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. 2023 Jan;50(1):397-409.
doi: 10.1002/mp.16028. Epub 2022 Oct 24.

On the feasibility of cardiac substructure sparing in magnetic resonance imaging guided stereotactic lung radiotherapy

Luuk H G van der Pol  1 , Sara L Hackett  1 , Firdaus A A Mohamed Hoesein  2 , Louk M W Snoeren  1 , Jacqueline Pomp  1 , Bas W Raaymakers  1 , Joost J C Verhoeff  1 , Martin F Fast  1
Affiliations

On the feasibility of cardiac substructure sparing in magnetic resonance imaging guided stereotactic lung radiotherapy

Luuk H G van der Pol et al. Med Phys. 2023 Jan.

Abstract

Background: Lung stereotactic body radiotherapy (SBRT) has proven an effective treatment for medically inoperable lung tumors, even for (ultra-)central tumors. Recently, there has been growing interest in radiation-induced cardiac toxicity in lung radiotherapy. More specifically, dose to cardiac (sub-)structures (CS) was found to correlate with survival after radiotherapy.

Purpose: Our goal is first, to investigate the percentage of patients who require CS sparing in an magnetic resonance imaging guided lung SBRT workflow, and second, to quantify how successful implementation of cardiac sparing would be.

Methods: The patient cohort consists of 34 patients with stage II-IV lung cancer who were treated with SBRT between 2017 and 2020. A mid-position computed tomography (CT) image was used to create treatment plans for the 1.5 T Unity MR-linac (Elekta AB, Stockholm, Sweden) following clinical templates. Under guidance of a cardio-thoracic radiologist, 11 CS were contoured manually for each patient. Dose constraints for five CS were extracted from the literature. Patients were stratified according to their need for cardiac sparing depending on the CS dose in their non-CS constrained MR-linac treatment plans. Cardiac sparing treatment plans (CSPs) were then created and dosimetrically compared with their non-CS constrained treatment plan counterparts. CSPs complied with the departmental constraints and were considered successful when fulfilling all CS constraints, and partially successful if some CS constraints could be fulfilled. Predictors for the need for and feasibility of cardiac sparing were explored, specifically planning target volume (PTV) size, cranio-caudal (CC) distance, 3D distance, and in-field overlap volume histograms (iOVH).

Results: 47% of the patients (16 out of 34) were in need of cardiac sparing. A successful CSP could be created for 62.5% (10 out of 16) of these patients. Partially successful CSPs still complied with two to four CS constraints. No significant difference in dose to organs at risk (OARs) or targets was identified between CSPs and the corresponding non-CS constrained MR-linac plans. The need for cardiac sparing was found to correlate with distance in the CC direction between target and all of the individual CS (Mann-Whitney U-test p-values <10-6 ). iOVHs revealed that complying with dose constraints for CS is primarily determined by in-plane distance and secondarily by PTV size.

Conclusion: We demonstrated that CS can be successfully spared in lung SBRT on the MR-linac for most of this patient cohort, without compromising doses to the tumor or to other OARs. CC distance between the target and CS can be used to predict the need for cardiac sparing. iOVHs, in combination with PTV size, can be used to predict if cardiac sparing will be successful for all constrained CS except the left ventricle.

Keywords: MR-linac; cardiac (sub-)structures; cardiac sparing; lung SBRT; treatment planning.

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Conflict of interest statement

The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported.

Figures

FIGURE 1
FIGURE 1
Schematic representation of the heart with CS constraint references displayed. The blue and red color indicate poorly oxygenated and well‐oxygenated blood flow, respectively. The selected constraints are underlined. Abbreviations: IVC, inferior vena cava; SVC, superior vena cava. The base of the heart is defined as the union of the right atrium and ascending aorta. The anatomical definition is given in Supplementary information IX B.
FIGURE 2
FIGURE 2
Overview showing the number of patients with successful cardiac sparing. Fractions in the legend indicate the number of CS constraints (out of the total of 5) that were violated.
FIGURE 3
FIGURE 3
Examples of patient cases for cardiac sparing not required (NR*), not feasible (NF*), and successful (S*), respectively. Computed tomography (CT) scans are shown with a window setting of 500 Hounsfield units (HU) and level of 50 HU.
FIGURE 4
FIGURE 4
Dose to cardiac (sub‐)structures relative to their constraint level. The left column for each constraint consists of all 34 patients, of which 16 are also present in the right column with their Cardiac sparing treatment plan (CSP). The green and red squares, as well as the red and orange triangles represent different treatment plans for the same patients.
FIGURE 5
FIGURE 5
Dose to conventional organs at risk (OARs) relative to their constraint level.
FIGURE 6
FIGURE 6
Dose to target structures at their objective (GTV: D98% and PTV: D95%) or constraint (GTV: D2%) level.
FIGURE 7
FIGURE 7
Receiver operating characteristic (ROC) curves for PTV size. The diagonal (double arrow headed) line indicates a random classifier.
FIGURE 8
FIGURE 8
In‐field overlap volume histograms (iOVHs) for the left atrium (a), base of the heart (b), left ventricle (c), right ventricle (d), and heart (e), including all patients that require sparing for one or more CS
See this image and copyright information in PMC

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