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Purpose The aim of this study was to investigate dosimetric effects of scattering filter on the stand‐up technique for total skin irradiation (TSI) with a single electron field by Monte Carlo (MC) simulation. Methods MC simulations were performed with BEAMnrc and DOSXYZnrc packages under EGSnrc environment. Scattering filter of a metal disc was mounted in the accessory slot. The filter materials (Cu, Fe, Au, Zn, Ag) were investigated, with thickness ranging from 0.05 to 0.55 mm, depending on material. The extended source to skin distance (SSD) ranging from 250 to 350 cm was studied. The following dosimetric quantities were evaluated: percent depth dose (PDD), profiles and output factor at depth of maximum, and composite dose distribution on a 30‐cm diameter cylindrical phantom. They were compared with the standard dual beam technique used at our clinic. The effects on different patient sizes were also studied. Results No filter produced acceptable profile flatness (±10% within the central 160 cm) at 250 cm SSD. At 300 cm SSD, Au (0.1 mm), Ag (0.25 mm), Cu (0.5 mm) produced acceptable flatness while Zn (0.45 mm) required 325 cm SSD. For these four configurations, the dmax was 0.90–0.99 cm, similar to dual beam (0.97 cm); R50 was 1.85–1.91 cm, compared with dual beam of 2.06 cm; the output factor ranged from 0.025 to 0.029, lower than the dual beam (0.080). With the composite fields for four configurations, the dmax was 0.10 cm, compared with dual beam (0.16 cm). The surface dose was 97%, similar to dual beam (96%). B‐factor was 3.3–3.4, compared with dual beam of 3.1. The maximum X‐ray contamination was 3%, higher than dual beam (1%). Conclusions The investigation suggests the TSI stand‐up technique can be implemented using a single electron beam if a customized filter is used. More dosimetric measurements are needed to validate the MC results and clinical implementation.

Amaç: Tüm cilt elektron ışınlamalarında (TSEI) abdominal bölge kalınlığı fazla olan hastalarda abdomen bölgesi dışındaki anterior cilt bölgeleri tedavi cihazından uzaklaşarak istenilenden farklı doz alır. Bu çalışmayla anterior cilt bölgelerindeki doz düzensizliğinin geliştirilecek olan yoğunluk ayarlı tüm cilt elektron ışınlama yöntemi (IM-TSEI) ile ortadan kaldırılması amaçlanmıştır. Yöntem: İlk olarak siyah ABS materyalinin doku eşdeğerliği belirlendi. Abdominal bölge kalınlığının 3, 6, 9, 12 ve 15 cm olduğu durumlar ABS ile üretilen farklı kalınlıklarda abdomen fantomları ve insan fantomu kullanılarak simüle edildi. Klasik Stanford TSEI tekniğinin özellikleri belirlenerek IM-TSEI tekniğinin özellikleri tanımlandı. Buna göre IM-TSEI tekniğinde hastaların önden ışın aldığı pozisyonlarda abdominal bölge kalınlığına bağlı olarak yeni bir açı çifti, 12AF ve tedavi süresi belirlenirken SSD sternum noktası baz alınarak ayarlandı. Arka vücut bölgelerinin ışınlandığı duruş pozisyonlarında abdominal bölge kalınlığı gibi kontur düzensizlikleri olmadığından klasik TSEI’daki gibi SSD bel arka noktasına göre ayarlanarak homojen profilin sağlandığı tedavi açı çifti ve tedavi süreleri kullanıldı. Abdomen kalınlığından etkilenen parametreler belirlenerek derinliğe bağlı doz değişim ölçümleri yapıldı. Farklı abdomen kalınlıkları için IM-TSEI uygulama parametreleri birleşik doz ölçümü ve tedavi sürelerinin hesaplanmasıyla belirlendi. Geliştirilen IM-TSEI yönteminin güvenilirliği antropomorfik insan fantomunun 3, 6, 9, 12 ve 15 cm’lik abdominal bölge kalınlıkları için TLD cilt dozu ve fantom içi film ölçümleriyle belirlendi. Bulgular: Yapılan istatistiksel analizler sonucunda profil ölçümleriyle IM-TSEI yönteminin TSEI tekniğine göre 6 cm ve üzerindeki abdomen kalınlıklarında vücut ön bölgesindeki dış kontur düzensizliğini telafi ederek vertikal eksen boyunca abdomen bölgesi de dahil olmak üzere daha homojen dozlar sağladığı belirlendi. İnsan fantomu cilt dozu ölçümleri ile tüm abdomen kalınlıkları için geliştirilen IM-TSEI yöntemiyle klasik TSEI yöntemine göre daha homojen doz dağılımları elde edildiği, TSEI yönteminin 9 cm ve üzerindeki abdomen kalınlıklarında homojenite kriterlerini sağlayamadığı görüldü. İki tekniğe ait tam tedavi yüzde derin doz eğrilerinin birbiriyle uyumlu olduğu ve fantom içi dozların TSEI tekniği dozlarıyla uyuştuğu görülmüştür. Ayrıca 9 cm’in altındaki abdomen kalınlıklarında hem ortalama tüm cilt dozları hem de vücut ön bölge cilt dozları için iki teknik arasında istatistiksel anlamlı farkın olmadığı ve ikisinin de kullanılabileceği belirlendi. IM-TSEI tekniğinde 9 cm ve üzerindeki abdomen kalınlıklarında vücut ön bölge ve ortalama cilt dozlarının tedavi dozundan mutlak farkının daha az olduğu belirlendi. TLD ölçümleri ile IM-TSEI tekniğiyle yoğunluk ayarı yapılmasının abdomen kalınlığına bağlı olarak vücut ön yüzeyi dozlarında oluşan düzensizliği giderdiği gösterildi. Sonuç: Geliştirilen IM-TSEI tekniği tüm abdomen kalınlıklarında güvenle kullanılabilir olmakla beraber 9 cm ve üzerindeki kalınlıklarda TSEI tekniğine göre daha avantajlıdır., Objective: In total skin electron irradiation(TSEI) of the patients with higher abdominal thickness, the anterior skin areas other than the abdominal area move away from the treatment device and receive a different dose than that of the prescribed dose. It is aimed to eliminate the dose irregularity in the anterior skin areas by the intensity modulated total skin electron irradiation(IM-TSEI) method which is developed in this study. Method: First, the tissue equivalence of the black ABS material was determined. Abdominal thicknesses of 3, 6, 9, 12 and 15cm were simulated using human phantom and abdominal phantoms of different thicknesses produced with ABS. The properties of classical Stanford TSEI technique were determined and the characteristics of IMTSEI technique were defined. In the IM-TSEI technique a new angle pair, 12FF, and treatment time were determined based on the thickness of the abdominal region at the anterior beam positions and the SSD was adjusted based on the sternum point. Since there were no contour irregularities such as abdominal thickness in the posterior treatment positions SSD was adjusted to the posterior waist point as in the conventional TSEI, and the treatment angle pair and treatment times creating a homogeneous profile were used. The parameters affected by abdominal thickness were determined and depth-dependent dose change measurements were performed. IMTSEI application parameters for different abdominal thicknesses were determined by combined dose measurements and calculation of treatment times. The quality assurance of the IM-TSEI method was done using an anthropomorphic human phantom with 3, 6, 9, 12 and 15 cm abdominal thicknesses by TLD skin dose and intraphantom film measurements. Results: As a result of statistical analyzes, it was determined that IM-TSEI method provided more homogeneous doses including abdominal region along the vertical axis by compensating for the external contour irregularities in the anterior region of the body at the thicknesses of 6cm and above according to TSEI technique with profile measurements.Human phantom skin dose measurements showed that IM-TSEI method provided more homogeneous dose distributions for all abdominal thicknesses compared to classical TSEI method and TSEI method did not meet homogeneity criteria for abdominal thicknesses of 9cm and above. The full treatment percentage depth dose curves of the two techniques were consistent with each other, and the intraphantom doses were consistent with the TSEI technique doses. In addition, it was determined that there was no statistically significant difference between the two techniques in abdominal thicknesses of less than 9 cm regarding average skin doses and body anterior skin doses, so both techniques could be used. In the IM-TSEI technique, the absolute dose difference between the anterior body region and the average skin doses was less than the prescribed dose at abdominal thicknesses of 9 cm and above. With TLD measurements, it was determined that intensity modulation by IM-TSEI technique corrected irregularities in the anterior body surface doses due to abdominal thickness. Conclusion: Although IM-TSEI technique can be used safely in all abdominal thicknesses, it is more advantageous than TSEI technique in thicknesses of 9 cm and above.

Purpose To validate the dose measurements for two total skin irradiation techniques with Monte Carlo simulation, providing more information on dose distributions, and guidance on further technique optimization. Methods Two total skin irradiation techniques (stand‐up and lay‐down) with different setup were simulated and validated. The Monte Carlo simulation was primarily performed within the EGSnrc environment. Parameters of jaws, MLCs, and a customized copper (Cu) filter were first tuned to match the profiles and output measured at source‐to‐skin distance (SSD) of 100 cm where the secondary source is defined. The secondary source was rotated to simulate gantry rotation. VirtuaLinac, a cloud‐based Monte Carlo package, was used for Linac head simulation as a secondary validation. The following quantities were compared with measurements: for each field/direction at the treatment SSDs, the percent depth dose (PDD), the profiles at the depth of maximum, and the absolute dosimetric output; the composite dose distribution on cylindrical phantoms of 20 to 40 cm diameters. Results Cu filter broadened the FWHM of the electron beam by 44% and degraded the mean energy by 0.7 MeV. At SSD = 100 cm, MC calculated PDDs agreed with measured data within 2%/2 mm (except for the surface voxel) and lateral profiles agreed within 3%. At the treatment SSD, profiles and output factors of individual field matched within 4%; dmax and R80 of the simulated PDDs also matched with measurement within 2 mm. When all fields were combined on the cylindrical phantom, the dmax shifted toward the surface. For lay‐down technique, the maximum x‐ray contamination at the central axis was (MC: 2.2; Measurement: 2.1)% and reduced to 0.2% at 40 cm off the central axis. Conclusions The Monte Carlo results in general agree well with the measurement, which provides support in our commissioning procedure, as well as the full three‐dimensional dose distribution of the patient phantom.

We report our technique for hemibody skin electron irradiation with a custom‐made plywood shield. The technique is similar to our clinical total skin electron irradiation (TSEI), performed with a six‐pair dual field (Stanford technique) at an extended source‐to‐skin distance (SSD) of 377 cm, with the addition of a plywood shield placed at 50 cm from the patient. The shield is made of three layers of standard 5/8” thick plywood (total thickness of 4.75 cm) that are clamped securely on an adjustable‐height stand. Gafchromic EBT3 films were used in assessing the shield's transmission factor and the extent of the dose penumbra region for two different shield‐phantom gaps. The shield transmission factor was found to be about 10%. The width of the penumbra (80%‐to‐20% dose falloff) was measured to be 12 cm for a 50 cm shield‐phantom gap, and reduced slightly to 10 cm for a 35 cm shield‐phantom gap. In vivo dosimetry of a real case confirmed the expected shielded area dose. PACS number(s): 87.53.Bn

Total skin electron irradiation (TSEI) is a complex technique which requires many nonstandard measurements and dosimetric procedures. The purpose of this work was to validate measured dosimetry data by Monte Carlo (MC) simulations using EGSnrc‐based codes (BEAMnrc and DOSXYZnrc). Our MC simulations consisted of two major steps. In the first step, the incident electron beam parameters (energy spectrum, FWHM, mean angular spread) were adjusted to match the measured data (PDD and profile) at SSD=100 cm for an open field. In the second step, these parameters were used to calculate dose distributions at the treatment distance of 400 cm. MC simulations of dose distributions from single and dual fields at the treatment distance were performed in a water phantom. Dose distribution from the full treatment with six dual fields was simulated in a CT‐based anthropomorphic phantom. MC calculations were compared to the available set of measurements used in clinical practice. For one direct field, MC calculated PDDs agreed within 3%/1 mm with the measurements, and lateral profiles agreed within 3% with the measured data. For the OF, the measured and calculated results were within 2% agreement. The optimal angle of 17° was confirmed for the dual field setup. Dose distribution from the full treatment with six dual fields was simulated in a CT‐based anthropomorphic phantom. The MC‐calculated multiplication factor (B12‐factor), which relates the skin dose for the whole treatment to the dose from one calibration field, for setups with and without degrader was 2.9 and 2.8, respectively. The measured B12‐factor was 2.8 for both setups. The difference between calculated and measured values was within 3.5%. It was found that a degrader provides more homogeneous dose distribution. The measured X‐ray contamination for the full treatment was 0.4%; this is compared to the 0.5% X‐ray contamination obtained with the MC calculation. Feasibility of MC simulation in an anthropomorphic phantom for a full TSEI treatment was proved and is reported for the first time in the literature. The results of our MC calculations were found to be in general agreement with the measurements, providing a promising tool for further studies of dose distribution calculations in TSEI. PACS number(s): 87.10. Rt, 87.55.K, 87.55.ne

Purpose We aimed to evaluate clinical outcomes including progression-free survival (PFS), overall survival (OS), partial response, and complete response in patients who underwent radiation therapy (RT) for mycosis fungoides (MF). Also, we sought to find prognostic factors for clinical outcomes. Materials and. Methods Total 19 patients confirmed with MF between 1999-2015 were retrospectively reviewed. Clinical and treatment characteristics, clinical outcomes, and and toxicities were analyzed. Results Eleven patients were treated with total skin electron beam radiotherapy (TSEBT) and 8 patients with involved field radiation therapy (IFRT) with median dose of 30 Gy, respectively. The median time interval from diagnosis to RT was 2.6 months (range, 0.4 to 87.3 months). The overall response rate was 100%; 11 patients (57.9%) had a complete response and 8 patients (42.1%) a partial response. The presence of positive lymph node at the time of consultation of RT was associated with lower OS (p = 0.043). In multivariate analysis, PFS was significantly lower for patients with increased previous therapies experienced following RT (p = 0.019) and for patients showing PR during RT (p = 0.044). There were no reported grade 3 or more skin toxicities related with RT. Conclusion Both IFRT and TSEBT are effective treatment for MF patients. Patients with short disease course before RT or complete response during RT are expected to have longer PFS. Positive lymph node status at the initiation of RT was associated woth poor OS, suggesting other treatment modalities such as low-dose RT for patients with low life-expectancy.

AimTo evaluate the current treatment of mycosis fungoides (MF) and Sézary syndrome (SS) focusing on the role of radiotherapy (RT), its principles and indications, and the perspectives of the novel irradiation technologies.BackgroundMF and SS are rare lymphoproliferative diseases whose incidence is increasing. For a long time RT has been used as a single modality or in integrated treatment programs for these diseases.Materials and methodsThe latest systematic reviews, primary studies and new diagnostic and treatment guidelines on MF and SS were analyzed. Clinical outcomes together with the technical aspects and the role of RT were also evaluated.ResultsNew data are available on pathogenesis, diagnostic criteria, classification and staging procedures for MF and SS and several local and systemic therapies are proposed. Localized RT can cure “minimal stage” MF while total skin electron beam irradiation (TSEI) may cure initial-stage disease and may offer important symptom relief (itch, erythroderma) in a more advanced setting. Despite its efficacy, RT is not largely used, mainly because of some technical difficulties but new RT technologies may be proposed to treat large skin surfaces.ConclusionsNew treatment programs offer good results, with median survival of more than 12 years in early-stage MF, but the median survival of 2.5 years or less in advanced stages is still a challenge. RT remains an option for all stages with a good cost/effectiveness ratio in a curative or palliative setting. New RT technologies can overcome some technical problems of treating large skin surfaces.

Total skin electron irradiation (TSEI) for patients with cutaneous lymphomas is technically challenging, and numerous approaches have been developed to overcome the many field matching problems associated with such a large and complex treatment volume. Since 1981 we have delivered TSEI using a rotational total skin electron irradiation (RTSEI) technique in conjunction with patch, treat and boost fields in order to provide complete skin and dose coverage. Initially we used a 6MeV electron beam at an extended source-skin distance (SSD) on a modified linear accelerator. More recently we began using a high dose rate electron mode on a commercially available linear accelerator. The RTSEI technique allows the delivery of a seamless surface dose to the majority of the patient's skin surface in a single treatment. In this review paper we present our three-decade experience with the technical development, dosimetry, treatment delivery and clinical outcomes of our RTSEI technique.

Traditionally, total skin electron patients remove all clothing for treatment. It is generally assumed that this is best for the treatment of superficial skin lesions out of concern clothing may significantly perturb dose. We investigate the dosimetric effect of patient gowns and determine the necessity of treating patients naked. Using GAFCHROMIC EBT2 film, dose to a cylindrical phantom was measured with cloth, paper, and tri‐layer cloth gowns, compared to no covering. A 6 MeV electron beam with spoiler accessory was used at ∼4 meters source‐to‐skin distance. The gantry was angled at 248° and 292°. The phantom was rotated at ‐60°, 0°, and 60° relative to the beam's central axis, simulating the Stanford technique. This was also repeated for films sandwiched between the phantom's discs. Using a Markus chamber, the effect of air gaps of 0 to 5 cm in cloth and paper gowns was measured. The water‐equivalent attenuation of the gowns was determined through transmission studies. Compared to no covering, films placed on the phantom surface revealed an average increase of 0.8% in dose for cloth, 1.8% for tri‐layered cloth, and 0.7% for paper. Films sandwiched within the phantom showed only slight shift of the percent depth‐dose curves. Markus chamber readings revealed 1.4% for tri‐layered cloth, and

Total skin electron irradiation (TSEI) has been employed as one of the methods of mycosis fungoides treatment since the mid-twentieth century. In order to improve the effects and limit the complications following radiotherapy, a number of varieties of the TSEI method, frequently differing in the implementation mode have been developed. The paper provides a systematic review of the different varieties of TSEI. The discussed differences concerned especially: (i) technological requirements and geometric conditions, (ii) the alignment of the patient, (iii) the number of treatment fields, and (iv) dose fractionation scheme.

Purpose Total Skin Electron Irradiation (TSEI) is a complex technique which usually involves the use of large electron fields and the dual‐field approach. In this situation, many electrons scattered from the treatment room floor are produced. However, no investigations of the effect of scattered electrons in TSEI treatments have been reported. The purpose of this work was to study the contribution of floor scattered electrons to skin dose during TSEI treatment using Monte Carlo (MC) simulations. Methods All MC simulations were performed with the EGSnrc code. Influence of beam energy, dual‐field angle, and floor material on the contribution of floor scatter was investigated. Spectrum of the scattered electrons was calculated. Measurements of dose profile were performed in order to verify MC calculations. Results Floor scatter dependency on the floor material was observed (at 20 cm from the floor, scatter contribution was about 21%, 18%, 15%, and 12% for iron, concrete, PVC, and water, respectively). Although total dose profiles exhibited slight variation as functions of beam energy and dual‐field angle, no dependence of the floor scatter contribution on the beam energy or dual‐field angle was found. The spectrum of the scattered electrons was almost uniform between a few hundred KeV to 4 MeV, and then decreased linearly to 6 MeV. Conclusions For the TSEI technique, dose contribution due to the electrons scattered from the room floor may be clinically significant and should be taken into account during design and commissioning phases. MC calculations can be used for this task.

The rotational total skin electron irradiation (RTSEI) technique at our institution has undergone several developments over the past few years. Replacement of the formerly used linear accelerator has prompted many modifications to the previous technique. With the current technique, the patient is treated with a single large field while standing on a rotating platform, at a source‐to‐surface distance of 380 cm. The electron field is produced by a Varian 21EX linear accelerator using the commercially available 6 MeV high dose rate total skin electron mode, along with a custom‐built flattening filter. Ionization chambers, radiochromic film, and MOSFET (metal oxide semiconductor field effect transistor) detectors have been used to determine the dosimetric properties of this technique. Measurements investigating the stationary beam properties, the effects of full rotation, and the dose distributions to a humanoid phantom are reported. The current treatment technique and dose regimen are also described. PACS numbers: 87.55.ne, 87.53.Hv, 87.53.Mr

SummaryBackgroundThe aim of total skin electron irradiation in the therapy of mycosis fungoides is to obtain total or partial remission of the disease while concurrently lowering the toxicity level. Total skin electron radiotherapy should fulfill the EORTC requirements from the year 2002.AimThe aim of this work is to present the rotary dual method for total skin electron irradiation used in the Great Poland Cancer Centre including its specific technical and physical aspects. These aspects include the parameters and conditions for the treatment (geometrical conditions of the technique, homogeneity of the dose, patient's positioning, additional fields and the method of dose fractionation), the absolute dosimetry (parameters and functions characterizing the electron irradiation beam used in the rotary dual method of the rotary dual method of the total skin electron irradiation) and in-vivo dosimetry conducted during the therapy with rotary dual fields, which allowed to control the dose distribution and to indicate additional skin areas, which should be irradiated with local fields.Materials/MethodsThe rotary dual method of the Total Skin Electron Irradiation proposed by the author, is based on the combination of the two most frequently used methods: 1) six dual fields’ method and 2) rotary method. The original, calculation algorithm elaborated by the author was based on the two dimensional algorithm proposed by Podgorsak et al.ResultsThe new algorithm, proposed by the author takes into the changes in doses along the vertical and horizontal axis of the object. Moreover the elliptical; shape of the object proposed by the author, is more approximated to the shape of the patient than the cylindrical object proposed by Podgorsak et al.DiscussionParameters used by authors: nominal energy – 6 MeV, mean energy at the skin surface – 2.8 MeV, values of the PDD (at the skin surface – 100%, at the 4 mm – 83.6% and at the 20 mm – 1.8%), SSD – 300 cm, field size equal to 230 cm by 115 cm, fraction dose – 1.5Gy, total time of treatment from 6 to 7 weeks were compatibility with EORTC recommendations.The elaboration of the rotary – dual method of total skin electron irradiation allowed to reduce SSD distance compared to irradiation with the rotary method. Moreover the dimension of most therapeutic rooms is usually too small to realize the total skin electron irradiation methods with SSD distance higher than 350 cm. Patient set-up and rotation during irradiation (based on rotary method) allow to reduce the number of dual fields used in Stanford method from six fields to one dual field (rotary-dual method). This modification leads to reduction of time assigned for set-up verification). Presented results of in-vivo dosimetry for selected patients treated in Great Poland Cancer Centre confirm the needfulness for using additional local for areas of perineum, scalp vertex, hands, neck-shoulders, plants.Moreover, obtained results indicate that total dose prescribed for additional fields depends on height and obesity and should be prescribed individually for each patient.

BackgroundMycosis Fungoides (MF) is an indolent lymphoproliferative disorder affecting dermis caused by abnormal proliferation of CD4+ T-cells. Radiation therapy is the most effective modality of treatment for MF which offers cure in limited stage disease and desirable palliation in advance stage disease. Treating entire skin having many curved surfaces and folds with radiation is the real challenge for the radiation oncologist. Many techniques, dose schedules and modifications in total skin electron irradiation (TSEI) have been tried since 1950s. TSEI treatment is a very time consuming, inconvenient and physically challenging to both patient as well as oncologist.AimAt our center TSEI was performed since 1983 with conventional linear accelerator where the treatment time was prolonged beyond two hours, which was very difficult or the patient, oncologist, technical officer and eating away the machine time hampering the treatment of other patients. From 1998 we shifted to high dose rate (HDR) mode, in order to bring down the treatment time of a single patient every day from two and half hour to 15min. The reduction of treatment time increases patient compliance and at the same time saved machine time.Materials and methodsBetween 1998 and 2003, eleven pathological diagnosed MF patients were treated using HDR TSEI. All the patients were male between 40 and 70 years of age, who had the history of having the disease for 7–22 months. Four patients had T2 and seven patients had T3 stage disease with more than 90% skin surface involvement. TSEI was performed with 4MeV electrons with a daily fraction size of 120cGy to a total dose of 36Gy. At the end of 36Gy, boost dose of 10Gy was delivered to self shielding regions like sole, scalp and perineum. Considering the treatment related toxicities and consequent treatment interruptions, in the first seven patients, the last four patients were treated using similar HDR TSEI technique with modified treatment schedule, where the treatment was given on an alternate day basis following 2nd week of initiation of treatment.ResultsThe patients were followed over a period of 144 months with a median of 72 months. Nine patients are alive without any evidence of disease, one patient relapsed and one died due to progression of disease. The most common radiation related morbidities are erythema, skin blisters, various degree of desquamations, swelling of joints (specially small joints) etc. which are controlled by treatment interruptions and conservative measures. By modifying the treatment schedule, the incidence of toxicity as well as treatment interruptions were brought down.ConclusionsWe can conclude that HDR-TSEI is an excellent and safe therapeutic modality for the patients with MF both curative as well as palliative without any added toxicity profile, provided patient positioning is done properly.

The purpose of this study was to compare anatomical and dosimetric variations in first 15 fractions, and between fractions 16 and 25, during intensity‐modulated radiotherapy (IMRT) for nasopharyngeal carcinoma (NPC). Twenty‐three NPC patients who received IMRT in 33 fractions were enrolled. Each patient had two repeat computed tomography (CT) scans before the 16th and 25th fraction. Hybrid IMRT plans were generated to evaluate the dosimetric changes. There was a significant decrease of the transverse diameter of nasopharyngeal and neck as well as gross tumor volume (GTV) in the primary nasopharyngeal carcinoma (GTVnx) and involved lymph nodes (GTVnd) during the first 15 fractions, and between fraction 16 and 25 (p