Abstract

Ezgi Oymak¹, Muharrem Polat²

¹Iskenderun Gelisim Hospital, Radiation Oncology, Hatay, Turkey; ²Iskenderun Gelisim Hospital, Radation Oncology, Hatay, Turkey

To evaluate doses for organs at risk (OARs) in whole brain radiotherapy (WBRT) in the standard opposed lateral fields (OLF) compared with MLC with IMRT and VMAT plans.

A single-institution retrospective analysis was conducted. Planning CT images of patients treated with WBRT were obtained for comparative plans. All patients were replanned with 3 Gy/fr in 10 fraction dose in standard OLF, IMRT and VMAT. Parotid glands, eyes and lenses were contoured as OARs. In OLF planning, inferior border was between C1 and C2. MLCs were adjusted according to the bone structures as follows: No shielding of frontal sinuses, shielding upto the posterior border of orbita, anterior border of sphenoid sinus and anterior border of C1-C2 vertebrae (Figure 1). No OARs were taken into consideration in OLF dose calculation. IMRT plans were delivered in 5 fields in 70, 140, 0, 290 and 20 degrees. VMAT plans were delivered in two full arcs. OAR doses and treatment times for each modality were compared in relation to deparmental protocols (Table 1).

43 patients who received previous WBRT were replanned. In OLF, protocol violation was observed in all patients for parotid glands and eye doses, in 28 (65%) for lens max doses. No patients achieved parotid max < 30 Gy. Mean parotid gland V25 was 29.2% (7.8-52.4). Compared to OLF plans, both IMRT and VMAT plans obtained significantly lower parotid mean doses, 14.1 (6.6-20.7), 4.2 (1.7-7.5) and 4.2 (1.6-10.7), respectively (p<0.0001) (Table 2). Same sparing benefit was seen for eye max and lens max doses. Mean eye max dose was 31.8 (25.2-32.9) for OLF, 16.6 (9.9-26.3) for IMRT, and 13.5 (7.8-23.8) for VMAT, respectively (p<0.0001). Mean lens max dose was 11.9 (2.4-31.6) for OLF, 4.0 (3.2-6.4) for IMRT, and 5.1 (3.3-10.9) for VMAT, respectively (p<0.0001). Mean treatment time in OLF plans was 0.7 minutes. In IMRT and VMAT plans, treatment duration was 3.9 min and 3.2 min, respectively. 

Brain metastasis causes inferior survival for all cancer types. WBRT remains the standard of care for disseminated intracranial metastasis. Commonly referred standard for WBRT in OLF is a 2D concept and is unable to spare the OARs including parotid glands, eyes and lenses. Parotid gland V25 of 29.2% is similar to the doses attained in curative HNC RT, and could lead to xerostomia. Data suggests that around these dose levels, it can take up to 12 months for patients to recover from xerostomia, a survival period this group may not have. Additionally, eyes and lenses are not spared sufficiently in OLF, which could lead to acute visual deficits during patients survival period. Our results shows that OLF inadequate in sparing the OARs and could cause toxicity affecting quality of life. Additional 3 minutes treatment time in IMRT and VMAT is a time worth spending to provide patients low toxicity. We believe OLF WBRT should be avoided if conformal techniques are available.