Anal Cancer: Radiation Therapy Planning



Fig. 16.1
Representative plan with and without contours (color wash) of the low pelvis of a woman with T1 N0 anal cancer treated with the MD Anderson SIB technique. This case highlights the benefit of a vaginal dilator as the external genitalia and anterior vaginal wall receive less than 30 Gy. Elective nodal CTV = yellow, elective nodal PTV = turquoise. Anal tumor = maroon, anal tumor CTV = khaki, anal tumor PTV = blue



A331976_1_En_16_Fig2_HTML.jpg


Fig. 16.2
Representative plan of a woman with locoregionally advanced, T2 N3 anal cancer treated with the MD Anderson SIB technique. She had small left external iliac and perirectal lymph nodes that were treated to 50 Gy. She was treated with a vaginal dilator. Elective nodal CTV = yellow, elective nodal PTV = turquoise. Gross nodal GTV = purple, gross nodal CTV = light green, gross nodal PTV = dark green. Anal tumor = maroon, anal tumor CTV = khaki, anal tumor PTV = blue


A331976_1_En_16_Fig3_HTML.jpg


Fig. 16.3
Representative plan of a man with T3 N0 anal cancer treated with the MD Anderson SIB technique and a scrotal shelf. This case highlights how the scrotal shelf elevates the external genitalia out of the high-dose region. Elective nodal CTV = yellow, elective nodal PTV = turquoise. Anal tumor = maroon, anal tumor CTV = khaki, anal tumor PTV = blue




 






     

  1. (iv)


    IMRT with sequential courses: An alternative IMRT technique is to use sequential courses with field reductions, more closely replicating the doses and fractionation schemes from the AP/PA and 3D conformal techniques discussed above [12]. Patients are initially treated with 30.6 Gy in 1.8 Gy fractions to the pelvic and inguinal regions, followed by an additional 14.4 Gy (total 45 Gy) in 1.8 fractions to the low pelvic region, followed by a total dose of at least 54 Gy to gross disease. Developing multiple sequential IMRT plans can make treatment planning more challenging. However, an advantage of this technique is that dose is limited to 30.6 Gy in part of the treatment volume.

     






       

    1. (b)


      Contours: Consensus contouring guidelines for IMRT-based treatment planning of anal tumors have been compiled by two international organizations – the Radiation Therapy Oncology Group (RTOG) and Australasian Gastrointestinal Trials Group (AGITG).


      1. (i)


        RTOG [13]


        1. 1.


          Boost CTV: Includes the primary anal tumor and any enlarged pelvic or inguinal lymph nodes. For the boost CTV, RTOG recommended a 2.5 cm expansion on the primary GTV and 1 cm expansion on the nodal GTV.

           

        2. 2.


          Elective nodal volumes: The RTOG consensus guidelines (Fig. 16.4) describe three separate elective nodal CTV, named CTVA, CTVB, and CTVC, all of which should be covered for anal cancer. In general, all CTV volumes should include at least a 7 mm margin on the target vessels [14].


          1. (a)


            CTVA: presacral, perirectal, and bilateral internal iliac lymph nodes.


            1. (i)


              Cranial border: The sacral promontory/bifurcation of the common iliacs branching into the internal and external iliacs.

               

            2. (ii)


              Caudal border: 2 cm beyond the anal verge or most distal aspect of gross disease.

               

            3. (iii)


              Anterior border:


              1. 1.


                High pelvis: at least 1 cm anterior to the sacrum

                 

              2. 2.


                Mid/Low pelvis: 1 cm into the posterior bladder

                 

               

            4. (iv)


              Posterior and lateral borders: Extend contours to the pelvic sidewall muscles and sacrum, carving out of bone and muscles. Levators should be included.

               

            5. (v)


              For T4 disease, CTVA should include a 1–2 cm margin around the area of invasion.

               

            6. (vi)


              Although the RTOG consensus guidelines do not recommend inclusion of the ischiorectal fossa, many radiation oncologists recommend including this region in patients with anal cancer.

               

             

          2. (b)


            CTVB: external iliac lymph nodes


            1. (i)


              Cranial border: Branching of the external iliac vessels from common iliac vessels

               

            2. (ii)


              Caudal border: Distal extent of the internal obturator vessels, with the superior pubic rami serving as a bony landmark

               

            3. (iii)


              Anterior/posterior and lateral borders: 7–8 mm margin on external iliac vessels, usually with ~10 mm anterolateral margin

               

             

          3. (c)


            CTVC: Inguinal lymph nodes


            1. (i)


              Cranial border: Distal extent of the internal obturator vessels, with the superior pubic rami serving as an approximate bony landmark.

               

            2. (ii)


              Caudal border: 2 cm distal to the saphenous/femoral junction which is approximately at the level of the lesser trochanter.

               

            3. (iii)


              Anterior/posterior and lateral borders: The inguinal region should be contoured as a compartment, including any visible nodes, while carving out of bone and muscles.

               

             

           

        3. 3.


          PTV: A 0.7–1 cm margin was recommended.

           


        A331976_1_En_16_Fig4a_HTML.jpgA331976_1_En_16_Fig4b_HTML.jpgA331976_1_En_16_Fig4c_HTML.jpg


        Fig. 16.4
        RTOG anorectal consensus contours for elective nodal coverage in the (a) upper pelvis, (b) mid-pelvis, and (c) low pelvis. Brown = CTVA. Blue = CTVB. Red = CTVC (Adapted from Myerson et al. [13])

         

      2. (ii)


        AGITG [15]


        1. 1.


          Boost CTV: Includes the primary tumor, the entire anal canal, and the internal and external anal sphincters with a 2 cm margin, while following anatomical boundaries. Involved nodes or nodal regions should be included with a 1–2 cm margin, also while following anatomical boundaries.

           

        2. 2.


          Elective nodal CTV: The AGITG guidelines (Fig. 16.5) describe specific nodal regions including the mesorectum, presacral space, internal iliac nodes, ischiorectal fossa, obturator nodes, external iliac nodes, and inguinal nodes.


          1. (a)


            Mesorectum: Extends from the rectosigmoid junction cranially to the anorectal junction caudally (where the mesorectal fat space ends). Anteriorly, extends to the penile bulb, prostate, seminal vesicles, and bladder in men and to the vagina, cervix, uterus, and bladder in women. An additional 1 cm anterior margin is recommended to account for changes in bladder filling. The mesorectum extends posteriorly to the presacral space and laterally to the levator ani in the lower pelvis and to the internal iliac region in the upper pelvis.

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      Oct 18, 2017 | Posted by in GASTROENTEROLOGY | Comments Off on Anal Cancer: Radiation Therapy Planning

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