Daily CT Guidance Improves Target Coverage During Definitive Radiation Therapy for Gastric MALT Lymphoma

Publication date: Available online 6 March 2017
Source:Practical Radiation Oncology
Author(s): He Wang, Sarah A. Milgrom, Bouthaina S. Dabaja, Grace L. Smith, Mary Martel, Chelsea C. Pinnix
PurposeRadiation therapy (RT) for gastric mucosa-associated lymphoid tissue (MALT) lymphoma is challenging, due to variation in the stomach's position, size, and shape. We investigated the inter-fractional changes in stomach location, consequent dosimetric effects, and impact of daily computed tomography image guidance (CT-IGRT).Methods and MaterialsTwelve patients treated for gastric MALT lymphoma with intensity modulated radiation therapy (IMRT), using a breath-hold technique and restriction of oral intake, were studied retrospectively. The planning target volume (PTV) comprised a 0.5–1.0cm expansion of the stomach. The prescription dose was 30Gy in 15–20 fractions. CT-IGRT was performed daily using CT-on-rails. Dosimetry was calculated on 229 daily CT images after bony vs. CT-based soft tissue alignment, and doses delivered to the target and adjacent structures were compared to the treatment plan. Target coverage was expressed as the percent of the clinical target volume (CTV) and PTV receiving ≥95% of the prescribed dose (V95%).ResultsThe average change in stomach volume was −12.4% (range: −47.6 to 38.6%). The average shift required for target coverage was 1.0 cm (maximum: 2.2 cm). With CT-based alignment to the stomach, the average V95% was 98.5% for CTV and 94.9% for PTV; with bony alignment, these values were 94.5% and 90.4%, respectively (P<0.01 for CTV and PTV). With bony alignment, the PTV V95% was ≤90% in 4 patients (33%) over the course of treatment and was as low as 72.5% for one fraction. The kidney position varied with respect to the stomach and bony anatomy. Consequently, the dose to the left kidney was higher based on daily CT scans than planning scans. Dose to other OARs did not vary significantly.ConclusionsSubstantial inter-fractional variation in stomach volume was observed, despite treatment with breath-hold and restriction of oral intake. Daily CT-IGRT improved target coverage, enabling excellent coverage despite the use of small PTV margins.



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