views. This figure allows visualisation of the main characteristics of RapidArc compared to IMRT, with a systematic reduction of bladder and rectum involvement and a reduced dose bath. Similar results were obtained for the other patients and through the entire extension of the target volume.

Fig. 2 shows the average DVH for the PTV comparing in 2a RapidArc (solid lines) to IMRT (dashed lines) while Fig. 2b (RapidArc)

Discussion

This study reports a comparison of the RapidArc technique compared to fixed beam IMRT. Similar investigations on different indications are appearing. Palma et al. investigated RapidArc progenitor on prostate showing that variable dose rate volumetric arc modulation is beneficial compared to IMRT or constant dose rate [29]. Fogliata et al. investigated RapidArc on small benign brain tumors compared to IMRT and helical tomotherapy [16]. It is expected that in a relatively short time-frame a

Conclusions

RapidArc was investigated for cervix uteri cancer and led to highly significant improvements in OAR and healthy tissue sparing with uncompromised target coverage compared to conventional IMRT. This is equivalent to a highly improved degree of conformal avoidance treatment. In addition, healthy tissue involvement, at short and long distancenear and far from the target and at both high and very low dose levels was improved with RapidArc compared to IMRT. The potential benefit of the better

Acknowledgements

The study was conducted in strict cooperation with many persons at Varian Medical Systems involved in the development of this novel technology. Among them we are particularly grateful for support and assistance to Martin Sabel and Yves Archambault for planning and to Jiri Bocanek for delivery issues. The Phase II randomised trial mentioned in the introduction is part of a Research Cooperation Agreement between TMH and Varian Medical Systems.

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