Stereotactic radiotherapy for lung cancer using a flattening filter free Clinac

Abstract

The objective of this study was to assess the feasibility of stereotactic radiotherapy for early stage lung cancer using photon beams from a Varian Clinac accelerator operated without a flattening filter. Treatment plans were generated for 10 lung cancer patients with isolated lesions less than 3 cm in diameter. For each patient, two plans were generated, one with and one without the flattening filter. Plans were generated with Eclipse 8.0 (Varian Medical Systems) commissioned with beam data measured on a Clinac 21EX (Varian Medical Systems) operated with and without the flattening filter. Removal of the flattening filter increased the dose rate. The median beam-on time per field was reduced from 25 sec (with the filter) to 11 sec (without the filter), increasing the feasibility of breath-hold treatments and the efficiency of gated treatments. Differences in a dose heterogeneity index for the planning target volume between plans with flattened and unflattened beams were statistically insignificant. Differences in mean doses to organs at risk were small, typically about 10 cGy over the entire treatment. The study concludes that radiotherapy with unflattened beams is feasible and requires substantially less beam-on time, facilitating breath-hold and gating techniques.

Figure 1

Cross‐field profiles of unflattened beams compared to the median PTV size of 5.2 cm. The solid line represents 6 MV photons, the dashed line 18 MV photons, and the dash/dot line represents the median PTV size of 5.2 cm.

Figure 2

Planning CT images with isodose lines for two patients. Images A and B are from patient 1, C and D from patient 2. Images A and C show plans with flattened 6 MV beams while B and D show plans with unflattened 6 MV beams. Isodose lines represent planned doses of 50 Gy (green), 40 Gy (red), 30 Gy (cyan), 20 Gy (orange), 10 Gy (blue). The maximum dose in these plans is less then 60 Gy. The PTV is shown in yellow.

Figure 3

Dose volume histograms for patients 1 and 2, images A and B, respectively. Data for plans with flattened beams are indicated by squares. Data for plans with unflattened beams are indicated by triangles. Dark gray lines represent the PTV; cyan, ipsilateral lung; dark green, contralateral lung; light green, esophagus; red, spinal cord; dark blue, carina; magenta, the heart.