Treatment of Cervical Cancer With a Prototype Flattening Filter-Free Straight-Through LINAC With Fast Jawless MLC Collimator

Treatment planning comparison of volumetric modulated arc therapy employing a dual-layer stacked multi-leaf collimator and helical tomotherapy for cervix uteri

Purpose

To ascertain the dosimetric performance of a new delivery system (the Halcyon system, H) equipped with dual-layer stacked multi-leaf collimator (MLC) for risk-adapted targets in cervix uteri cancer patients compared to another ring-based system in clinical operation (Helical Tomotherapy, HT).

Methods

Twenty patients were retrospectively included in a treatment planning study (10 with positive lymph nodes and 10 without). The dose prescription (45Gy to the primary tumour volume and a simultaneously integrated boost up to 55Gy for the positive patients) and the clinical planning objectives were defined consistently as recommended by an ongoing multicentric clinical trial. Halcyon plans were optimised for the volumetric modulated arc therapy. The plan comparison was performed employing the quantitative analysis of the dose-volume histograms.

Results

The coverage of the primary and nodal target volumes was comparable for both techniques and both subsets of patients. The primary planning target volume (PTV) receiving at least 95% of the prescription isodose ranged from 97.2 ± 1.1% (node-negative) to 99.1 ± 1.2% (node-positive) for H and from 96.5 ± 1.9% (node-negative) to 98.3 ± 0.9% (node-positive) for HT. The uncertainty is expressed at one standard deviation from the cohort of patient per each group. For the nodal clinical target volumes, the dose received by 98% of the planning target volume ranged 55.5 ± 0.1 to 56.0 ± 0.8Gy for H and HT, respectively. The only significant and potentially relevant differences were observed for the bowels. In this case, V_40Gy resulted 226.3 ± 35.9 and 186.9 ± 115.9 cm³ for the node-positive and node-negative patients respectively for Halcyon. The corresponding findings for HT were: 258.9 ± 60.5 and 224.9 ± 102.2 cm³. On the contrary, V_15Gy resulted 1279.7 ± 296.5 and 1557.2 ± 359.9 cm³ for HT and H respectively for node-positive and 1010.8 ± 320.9 versus 1203.8 ± 332.8 cm³ for node-negative.

Conclusion

This retrospective treatment planning study, based on the dose constraints derived from the Embrace II study protocol, suggested the essential equivalence between Halcyon based and Helical Tomotherapy based plans for the intensity-modulated rotational treatment of cervix uteri cancer. Different levels of sparing were observed for the bowels with H better protecting in the high-dose region and HT in the mid-low dose regions. The clinical impact of these differences should be further addressed.

Critical Appraisal of the Treatment Planning Performance of Volumetric Modulated Arc Therapy by Means of a Dual Layer Stacked Multileaf Collimator for Head and Neck, Breast, and Prostate

Purpose:

To ascertain whether a new delivery system (the Halcyon system) equipped with dual-layer stacked multileaf collimator operating in a mode, which allows independent, fully interdigitating motion of both layers and 6 flattening filter free energy, could generate plans of high clinical quality compared to a well-established delivery system with single layer multileaf collimator.

Methods:

Twenty patients in each of the 3 groups (advanced head and neck, breast, and high-risk prostate) were selected for an in silico planning study. For each patient, reference plans were developed for volumetric modulated arc therapy technique with 6 MV photon beams from a TrueBeam linear accelerator and compared against the corresponding plans for the Halcyon system. Plan comparison was performed in terms of dose volume histogram quantitative analysis.

Results:

Concerning the planning target volumes, with identical dose calculation and optimization algorithms and with identical planning techniques, no clinically relevant difference in coverage (D_98%), hot spot (D_2%), or homogeneity was observed. Similarly, for all the organs at risk, the dosimetric findings showed that (1) all planning constraints were met by the 2 delivery systems and (2) although statistical significant differences were reported for most of the parameters but none of these were judged of potential clinical relevance.

Conclusion:

The data presented confirmed that the new delivery system can generate treatment plans for volumetric modulated arc therapy with the same dosimetric quality of what is achievable with other systems routinely used in the clinics without significantly changing the current practice. Additional studies which customize the optimization parameters for each delivery device would complement the spectrum of investigations.