Adaptive Radiotherapy in Locally Advanced Lung Cancers – Real World Scenario from a Tertiary Cancer Center

Abstract

Concurrent chemo-radiation (CTRT) for locally advanced lung cancer (LALC) leads to dynamic changes in tumor volume and location especially in patients with large tumor or atelectasis. Using adaptive radiotherapy (ART) prevents under dosage of the tumor and over dosage of the organs at risk (OAR). One hundred forty patients of LALC from Jan 2017 to Dec 2019 were treated with concurrent CTRT. A four-dimensional computed tomography (4D CT) scan was acquired and contouring of gross target volume (GTV) was guided by PET-scan fusion. All patients were planned to receive 60Gy/30 daily fractions with volumetric modulated arc therapy. Cone-beam CT (CBCT) for set-up verification was performed at least twice weekly. Change in anatomy and tumor response was determined on CBCT during weekly radiotherapy audit. Patients suitable for ART were re-simulated and re-planned. GTV now consisted of residual tumor and 5mm isotropic clinical target volume (CTV) was generated and manually extended to include areas suspicious for residual disease. Dose delivered prior to ART and time needed to execute ART (calculated from the day of re-simulation to the day of starting ART) were documented. The volumetric and dosimetric changes were analyzed using paired t-test between the 2 plans for full prescription dose. We also evaluated local control for this cohort. Out of 140 patients, 35 (25%) required re-planning for ART. Median age was 61 years (range 50 - 65). Twenty-six patients (75%) had upper lobe tumors. Fifteen (43%) patients had squamous carcinoma, 13 (37%) had adenocarcinoma, and 7 (20%) had small cell carcinoma. Requirement for ART was anticipated in 10/35 (28%) patients prior to CTRT. The triggers for ART were resolution of atelectasis in 7 (20%) patients, tumor response in 26 (74%) and progression in 2 (6%) patients. A median of 6 CBCTs prior to ART were acquired. A median of 26Gy (IQR 14-36Gy) was delivered prior to ART. While it required 5 days (IQR:3.5-7.5) for executing ART. There was a reduction in mean planning target volume of 23% (544cc - 418 cc; p = 0.003), mean GTV primary of 47% (153cc - 80.8cc; p = 0.024), and GTV nodes of 33.7% (8 cc - 5.3 cc; p = 0.033). The V20 of total lung minus PTV and the mean lung dose increased by 3.7% (18.8% - 19.5%; p = 0.158) and 4.5% (10.9Gy - 11.4Gy; p = 0.274) respectively whereas there was a reduction in heart V20 and Spinal cord Dmax by 18.3% (9.7 Gy to 8.2 Gy; p = 0.005) and 6.7% (36.6 Gy to 34.3 Gy; p = 0.100) respectively. At a median follow up of 14.7 months, the loco regional control rate was 70% and no patient experienced ³ grade 3 toxicity. In our practice approximately 25% patients of locally advanced lung cancer required ART, with tumor response and resolution of atelectasis being the most common triggers. Anticipation of clinically significant change, frequent use of on-board imaging and implementation of ART could prevent under dosage of the tumor and over dosage of OAR.