Simplified estimation method for dose distributions around field junctions in proton craniospinal irradiation

In radiotherapy involving craniospinal irradiation (CSI), field junctions of therapeutic beams are necessary, because a CSI target is generally several times larger than the maximum field size of the beams. The purpose of this study was to develop a simplified method for estimating dose uniformity around the field junctions in proton CSI. We estimated the dose profiles around the field junctions of proton beams using a simplified field-junction model, in which partial lateral dose distributions around the field edge were assumed to be approximated using the error function. We measured the lateral dose distributions of the proton beams planned for the CSI treatment using a two-dimensional (2D) ionization chamber array. Although dose hot spots and cold spots tend to be underestimated by a chamber array because of the partial volume effect of the sensitive volume and discrete chamber positions, the model estimation results were fairly consistent with the measurements obtained using a 2D chamber array subjected to CSI-simulated serial irradiation. The simplified junction model enabled us to estimate the dose distributions and dependence of the setup position gap on the dose uniformity around the field junctions on the basis of the field-by-field dose profiles measured using the 2D chamber array.     

Intensity-modulated radiation therapy and volumetric-modulated arc therapy for adult craniospinal irradiation—A comparison with traditional techniques

Craniospinal irradiation (CSI) poses a challenging planning process because of the complex target volume. Traditional 3D conformal CSI does not spare any critical organs, resulting in toxicity in patients. Here the dosimetric advantages of intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) are compared with classic conformal planning in adults for both cranial and spine fields to develop a clinically feasible technique that is both effective and efficient. Ten adult patients treated with CSI were retrospectively identified. For the cranial fields, 5-field IMRT and dual 356° VMAT arcs were compared with opposed lateral 3D conformal radiotherapy (3D-CRT) fields. For the spine fields, traditional posterior-anterior (PA) PA fields were compared with isocentric 5-field IMRT plans and single 200° VMAT arcs. Two adult patients have been treated using this IMRT technique to date and extensive quality assurance, especially for the junction regions, was performed. For the cranial fields, the IMRT technique had the highest planned target volume (PTV) maximum and was the least efficient, whereas the VMAT technique provided the greatest parotid sparing with better efficiency. 3D-CRT provided the most efficient delivery but with the highest parotid dose. For the spine fields, VMAT provided the best PTV coverage but had the highest mean dose to all organs at risk (OAR). 3D-CRT had the highest PTV and OAR maximum doses but was the most efficient. IMRT provides the greatest OAR sparing but the longest delivery time. For those patients with unresectable disease that can benefit from a higher, definitive dose, 3D-CRT–opposed laterals are the most clinically feasible technique for cranial fields and for spine fields. Although inefficient, the IMRT technique is the most clinically feasible because of the increased mean OAR dose with the VMAT technique. Quality assurance of the beams, especially the junction regions, is essential.     

A robust volumetric arc therapy planning approach for breast cancer involving the axillary nodes

We quantify the robustness of a proposed volumetric-modulated arc therapy (VMAT) planning and treatment technique for radiotherapy of breast cancer involving the axillary nodes. The proposed VMAT technique is expected to be more robust to breast shape changes and setup errors, yet maintain the improved conformity of VMAT compared to our current standard technique that uses tangential intensity-modulated radiation therapy (IMRT) fields. Treatment plans were created for 10 patients. To account for anatomical variation, planning was carried out on a computed tomography (CT) with an expanded breast, followed by segment weight optimization (SWO) on the original planning CT (VMAT + SWO). For comparison purposes, tangential field IMRT plans and conventional VMAT (cVMAT) plans were also created. Anatomical changes (expansion and contraction of the breast) and setup errors were simulated to quantify changes in target coverage, target maximum, and organ-at-risk (OAR) doses. Finally, robustness was assessed by calculating the actual delivered dose for each fraction using cone-beam CT images acquired during treatment. Target coverage of VMAT + SWO was shown to be significantly more robust compared to cVMAT technique, against anatomical variations and setup errors. Sensitivity of the clinical target volume (CTV) V95% is ?5%/cm of expansion for the proposed technique, which is identical to the IMRT technique and much lower than the ?22%/cm for cVMAT. Results are similar for setup errors. OAR doses are mostly insensitive to anatomical variations and the OAR sensitivity to setup variations does not depend on the planning technique. The results are confirmed by dose distributions recalculated on cone-beam CT, showing that for VMAT + SWO the CTV V95% remains within 2.5% of the planned value, whereas it deviates by up to 7% for cVMAT. A practical VMAT planning technique is developed, which is robust to daily anatomical variations and setup errors.     

Verification techniques and dose distribution for computed tomographic planned supine craniospinal radiation therapy.

A modified 3-field technique was designed with opposed cranial fields and a single spinal field encompassing the entire spinal axis. Two methods of plan verifications were performed before the first treatment. First, a system of orthogonal rulers plus the thermoplastic head holder was used to visualize the light fields at the craniospinal junction. Second, film phantom measurements were taken to visualize the gap between the fields at the level of the spinal cord. Treatment verification entailed use of a posterior-anterior (PA) portal film and placement of radiopaque wire on the inferior border of the cranial field. More rigorous verification required a custom-fabricated orthogonal film holder. The isocenter positions of both fields when they matched were recorded using a record-and-verify system. A single extended distance spinal field collimated at 42 degrees encompassed the entire spinal neuraxis. Data were collected from 40 fractions of craniospinal irradiation (CSI). The systematic error observed for the actual daily treatments was -0.5 mm (underlap), while the stochastic error was represented by a standard deviation of 5.39 mm. Measured data across the gapped craniospinal junction with junction shifts included revealed a dose ranging from 89.3% to 108%. CSI can be performed without direct visualization of the craniospinal junction by using the verification methods described. While the use of rigorous film verification for supine technique may have reduced the systematic error, the inability to visualize the supine craniospinal junction on skin appears to have increased the stochastic error compared to published data on such errors associated with prone craniospinal irradiation.     

Conventional Craniospinal Irradiation with Patient Supine and Source-Skin Distance (SSD) 100 cm for Spinal Field

We describe a method of craniospinal irradiation (CSI) in the supine position and at a source-skin distance (SSD) of 100 cm for the spinal fields. The procedure is carried out with a 100-cm isocenter linear accelerator and conventional simulator, and the treatment is delivered with 2 opposed lateral cranial fields at source-axis distance (SAD) of 100 cm and 1 or 2 direct posterior spinal fields at SSD, 100 cm. The half beam–blocked cranial fields with a collimator rotation is used to match the superior border of the spinal field at the level of C2 vertebral body. The length of the spinal field is fixed, and is the same if 2 spinal fields are used. The position of the isocenter of the spine field is defined by longitudinally moving the couch a distance from the isocenter of the cranial fields and adjusting the SSD = 100 cm to the surface of the couch with the gantry rotated to the angle of 180° (posteroanterior position), and the distance can be calculated easily according to a few parameters. It only needs a simple calculation without couch rotation, extended SSD, or markers. The inferior and superior borders of the spinal field do not require visualization under fluoroscopy when it is beyond the visual field of the simulator. The entire simulation takes no more than 20 minutes. Supine craniospinal treatment using this technique may substitute the traditional prone position as a potentially beneficial alternative to CSI.     

A dosimetric study using split x-jaw planning technique for the treatment of endometrial carcinoma

The aim of this retrospective study was to determine if the split x-jaw planning technique could be used with Varian linear accelerators to improve plan conformity and limit dose to organs at risk (OAR) for planning target volumes that require field sizes larger than the 15 cm extent of the multileaf collimator in the x-jaw position. Traditional planning techniques include limited and open x-jaw methods. The study population included 20 randomly selected patients with endometrial carcinoma. Treatment plans for each patient were designed using split, limited, and open x-jaw volumetric modulated arc radiotherapy for comparison purposes. Dose statistics including the PTV conformity index and dose to OAR were used to evaluate plan performance. The split x-jaw planning method had the most consistent conformity index (0.98 ± 0.02), followed by the open (01 ± 0.03), and the limited (1.04 ± 0.05) techniques. On average, the split method better spared the OAR. In comparison to the limited and open techniques, the split method reduced the dose to the bowel by 3.8%, rectum by 3.2%, sigmoid by 2.1%, right femoral head by 3.5%, and left femoral head by 3.9%. The split and open techniques showed comparable bladder results and were superior over the limited method. The monitor units were highest with the split method leading to increased treatment times. The split x-jaw planning technique should be used with Varian linear accelerators to produce superior volumetric modulated arc radiotherapy plans for planning target volumes larger than the maximum extent of the multileaf collimator in the x-jaw direction.     

An optimized posterior axillary boost technique in radiation therapy to supraclavicular and axillary lymph nodes: A comparative study

To assess the advantages of an optimized posterior axillary (AX) boost technique for the irradiation of supraclavicular (SC) and AX lymph nodes. Five techniques for the treatment of SC and levels I, II, and III AX lymph nodes were evaluated for 10 patients selected at random: a direct anterior field (AP); an anterior to posterior parallel pair (AP-PA); an anterior field with a posterior axillary boost (PAB); an anterior field with an anterior axillary boost (AAB); and an optimized PAB technique (OptPAB). The target coverage, hot spots, irradiated volume, and dose to organs at risk were evaluated and a statistical analysis comparison was performed. The AP technique delivered insufficient dose to the deeper AX nodes. The AP-PA technique produced larger irradiated volumes and higher mean lung doses than the other techniques. The PAB and AAB techniques originated excessive hot spots in most of the cases. The OptPAB technique produced moderate hot spots while maintaining a similar planning target volume (PTV) coverage, irradiated volume, and dose to organs at risk. This optimized technique combines the advantages of the PAB and AP-PA techniques, with moderate hot spots, sufficient target coverage, and adequate sparing of normal tissues. The presented technique is simple, fast, and easy to implement in routine clinical practice and is superior to the techniques historically used for the treatment of SC and AX lymph nodes.     

The midline dose distribution for a three-field radiotherapy technique.

At the University of Florida, head and neck cancer often is irradiated using parallel opposed lateral fields (with inferior borders slanted superiorly) and an anterior low neck field. A common criticism is that overlap may occur at the match-line junction of the three fields, resulting in an increased risk of radiation myelitis. One setup for treatment of the oropharynx and two for the larynx were irradiated in an anthropomorphic head and neck phantom made of tissue-equivalent polyacrylamide gel with a two-dimensional thermoluminescent dosimeter array in its sagittal midplane. The results showed that no excess radiation dose was measured at the junction of the three fields. The "spinal cord dose," as percentage of dose to the central axis of the primary field, was as follows: oropharynx setup, 15% to 100%; larynx setup with midline tracheal block, 10% to 90%; larynx setup without tracheal block, 10% to 90%. In conclusion, the University of Florida three-field technique for head and neck cancer produces no measured increase in dose at field junctions.     

腮腺癌术后高危复发区放疗的不同治疗计划剂量学比较

目的 探讨腮腺癌术后高危复发区用何种照射方法可以更有效的使靶区剂量均匀及更好的保护危及器官.方法 对8例腮腺癌术后患者设计治疗计划,处方剂量为95%计划靶区(PTV)60 Gy/30次.对常规放疗、二维适形放疗(2D-CRT)、三维适形放疗(3D-CRT)和调强放疗(IMRT)等放射治疗技术的腮腺癌术后靶区进行放疗计划设计,分析比较各种治疗计划靶区适形度和在保护危及器官等方面的优劣.结果 在2D-CRT时,以计算点深度取3.5 cm,电子线能量采取12 MeV及X射线/电子射线(X/E)剂量比为1∶2时靶区的适形度和均匀度较好,危及器官的受量较低.与2D-CRT比较,常规放疗照射野能够较好地包括CT断层图像上勾画的靶区.与2D-CRT及3D-CRT相比,IMRT计划有最好的靶区适形度及均匀度,同时对危及器官有较好的保护作用.结论 X射线与电子线混合线束照射时,剂量计算点深度取3.5 cm左右、电子线能量采取12 MeV及X/E剂量比为1∶2时,靶区的适形度和均匀度较好,对正常组织的保护较好,但具体患者最好用计划系统来选择以上指标.常规放疗按解剖标志确定的照射野能够较好地包括三维靶区.IMRT计划的靶区适形度及均匀度最好,并且危及器官受量较低,在腮腺癌术后放射治疗中IMRT技术是值得推广并普及的放射治疗技术.     

Volumetric modulated arc radiotherapy for esophageal cancer

A treatment planning study was performed to evaluate the performance of volumetric arc modulation with RapidArc (RA) against 3D conformal radiation therapy (3D-CRT) and conventional intensity-modulated radiation therapy (IMRT) techniques for esophageal cancer. Computed tomgraphy scans of 10 patients were included in the study. 3D-CRT, 4-field IMRT, and single-arc and double-arc RA plans were generated with the aim to spare organs at risk (OAR) and healthy tissue while enforcing highly conformal target coverage. The planning objective was to deliver 54 Gy to the planning target volume (PTV) in 30 fractions. Plans were evaluated based on target conformity and dose-volume histograms of organs at risk (lung, spinal cord, and heart). The monitor unit (MU) and treatment delivery time were also evaluated to measure the treatment efficiency. The IMRT plan improves target conformity and spares OAR when compared with 3D-CRT. Target conformity improved with RA plans compared with IMRT. The mean lung dose was similar in all techniques. However, RA plans showed a reduction in the volume of the lung irradiated at V_20Gy and V_30Gy dose levels (range, 4.62–17.98%) compared with IMRT plans. The mean dose and D_35% of heart for the RA plans were better than the IMRT by 0.5–5.8%. Mean V_10Gy and integral dose to healthy tissue were almost similar in all techniques. But RA plans resulted in a reduced low-level dose bath (15–20 Gy) in the range of 14–16% compared with IMRT plans. The average MU needed to deliver the prescribed dose by RA technique was reduced by 20–25% compared with IMRT technique. The preliminary study on RA for esophageal cancers showed improvements in sparing OAR and healthy tissue with reduced beam-on time, whereas only double-arc RA offered improved target coverage compared with IMRT and 3D-CRT plans.     

A Novel Method of Island Blocking in Whole Abdominal Radiotherapy Using a Modified Electronic Tissue Compensation Technique

Traditionally, large fields requiring island blocking used external beam radiation therapy (EBRT) with Cerrobend blocks to limit dose to the critical structures. It is laborious to construct blocks and use them on a daily basis. We present a novel technique for island blocking using a modified electronic tissue compensation (MECOMP) technique. Five patients treated at our institution were selected for this study. The study compared two planning techniques: a novel MECOMP and a conventional EBRT technique. Conventional fields were defined using anterior-posterior and posterior-anterior (PA) fields. The kidneys were contoured and an aperture cut-out block was fitted to the OAR with a 1-cm margin (OAR_CTV) and placed in the PA field. A dynamic multileaf collimation (DMLC) plan with ECOMP was developed using identical beam and blocking strategy; this tissue compensation–based fluence map was modified to deliver a “zero” dose to the CTV_OAR from the PA field. There were no significant differences in the mean, maximum, and minimum doses to the right or left kidney between the two methods. The mean, maximum, and minimum doses to the peritoneal cavity were also not significantly different. The number of monitor units (MUs) required was increased using the MECOMP (273 vs. 1152, p < 0.01). The MECOMP is effectively able to deliver DMLC-based radiotherapy, even with island blocks present. This novel use of MECOMP for whole abdominal radiotherapy should substantially reduce the labor, daily treatment time, and treatment-related errors through the elimination of cerrobend blocks.     

High-precision radiotherapy for craniospinal irradiation: evaluation of three-dimensional conformal radiotherapy,intensity-modulated radiation therapy and helical TomoTherapy

This study aimed to establish the feasibility of intensity-modulated radiation therapy (IMRT) in craniospinal irradiation (CSI) using conventional linear accelerator (IMRT_LA) and compare it dosimetrically with helical TomoTherapy (IMRT_Tomo) and three-dimensional conformal radiotherapy (3DCRT). CT datasets of four previously treated patients with medulloblastoma were used to generate 3DCRT, IMRT_LA and IMRT_Tomo plans. A CSI dose of 35 Gy was prescribed to the planning target volume (PTV). IMRT_LA plans for tall patients were generated using an intensity feathering technique. All plans were compared dosimetrically using standardised parameters. The mean volume of each PTV receiving at least 95% of the prescribed dose (V_95%) was >98% for all plans. All plans resulted in a comparable dose homogeneity index (DHI) for PTV_brain. For PTV_spine, IMRT_Tomo achieved the highest mean DHI of 0.96, compared with 0.91 for IMRT_LA and 0.84 for 3DCRT. The best dose conformity index was achieved by IMRT_Tomo for PTV_brain (0.96) and IMRT_LA for PTV_spine (0.83). The IMRT_Tomo plan was superior in terms of reduction of the maximum, mean and integral doses to almost all organs at risk (OARs). It also reduced the volume of each OAR irradiated to various dose levels, except for the lowest dose volume. The beam-on time was significantly longer in IMRT_Tomo. In conclusion, IMRT_Tomo for CSI is technically easier and potentially dosimetrically favourable compared with IMRT_LA and 3DCRT. IMRT for CSI can also be realised on a conventional linear accelerator even for spinal lengths exceeding maximum allowable field sizes. The longer beam-on time in IMRT_Tomo raises concerns about intrafraction motion and whole-body integral doses.Medulloblastoma is the most common malignant neoplasm of the central nervous system in children, constituting approximately 20% of all paediatric brain tumours [1]. The last two decades have witnessed tremendous advances in technology and biology, resulting in an improved outcome for these children [2–4] owing to refinements in micro-neurosurgery, more effective chemotherapy regimens and modern radiotherapy techniques. A more mature understanding of the biology of disease has led to a contemporary clinico-biological risk stratification system for assigning prognosis and deciding treatment [5]. The current standard of care consists of maximal safe resection followed by radiotherapy and chemotherapy, yielding a 5-year survival rate of >80% for average-risk medulloblastoma and >50% for high-risk disease [4].Radiotherapy for medulloblastoma entails irradiation of the entire neuraxis, i.e. craniospinal irradiation (CSI) with a homogeneous dose. This still remains one of the most technically challenging processes in radiotherapy planning and delivery because of the need to irradiate a very large and complex shaped target volume uniformly. With continuous improvements in long-term survival, particularly in children with average-risk medulloblastoma, there is a growing concern regarding treatment-related long-term side effects. These include neurocognitive decline, hearing impairment, growth retardation, endocrine dysfunction, cataract formation, cardiomyopathy, impaired fertility and second malignancies. The majority of these late effects are dose- and volume-related, and form the basis of reduced dose CSI (23.4 Gy) for average-risk disease in conjunction with chemotherapy [6], and are the clinical motivation for investigating sophisticated emerging radiotherapy techniques to reduce doses to non-target tissues to ameliorate toxicity.Field shaping for CSI has evolved from traditional bony landmarks using two-dimensional (2D) planar radiographs to the more recent CT simulation techniques [7, 8]. In most of these techniques, field shaping and matching of cranial and spinal fields are done geometrically with no attempt to compute the dose–volume data of the target and/or organs at risk (OARs). Various modifications to treatment planning and delivery have been made in an effort to improve target volume coverage, dose homogeneity and conformity. Parker et al [9] have recently reported the feasibility of conventional linear accelerator (LA)-based intensity-modulated radiotherapy (IMRT) for CSI in small children. However, even with this advanced technique, matching of cranial and spinal fields is an unavoidable situation in LA-based IMRT for CSI. This problem is further compounded in older children and adolescents, in whom the spinal lengths often exceed the allowable maximum field sizes, necessitating a second spinal–spinal junction. Helical TomoTherapy has emerged as a revolutionary and novel approach to radiation treatment, whereby a 6 MV LA mounted on a ring gantry continuously rotates around the patient to deliver radiation in helical mode as the patient is transported through the ring, allowing treatment to large cylindrical volumes of up to 40 × 160 cm². This unique feature of TomoTherapy has been explored for CSI [10, 11], with promising dosimetric results. However, to the best of our knowledge, no formal dosimetric comparison has been made between conventional three-dimensional conformal radiotherapy (3DCRT) and IMRT, either with conventional LA (IMRT_LA) or Helical TomoTherapy (IMRT_Tomo), for CSI using the same patient dataset. The aim of this study was to establish the feasibility of performing IMRT using conventional LA for CSI in medulloblastoma patients of differing spinal lengths and to compare it dosimetrically with 3DCRT and Helical TomoTherapy.     

Border separation for adjacent orthogonal fields

Field border separations for adjacent orthogonal fields can be calculated geometrically, given the validity of some important assumptions such as beam alignment and field uniformity. Thermoluminescent dosimetry (TLD) measurements were used to investigate dose uniformity across field junctions as a function of field separation and, in particular, to review the CCSG recommendation for the treatment of medulloblastoma with separate head and spine fields.     

Multicentre dosimetric comparison of photon-junctioning techniques in head and neck radiotherapy

Because many head and neck radiotherapy treatment techniques rely on a junction between X-ray fields, it was the aim of the present study to investigate the use of different junctioning techniques and the affect on the dose across the junction. Techniques in use at nine radiotherapy centres in Australia were investigated using thermoluminescence dosimetry (TLD). The techniques could broadly be divided into two groups: (i) use of the light field to match the fields after moving the patient; and (ii) use of asymmetric collimation to create a single isocentre located in the junction. The mean dose at the junction and its reproducibility was studied in five consecutive treatments in each centre using 25 TLD chips placed throughout the junction in an anthropomorphic phantom. There was a tendency for the mono-isocentric technique to deliver a lower, more accurate mean dose at the junction (Group I: 1.22 Gy (n = 8) vs Group II: 0.96 Gy (n = 5) for 1 Gy planned, some centres contributed to both technique) with greater reproducibility (Group I: 9.6%, Group II: 5.1% of the mean dose). We conclude that a mono-isocentric treatment technique has the potential to deliver a more accurate and reproducible dose distribution at the field junction of photon beams in head and neck treatment.     

An approach to abutting adjacent fields

Problems arise in designing treatment techniques involving two pair of adjacent opposing fields where machine limitations require the patient to flip from supine to prone positions. Mantle and para-aortic treatments, in particular, can create challenging problems because of changes in patient position, different SSD's between adjacent fields, internal anatomical changes from supine to prone position, as well as field size and other treatment machine limitations. A simulator technique has been developed which takes cognizance of these limitations in specifying the gap between adjacent fields. It employs collinearity of the 50% decrement lines of adjacent-opposed field edges and the intersection of all four edges at an internal mid-plane match point. The technique maintains dose homogeneity and eliminates hot and cold triangles in the area of abutment. Simulation radiographs facilitate identification of collinearity with respect to a specific vertebra in the plane of abutment. In summary, this approach: Verifies abutment of coplanar fields by use of match film, improves isodose uniformity at mid-plane, evaluates dose distributions when abutment occurs at a point anterior or posterior to midline, prevents the possibility of spinal cord complications that might occur due to three field overlap.