This work introduces a new method for verifying MLC leaf positions with enough spatial resolution to replace film‐based methods in performing QA tests. It is implemented on a 2D ion chamber array, and it is based on the principle of varying signal response of a volumetric detector to partial irradiation. A PTW 2D‐ARRAY seven29 (PTW‐729 2D) array was used to assess a Siemens OPTIFOCUS MLC. Partial volume response curves for chambers in the array were obtained by irradiating them with the leaves of the MLC, progressively covering varying portions of the chambers correlated with the leaf positions. The readings from the array's chambers are processed with an in‐house program; it generates a reference response that translates readings into leaf positions. This principle allows discriminating errors in pairs of opposing leaves that could combine to cancel their detection with other tools.Patterns of leaf positions, similar to the Bayouth test but with different, purposefully introduced errors, were generated and used to test the effectiveness of the method. The same patterns were exposed on radiographic film and analyzed with the RIT software for validation. For four test patterns with a total of 100 errors of ±1 mm, ±2 mm and ±3 mm, all were correctly determined with the proposed method. The analysis of the same pattern with film using the Bayouth routine in the RIT software resulted in either somewhat low true positives combined with a large fraction of false positives, or a low true positive rate with a low false positive ratio, the results being significantly affected by the threshold selected for the analysis.This method provides an effective, easy to use tool for quantitative MLC QA assessment, with excellent spatial resolution. It can be easily applied to other 2D arrays, as long as they exhibit a partial volume detector response.PACS number: 87.55.Qr

Purpose Verification of the accuracy of multileaf collimator (MLC) leaf positioning is a vital component in a quality control program in any Radiation Therapy Department. The decline of film‐based dosimetric systems reinforces the trend to use 2D detector arrays. This work introduces a new method, based on the principle of partial volume response of detectors that overcomes the problem of the somehow inferior resolution of a 2D ion chamber array. Method and Material The 2D array PTW‐729 was used for the verification of the 82 leaf‐MLC Optifocus of a Primus linear accelerator, Siemens. The partial volume response curve for each ion chamber was obtained by irradiating variable rectangular patterns, similar to that used in the Bayouth Test, so that each leaf covered a specific detector in a known proportion. Afterwards, it was developed an algorithm implemented into Matlab to predict the position of the leaf from the detector normalized response. Planes with intentionally introduced deviations of the leaf positions of ±1mm and ±2mm were irradiated upon the PTW‐729 and the predicted positions were compared with those included in the planification files. Result It was possible to detect, and correctly quantify, all the positioning errors into leaves positioning files, with a minimum number of false errors. The results obtained with this method compare favorably with those obtained with the Bayouth test using the same radiation patterns. Conclusion This method provides a superior substitute for film based QA method of MLC performance, with excellent spatial resolution. It detects and correctly quantified all the positioning errors intentionally introduced. It provides and effective and easy to use tool for quantitative measurement of MLC leaf positions, without compromising resolution. It can be easy applied to other 2D array as log as they exhibit a partial volume detector response.

Two comprehensive test cases are presented for dosimetric commissioning of a radiosurgery system, in a hospital where dedicated phantoms are not available. The system consisted of an Elekta Precise linear accelerator, an Apex micro multi-leaf collimator, and a Monaco treatment planning system (TPS). The purpose of Test I was to assess the dose accuracy with coplanar arc beams. Test II was an end-to-end type test, a rigid Leksell stereotactic frame was fixed to a watermelon phantom and Ergo++ TPS was used for stereotactic coordinates definition. The purpose of Test II was to assess the dose accuracy with non-coplanar arc beams and the influence of geometrical accuracy in the whole process. Ionization chambers were used for dose measurements. Results of Test I showed that discrepancies below 1% are achievable, while results of Test II allowed detection of geometric shifts < 1 mm with dose discrepancies lower than 1%. To the best of our knowledge, there are not published works reporting test cases for commissioning a stereotactic radiosurgery system like the one tested in this work. The designed test cases showed adequacy for assessment of TPS accuracy in complex treatment configurations, like those used in stereotactic radiosurgery.

Head and neck cancer (HNCC) is the sixth most common cancer with an incidence of approximately 600,000 cases per year and 300,000 annual deaths worldwide 1y2. In Cuba, cancer of the larynx is the fourth cause in incidence, being the fourth cause of cancer death in men. Radiotherapy constitutes an important modality in the control of these tumors and the Intensity Modulated Radiotherapy (IMRT) is a new advance in this field. With it, it is possible to improve dose distribution, decreasing the dose in adjacent healthy tissues and escalating dose in tumor. In this work we present 33 patients of National Institute of Oncology and Radiobiology in Cuba (INOR), in whom the IMRT was used as a treatment technique with a hypofractionation of the dose. Their response was observed at the end of the treatment and one month later. 56% (19) of the patients had a complete response to treatment at the primary site of the tumor and neck. 10% (3) had no response, progressed. 11 of the patients had no response at the lymph node site at the end of treatment, 8 of these 11 had complete remission one month after radiotherapy ended.