Impact of varying air cavity on planning dosimetry for rectum patients treated on a 1.5 T hybrid MR‐linac system. Issue 7 (23rd May 2020)

• Record Type: Journal Article
• Authors: Godoy Scripes, Paola; Subashi, Ergys; Burleson, Sarah; Liang, Jiayi; Romesser, Paul; Crane, Christopher; Mechalakos, James; Hunt, Margie; Tyagi, Neelam
• Is Part Of: Journal of applied clinical medical physics. Volume 21:Issue 7(2020)
• Page Start: 144
• Page End: 152
• Publication Date: 2020-05-23
• Subjects: 1.5 T magnetic field -- electron return effect (ERE) -- MR‐guided radiation therapy -- rectal cancer; Medical physics -- Periodicals; Clinical medicine -- Periodicals; Health Physics; Clinical Medicine; Electronic journals; Periodicals; Periodicals; Fulltext; Internet Resources; 610.153
• DOI: 10.1002/acm2.12903
• Languages: English
• ISSNs: 1526-9914

Abstract

Abstract: Purpose: To investigate the dosimetric impact of magnetic (B) field on varying air cavities in rectum patients treated on the hybrid 1.5 T MR‐linac. Methods: Artificial air cavities of varying diameters (0.0, 1.0, 1.5, 2.0, 2.5, 3.0, and 5.0 cm) were created for four rectum patients (two prone and two supine). A total of 56 plans using a 7 MV flattening filter‐free beam were generated with and without B‐field. Reference intensity‐modulated radiation therapy treatment plans without air cavity in the presence and absence of B‐field were generated to a total dose of 45/50 Gy. The reference plans were copied and recalculated for the varying air cavities. D95 (PTV45 –PTV50 ), D95 (PTV50 –aircavity), V50 (PTV50 –aircavity), Dmax (PTV50 –aircavity), and V110% (PTV50 –aircavity) were extracted for each patient. Annulus rings of 1‐mm‐diameter step size were generated for one of the air cavity plans (3.0 cm) for all four patients to determine Dmax (%) and V110% (cc) within each annulus. Results: In the presence of B‐field, hot spots at the cavity interface start to become visible at ~1 cm air cavity in both supine and prone positioning due to electron return effect (ERE). In the presence of B‐field Dmax and V110% varied from 5523 ± 49 cGy and 0.09 ± 0.16 cc for 0 cm air cavity size to 6050 ± 109 cGy and 11.6 ± 6.7 cc for 5 cm air cavity size. The hot spots were located within 3 mm inside the rectal‐air interface, where Dmax increased from 110.4 ± 0.5% without B‐field to 119.2 ± 0.8 % with B‐field. Conclusions: Air cavities inside rectum affects rectum plan dosimetry due ERE. Location and magnitude of hot spots are dependent on the size of the air cavity.