Introduction

Lung cancer is the leading cause of cancer deaths in the United States and around the world. ^{Reference Dela Cruz, Tanoue and Matthay1} Lung cancer causes more deaths in the United States almost every year than prostate, breast, colorectal and brain cancers combined. ^{Reference Siegel, Miller and Jemal2} The American Cancer Society’s estimates that for lung cancer in the United States for 2020, there were about 228,820 new cases of lung cancer with 135,720 deaths. ^{Reference Siegel, Miller and Jemal2} The rapid increase in the worldwide prevalence of lung cancer is attributed mostly to the increased use of cigarettes following World War I, though increases in environmental air pollution are suspected to have been a contributing factor as well.

About 85–90% of lung cancers are non-small-cell lung cancers (NSCLC) and 10–15% are small-cell lung carcinoma (SCLC). There are three main types of NSCLC: adeno carcinoma (AC), squamous cell carcinoma (SCC) and large-cell carcinoma (LCC). AC is the most common form of lung cancer found in the outer region of the lung. SCC is found centrally in the lung, where the larger bronchi joins the trachea to the lung, or in one of the main airway branches and is generally linked to smoking. LCC grows and spreads quickly and can be found anywhere in the lung.

It was shown that surgically un-resected NSCLC receiving radiotherapy, after induction chemotherapy, provided a statistically significant survival advantage. ^{Reference Sause, Scott and Taylor3} The role of radiotherapy in combined modality treatment of locally advanced NSCLC was shown to have significant long-term survival advantages. ^{Reference Kubota, Furuse and Kawahara4} Addition of radiotherapy to chemotherapy produced local control and survival advantage. ^{Reference Pignon, Arriagada, Ihde, Johnson, Perry and Souhami5} Better survival was shown using multi-field conformal therapy without increased toxicity. ^{Reference Pignon, Arriagada, Ihde, Johnson, Perry and Souhami5} Due to the proximity of lungs to oesophagus, heart and spinal cord, optimal dose delivery to target volume within thorax is challenging.

For radiotherapy treatment, patients need to be immobilised during the simulation as well as treatment delivery. Computed tomography (CT) simulations providing image sets (slice by slice) are usually used as patient data for treatment planning system (TPS) and dose calculations. PET-CT improves the outline of the gross tumour volume (GTV). Once patient data are acquired, the images are imported in DICOM format for the generation of treatment plan. Different anatomical structures or regions of interest (ROIs) are defined and delineated so that different tissue volume can be identified by the TPS and proceed with the appropriate planning procedures. Planning target volume (PTV) that covers clinical target volume (CTV) with a margin of about 2–2.5 cm due to respiratory tumour motion and setup is defined and delineated as a target structure of interest. Heart, breast, oesophagus and spinal cord are the main organs at risk (OAR) for NSCLC treatment. For 3D conformal therapy (3D-CRT) planning techniques, parallel-opposed fields, three fields or two-field wedge pairs, as well as multi-fields are used to cover the size of PTV. ^{Reference Bahri, Flickinger and Kalend6} MV beams are usually used and conformal treatment blocks are created in the beam’s eye view around the CTV with 2.5 cm margin. ^{Reference Das, Cheng, Anderson and Movsas7}

Intensity-modulated radiation therapy (IMRT) was first implemented in the early 1990s with the introduction of the first commercial IMRT delivery unit NOMOS Peacock system (NOMOS Corporation, Sewickley, Pennsylvania, USA), ^{Reference Woo, Sanders and Grant8} when computing capability required for complex inverse treatment planning algorithms became available commercially. The unique feature of IMRT is that the leaves are known as multileaf collimators (MLCs) that help create the complex shape of the beam to conform radiation to the shape of the tumour while minimising exposure of surrounding critical structures. IMRT technology has the ability to treat patients with several different modes that include the complex volumetric-modulated arc therapy (VMAT) mode, when the gantry of the linear accelerator rotates at a constant or variable speed around the patient for a partial or full arc, MLCs are in constant motion and dose rate is continuously varied. ^{Reference Herman, Schnell and Young9}

IMRT can be an effective treatment modality for managing advanced-stage NSCLC and SCLC. Target delineation and organ motion due to respiration need to be carefully considered during the simulation. Respiratory motion has a significant impact on the accuracy of tumour targeting with radiation and is of particular interest in the treatment of NSCLC and other thoracic malignancies. With the image-guided radiation therapy (IGRT) technology, 4D-CT treatment planning may be used for radiation dose escalation with tighter radiation fields and has the potential for improving outcomes in patients with thoracic malignancies. ^{Reference Vlachaki, Castellon, Leite, Perkins and Ahmad10} Irradiation to uninvolved lung and normal tissues need to be minimised by beam angle selections during planning. In addition to ROIs that are needed to 3D-CRT, IMRT needs inverse planning with fluence optimisation, MLC sequences, beam configuration, plan evaluations, etc. ^{Reference Malisan11}

IMRT plans are usually oriented up to nine beam angles and a low dose ‘bath’ of radiation is created outside the PTV. This effect (not spread out widely) also occurs in 3D-CRT where only two–four beam angles are usually used. Complex shapes of radiation with IMRT sometimes result in unwanted ‘hot spots’ or ‘cold spots’. Hotspots in OAR put patients at higher risk and cold spots within the PTV under-dose the tumour. IMRT is a technique where hundreds of small radiation beams with different intensities are delivered to provide precise tumour dose while minimising adjacent normal tissue doses and generate a conformal dose distribution with steep dose fall off at the boundary between tumour and normal structures. The aim of this brief literature review is to evaluate, which is the superior radiotherapy technique among 3D-CRT, IMRT and VMAT for the treatment of lung cancer.