[Stereotactic radiotherapy of brain metastases in complex situations]

Stereotactic Radiation Therapy of Single Brain Metastases: A Literature Review of Dosimetric Studies

Stereotactic radiotherapy (SRT) plays a major role in treating brain metastases (BMs) and can be delivered using various equipment and techniques. This review aims to identify the dosimetric factors of each technique to determine whether one should be preferred over another for single BMs treatment. A systematic literature review on articles published between January 2015 and January 2022 was conducted using the MEDLINE and ScienceDirect databases, following the PRISMA methodology, using the keywords "dosimetric comparison" and "brain metastases". The included articles compared two or more SRT techniques for treating single BM and considered at least two parameters among: conformity (CI), homogeneity (HI) and gradient (GI) indexes, delivery treatment time, and dose-volume of normal brain tissue. Eleven studies were analyzed. The heterogeneous lesions along with the different definitions of dosimetric indexes rendered the studied comparison almost unattainable. Gamma Knife (GK) and volumetric modulated arc therapy (VMAT) provide better CI and GI and ensure the sparing of healthy tissue. To conclude, it is crucial to optimize dosimetric indexes to minimize radiation exposure to healthy tissue, particularly in cases of reirradiation. Consequently, there is a need for future well-designed studies to establish guidelines for selecting the appropriate SRT technique based on the treated BMs' characteristics.

Acute toxicities and cumulative dose to the brain of repeated sessions of stereotactic radiotherapy (SRT) for brain metastases: a retrospective study of 184 patients

BACKGROUND

Stereotactic radiation therapy (SRT) is a focal treatment for brain metastases (BMs); thus, 20 to 40% of patients will require salvage treatment after an initial SRT session, either because of local or distant failure. SRT is not exempt from acute toxicity, and the acute toxicities of repeated SRT are not well known. The objective of this study was to analyze the acute toxicities of repeated courses of SRT and to determine whether repeated SRT could lead to cumulative brain doses equivalent to those of whole-brain radiotherapy (WBRT).

MATERIAL AND METHODS

Between 2010 and 2020, data from 184 patients treated for 915 BMs via two to six SRT sessions for local or distant BM recurrence without previous or intercurrent WBRT were retrospectively reviewed. Patients were seen via consultations during SRT, and the delivered dose, the use of corticosteroid therapy and neurological symptoms were recorded and rated according to the CTCAEv4. The dosimetric characteristics of 79% of BMs were collected, and summation plans of 76.6% of BMs were created.

RESULTS

36% of patients developed acute toxicity during at least one session. No grade three or four toxicity was registered, and grade one or two cephalalgy was the most frequently reported symptom. There was no significant difference in the occurrence of acute toxicity between consecutive SRT sessions. In the multivariate analysis, acute toxicity was associated with the use of corticosteroid therapy before irradiation (OR = 2.6; p = 0.01), BMV grade (high vs. low grade OR = 5.17; p = 0.02), and number of SRT sessions (3 SRT vs. 2 SRT: OR = 2.64; p = 0.01). The median volume equivalent to the WBRT dose (V_WBRT) was 47.9 ml. In the multivariate analysis, the V_WBRT was significantly associated with the total GTV (p < 0.001) and number of BMs (p < 0.001). Even for patients treated for more than ten cumulated BMs, the median BED to the brain was very low compared to the dose delivered during WBRT.

CONCLUSION

Repeated SRT for local or distant recurrent BM is well tolerated, without grade three or four toxicity, and does not cause more acute neurological toxicity with repeated SRT sessions. Moreover, even for patients treated for more than ten BMs, the V_WBRT is low.

Radionecrosis after repeated courses of radiotherapy under stereotactic conditions for brain metastases: Analysis of clinical and dosimetric data from a retrospective cohort of 184 patients

PURPOSE

Between 10 and 40% of patients with cancer will develop one or more brain metastases (BMs). Stereotactic radiotherapy (SRT) is part of the therapeutic arsenal for the treatment of de novo or recurrent BM. Its main interest is to delay whole brain radiation therapy (WBRT), which may cause cognitive toxicity. However, SRT is not exempt from long-term toxicity, and the most widely known SRT is radionecrosis (RN). The objective of this study was to analyze the occurrence of RN per BM and per patient.

MATERIAL AND METHODS

Between 2010 and 2020, data from 184 patients treated for 915 BMs by two to six SRT sessions for local or distant brain recurrence without previous or intercurrent WBRT were retrospectively reviewed. RN was examined on trimestral follow-up MRI and potentially confirmed by surgery or nuclear medicine. For each BM and SRT session plan, summation V_12Gy, V_14Gy, V_21Gy and V_23Gy isodoses were collected. Volumes of intersections were created between the 12Gy isodose at the first SRT and the 18Gy isodose of the following SRT (V_18-12Gy).

RESULTS

At the end of follow-up, 23.0% of patients presented RN, and 6.3% of BM presented RN. Median follow-up of BM was 13.3 months (95%CI 18.3-20.8). The median interval between BM irradiation and RN was 8.7 months (95% CI 9.2-14.7). Six-, 12- and 24-month RN-free survival rates per BM were 75%, 54% and 29%, respectively. The median RN-free survival per patient was 15.3 months (95% CI 13.6-18.1). In multivariate analysis, the occurrence of RN per BM was statistically associated with local reirradiation (P<0.001) and the number of SRTs (P<0.001). In univariate analysis, the occurrence of RN per patient was statistically associated with the sum of all V_18-12Gy (P=0.02). No statistical association was found in multivariate analysis. A sum of all V_18-12Gy of less than 1.5ml was associated with a 14.6% risk of RN, compared with 35.6% when the sum of all V_18-12Gy was superior to 1.5ml. The sum of all V_18-12Gy larger than 1.5ml was associated with a 74% specificity and 53% sensitivity of RN (P<0.001).

CONCLUSION

Based on these results, a small number of BMs show RN during repeated SRT for local or distant recurrent BMs. Local reirradiation was the most predictive factor of brain RN. A V_18-12Gy larger than 7.6ml in the case of local reirradiation or larger than 1.5ml in proximity reirradiation were prognostic factors of RN. The more BM patients need radiation therapy, and the longer they survive after irradiation, the higher their individual risk of developing RN.

Local recurrence and cerebral progression-free survival after multiple sessions of stereotactic radiotherapy of brain metastases: a retrospective study of 184 patients : Statistical analysis

PURPOSE

Forty to sixty percent of patients treated with focal therapy for brain metastasis (BM) will have distant brain recurrence (C-LR), while 10-25% of patients will have local recurrence (LR) within 1 year after stereotactic radiotherapy (SRT). The purpose of this study was to analyze cerebral progression-free survival (C-PFS) and LR of BM among patients treated with repeated courses of radiotherapy in stereotactic conditions.

METHODS AND MATERIALS

We retrospectively reviewed data from 184 patients treated for 915 BMs with at least two courses of SRT without previous WBRT. Initial patient characteristics, patient characteristics at each SRT, brain metastasis velocity (BMV), delay between SRT, MRI response, LR and C‑LR were analyzed.

RESULTS

In all, 123 (66.9%), 39 (21.2%), and 22 (12%) patients received 2, 3, or 4 or more SRT sessions, respectively. Ninety percent of BMs were irradiated without prior surgery, and 10% were irradiated after neurosurgery. The MRI response at 3, 6, 12 and 24 months after SRT was stable regardless of the SRT session. At 6, 12 and 24 months, the rates of local control were 96.3, 90.1, and 85.8%, respectively. In multivariate analysis, P‑LR was statistically associated with kidney (HR = 0.08) and lung cancer (HR = 0.3), ECOG 1 (HR = 0.5), and high BMV grade (HR = 5.6). The median C‑PFS after SRT1, SRT2, SRT3 and SRT4 and more were 6.6, 5.1, 6.7, and 7.7 months, respectively. C‑PFS after SRT2 was significantly longer among patients in good general condition (HR = 0.39), patients with high KPS (HR = 0.91), patients with no extracerebral progression (HR = 1.8), and patients with a low BMV grade (low vs. high: HR = 3.8).

CONCLUSION

Objective MRI response rate after repeated SRT is stable from session to session. Patients who survive longer, such as patients with breast cancer or with low BMV grade, are at risk of local reirradiation. C‑PFS after SRT2 is better in patients in good general condition, without extracerebral progression and with low BMV grade.

Collision prediction for intracranial stereotactic radiosurgery planning: An easy-to-implement analytical solution

PURPOSE

Gantry collision is a concern in linac-based stereotactic radiosurgery (SRS). Without collision screening, the planner may compromise optimal planning, unnecessary re-planning delays can occur, and incomplete treatments may be delivered. To address these concerns, we developed a software for collision prediction based on simple machine measurements.

MATERIALS AND METHODS

Three types of collision were identified; gantry-couch mount, gantry-couch and gantry-patient. Trigonometric formulas to calculate the distance from each potential point of collision to the gantry rotation axis were generated. For each point, collision occurs when that distance is greater than the gantry head to gantry rotational axis distance. The colliding arc for each point is calculated. A computer code incorporating these formulas was generated. The inputs required are the couch coordinates relative to the isocenter, the patient dimensions, and the presence or absence of a circular SRS collimator. The software outputs the collision-free gantry angles, and for each point, the shortest distance to the gantry or the colliding sector when collision is identified. The software was tested for accuracy on a TrueBEAM® machine equipped with BrainLab® accessories for 80 virtual isocenter-couch angle configurations with and without a circular collimator and a parallelepiped phantom.

RESULTS

The software predicted the absence of collision for 19 configurations. The mean absolute error between the measured and predicted gantry angle of collision for the remaining 61 cases was 0.86 (0.01-2.49).

CONCLUSION

This tool accurately predicted collisions for linac-based intracranial SRS and is easy to implement in any radiotherapy facility.