Physics Contribution
Head and Neck Margin Reduction With Adaptive Radiation Therapy: Robustness of Treatment Plans Against Anatomy Changes

https://doi.org/10.1016/j.ijrobp.2016.07.011Get rights and content

Purpose

We set out to investigate loss of target coverage from anatomy changes in head and neck cancer patients as a function of applied safety margins and to verify a cone beam computed tomography (CBCT)–based adaptive strategy with an average patient anatomy to overcome possible target underdosage.

Methods and Materials

For 19 oropharyngeal cancer patients, volumetric modulated arc therapy treatment plans (2 arcs; simultaneous integrated boost, 70 and 54.25 Gy; 35 fractions) were automatically optimized with uniform clinical target volume (CTV)–to–planning target volume margins of 5, 3, and 0 mm. We applied b-spline CBCT–to–computed tomography (CT) deformable registration to allow recalculation of the dose on modified CT scans (planning CT deformed to daily CBCT following online positioning) and dose accumulation in the planning CT scan. Patients with deviations in primary or elective CTV coverage >2 Gy were identified as candidates for adaptive replanning. For these patients, a single adaptive intervention was simulated with an average anatomy from the first 10 fractions.

Results

Margin reduction from 5 mm to 3 mm to 0 mm generally led to an organ-at-risk (OAR) mean dose (Dmean) sparing of approximately 1 Gy/mm. CTV shrinkage was mainly seen in the elective volumes (up to 10%), likely related to weight loss. Despite online repositioning, substantial systematic errors were present (>3 mm) in lymph node CTV, the parotid glands, and the larynx. Nevertheless, the average increase in OAR dose was small: maximum of 1.2 Gy (parotid glands, Dmean) for all applied margins. Loss of CTV coverage >2 Gy was found in 1, 3, and 7 of 73 CTVs, respectively. Adaptive intervention in 0-mm plans substantially improved coverage: in 5 of 7 CTVs (in 6 patients) to <2 Gy of initially planned.

Conclusions

Volumetric modulated arc therapy head and neck cancer treatment plans with 5-mm margins are robust for anatomy changes and show a modest increase in OAR dose. Margin reduction improves OAR sparing with approximately 1 Gy/mm at the expense of target coverage in a subgroup of patients. Patients at risk of CTV underdosage >2 Gy in 0-mm plans may be identified early in treatment using dose accumulation. A single intervention with an average anatomy derived from CBCT effectively mitigates discrepancies.

Introduction

Current planning and delivery techniques for head and neck cancer (HNC), such as intensity modulated radiation therapy and volumetric modulated arc therapy (VMAT), offer dose distributions conformal to the tumor with steep dose gradients, with superior sparing of organs at risk (OARs) (1). During treatment planning, safety margins around the targets are applied to account for geometric uncertainties such as delineation errors, uncorrected setup errors, and/or anatomy changes during treatment (2). With large margins, robust plans are created that ensure correct target dosage in case of large errors; however, substantial amounts of normal tissue may be irradiated. A reduction in geometric uncertainties would allow smaller safety margins and thereby increase the therapeutic window.

Image guided radiation therapy reduces geometric uncertainties. In-room imaging (eg, cone beam computed tomography [CBCT]) allows one to visualize (a surrogate of) the target volume in relation to OARs and derive a couch correction for alignment of the target volume or volumes with the treatment beams. In offline protocols, the systematic component of target positioning errors is estimated and corrected. Online protocols allow for near perfect correction of errors (3).

To further reduce uncertainties arising from nonrigid setup errors and anatomy changes (deformations), adaptive radiation therapy (ART) can be considered (4). Commonly, ART for HNC is based on a second computed tomography (CT) scan and used to address ad hoc observed treatment response (tumor regression, weight loss) 5, 6. Recently, however, ART based on an average anatomy model derived from CBCT has been proposed to reduce the impact of systematic deformations (7).

To improve the therapeutic ratio, we propose to use small margins, evaluate delivered dose with CBCT, and use adaptive strategies in case of target underdosage when needed. To test the feasibility of such an approach, we investigated (1) changes in dose to clinical target volumes (CTVs) and OARs from deformations in HNC patients as a function of applied safety margins; and (2) a CBCT-based adaptive strategy with an average anatomy model to overcome loss of coverage in selected patients.

Section snippets

Patient group

We retrospectively selected 19 patients with oropharyngeal cancers that were treated with curative intent. Patient data were accessed according to institutional guidelines. The T category distribution was T1 in 2 patients, T2 in 7, T3 in 8, and T4 in 2. Three patients did not have nodal involvement. The planning computed tomography (pCT) scan (Somatom Sensation Open; Siemens, Erlangen, Germany) had a voxel size of 1 × 1 × 3 mm3. Daily CBCT scans (Elekta Synergy; Elekta Oncology Systems,

Results

For the 19 selected patients, plans with 5-, 3-, and 0-mm margins were generated that fulfilled minimal requirements (Supplementary Material; available online at www.redjournal.org). A total of 651 CBCT scans underwent deformable registration to the corresponding pCT scans, and DVFs were visually inspected.

Discussion

In this study the potential of ART for anatomy changes during radiation therapy (RT) of HNC was assessed based on accumulated dose in daily CBCT scans. For 73 CTVs in 19 patients, VMAT treatment plans with 5-mm safety margins were adequate in all but 1 CTV. When margins were reduced, loss of target coverage (D99%<95% prescribed dose) increased to 27% of all CTVs (at 0-mm margins) but discrepancies with planned dose were small. Margin reduction led to sparing of approximately 1 Gy/mm in

Conclusions

In this study, the potential of ART for anatomy changes during RT of HNC was assessed based on accumulated dose in daily CBCT scans. For 73 target volumes in 19 patients, VMAT treatment plans with 5-mm safety margins were adequate in all but 1 target volume. Modest extra OAR dose was present. Margin reduction led to an improvement in OAR dose of approximately 1 Gy/mm. With 3- and 0-mm margins, dose to 2 targets and 20 targets, respectively, was compromised, although large discrepancies (>2 Gy)

References (27)

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    Most of the studies demonstrated results of only one replan during RT, mostly implemented in the middle fraction. However, in some studies, the number of replans was not clearly mentioned [27,29,34,36,42,43,57,63,71,72,77,84,86,87,105,107]. In nine studies (9/85), replanning was based mainly on hybrid plans performed before the decision to replan or simultaneously with the adapted plan [32,44,45,49,70,89,93,101,102].

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    Changes such as tumour shrinkage, weight loss and oedema during a course of RT leads to anatomical and positional variations that can be identified using 3D IGRT such as KVCBCT and MVCBCT [87,88]. Average weight loss of 6–10% over a H&N RT course has been reported [89,87,90] while parotid glands can shrink by up to 30% [91–93,62,94]. Mean TV reductions reported vary – Liu reported a 1.5–1.8% reductions in CTV and GTV [93] while Berwouts et al. reported a 72% and 46.3% reduction in GTV and PTV volumes, respectively, over an entire treatment course [95].

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Supported by a grant from the Dutch Cancer Society (NKI:2005-3378).

Conflict of interest: none.

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