Effect of organ size and position on out-of-field dose distributions during radiation therapy

Abstract

Mantle field irradiation has historically been the standard radiation treatment for Hodgkin lymphoma. It involves treating large regions of the chest and neck with high doses of radiation (up to 30 Gy). Previous epidemiological studies on the incidence of second malignancies following radiation therapy for Hodgkin lymphoma have revealed an increased incidence of second tumors in various organs, including lung, breast, thyroid and digestive tract. Multiple other studies, including the Surveillance, Epidemiology and End Results, indicated an increased incidence in digestive tract including stomach cancers following mantle field radiotherapy. Assessment of stomach dose is challenging because the stomach is outside the treatment field but very near the treatment border where there are steep dose gradients. In addition, the stomach can vary greatly in size and position. We sought to evaluate the dosimetric impact of the size and variable position of the stomach relative to the field border for a typical Hodgkin lymphoma mantle field irradiation. The mean stomach dose was measured using thermoluminescent dosimetry for nine variations in stomach size and position. The mean doses to the nine stomach variations ranged from 0.43 to 0.83 Gy when 30 Gy was delivered to the treatment isocenter. Statistical analyses indicated that there were no significant differences in the mean stomach dose when the stomach was symmetrically expanded up to 3 cm or shifted laterally (medial, anterior or posterior shifts) by up to 3 cm. There was, however, a significant (P > 0.01) difference in the mean dose when the stomach was shifted superiorly or inferiorly by ≥2.5 cm.

Figure 1

Three expansions and five shifts of typical stomach relative to the treatment field. (a) Three expansions: the typical stomach is the smallest contour, shaded dark gray. The progressively lighter shaded contours are the 1 cm, 2 cm and 3 cm 3D symmetric expansions. Five shifts of the typical stomach relative to the treatment field (typical stomach, cyan): (b) 3 cm posterior, green; 3 cm anterior, orange; (c) 3 cm medial, yellow; (d) 2.5 cm superior, white; and 2.5 cm inferior, pink.

Figure 2

The anthropomorphic male phantom used in this study placed in the treatment position. The figure shows gantry positioned for the anterior treatment field, custom block used to shape the treatment field and alignment bebes used to align phantom according to planned treatment isocenter. The phantom was loaded with lithium fluoride thermoluminescent dosimeters in the specified locations (in slices 20–27 of the phantom).

Figure 3

Axial plane from the CT scan of the phantom of the phantom showing the typical stomach contour in blue and TLD locations indicated by red squares. (a) Contour of typical stomach and all TLD locations within that contour used to estimate dose to the entire stomach. (b) Contour of typical stomach and TLD locations around the periphery used to estimate dose to the stomach wall.

Figure 4

The variation in the mean dose through the stomach region at a depth of 9.5 cm in tissue as a function of distance from the field edge. Data are compared with measured dose values from Stovall et al (1995) and from Fraass and van de Geijn (1983).

Figure 5

The mean dose for all TLD within each slice of the phantom for the largest stomach variation considered in this study (3 cm isotropic expansion). The error bars represent one standard deviation and the percent standard deviation for each phantom slice is indicated to the right of each data point (average deviation for all slices = 9.6%).