Supervisors info:
Παντελής Καραΐσκος, Καθηγητής Ιατρικής Φυσικής, Τμήμα Ιατρικής, ΕΚΠΑ
Παναγιώτης Παπαγιάννης, Αναπληρωτής Καθηγητής Ιατρικής Φυσικής,Τμήμα Ιατρικής, ΕΚΠΑ
Κωνσταντίνος Λουκάς, Επίκουρος Καθηγητής Ιατρικής Φυσικής,Τμήμα Ιατρικής, ΕΚΠΑ
Original Title:
Αξιολόγηση της υποβάθμισης ποιότητας πλάνων θεραπείας SRS VMAT με χρήση 2 ισοκέντρων, σε ασθενείς με πολλαπλές μεταστάσεις, εξαιτίας σφαλμάτων περιστροφής
Summary:
Metastases to the brain are the most common brain tumor and over the past decade, fundamental treatment approaches such as surgery and/or stereotactic radiosurgery (SRS) have been exploited in order to deal with the metastatic tumors. Multi-isocenter Volumetric Modulated Arc Therapy (VMAT) has been introduced as an effective SRS treatment technique. This technique delivers a precisely-sculpted 3D dose distribution with a 360-degree rotation of the gantry in a single or multi-arc treatment using a dynamic multileaf collimator (MLC), variable dose rate and variable gantry speed and requires reduced treatment time as compared with other techniques while it offers high-dose high conformality in target volumes with high protection of surrounding healthy tissue environment.
Recent in-depth investigations featured that multi-arc non-coplanar VMAT with a single isocenter equally provides high quality treatment plans. According to those investigations, single-isocenter VMAT technique further shortens the treatment time of SRS for multiple metastases, while preserving its high efficiency. However, the quality of the single-isocenter SRS VMAT technique in the treatment of multiple brain metastases can be degraded by rotational errors in the patient's head when placed on the couch. These rotational errors affect importantly metastases that are quite far from the isocenter. Studies have shown that such metastases are significantly underdosed and have significant spatial displacements. Also, for metastases that are close to critical organs, there is a risk that with a rotational error they may exceed their tolerance dose. One way to smooth out this negative effect of rotational errors in the SRS VMAT technique with a single isocenter is to produce plan with two isocenters with the VMAT technique.
However, the aim of this study is to evaluate the impact of rotational setup errors on SRS VMAT treatment plans of two isocenters (plan cohort of five patients, with 3 - 4 metastases per patient).
In particular, for each patient, two sub-plans were made up in treatment planning system Monaco(Elekta), of which, each sub-plan corresponds to a different isocenter, and then rotational errors of ±0,5°, ±1° and ±2° on the X, Y, Z and XYZ axes in each sub-plan were simulated via MATLAB. The dose distributions of sub-plans to which the rotations were applied were summed up and the cumulative plan after aplying the rotations with two isocenters was imported into BrachyGuide, a software developed in MATLAB environment.
In order to evaluate the degradation of the quality of the plans in the five patients, the Conformity Index, the Paddick Gradient Index (PCI, PGI), the Target Coverage and the D95% of the metastases were calculated, and for Organs At Risk: Dmax and D20mm3. For healthy brain parenchyma V7Gy(cc,%) was calculated, which is associated with irradiation side effects of healthy brain tissue. The spatial displacements of the metastases after the simulation of rotational errors were also calculated.
In summary, the results obtained were as follows:
Significant underdosage(>5%) in metastases was observed in all patients for 2 degrees on a single axis (X, Y, Z) and / or on all axes (XYZ).
Critical organs (brainstem, optic chiasm) that were near metastases exceeded the tolerance dose even after small(of the order of 0.5°) rotational errors.
In conclusion, rotational errors should be considered and corrected prior to treatment of patients with brain metastases with SRS VMAT especially when they correspond to 2 or more ± degrees.
Correction of rotational errors is done using robotic couches that correct errors in 6 degrees of freedom, something that cannot be done with conventional couches that only correct translational errors.
